Paper

Crystallization-induced dynamic resolution and analysis of the non-

covalent interactions in the crystal packing of peptide–biphenyl

hybrids{{

Bernardo Herradon,*a Ana Montero,a Enrique Manna and Miguel A. Maestrob

aInstituto de Quımica Organica General, C.S.I.C., Juan de la Cierva 3, 28006 Madrid,

Spain. E-mail: herradon@iqog.csic.es; Fax: 134915644853bDepartamento de Quımica Fundamental, Facultad de Ciencias, Universidade da Coruna,

Campus da zapateira, 15071 A Coruna, Spain

Received 4th May 2004, Accepted 3rd June 2004

First published as an Advance Article on the web 17th September 2004

Dynamic atropselective resolutions of the peptide–biphenyl hybrids 3–5 were achieved by crystallization from

methanol solution. These compounds were studied by single-crystal X-ray diffraction. These hybrids crystallize

in highly ordered structures kept by an extensive array of intramolecular and intermolecular hydrogen bonds.

The solid-state molecular structure is highly dependent on the substituents on the a-carbon of the amino acid.

The presence of additional aromatic rings in the side chain causes the molecules in the crystal to interact

through a variety of interactions involving the aromatic rings. The resulting supramolecular structures are

helices whose orientation depends on the senses of the chirality of the a-carbon of the amino acid residues

and the aryl–aryl bond.

Introduction

The ultimate goal of crystal engineering is the design andpreparation of solid materials with defined properties.1

Although important advances have been achieved in recentyears, the field is still far from maturity. In order to understandthe rules responsible for the construction of a specific solid statestructure some basic knowledge is required. Two importantrelated issues are: (i) which building blocks can be employedfor the generation of the desired structure; and (ii) whichnon-covalent interactions are responsible for the packing ofmolecules in the crystal.

Regarding the nature of the non-covalent interactions,2 theclassical strong hydrogen bond (i.e.: X–H…Y, where X and Yare electronegative atoms), due to its strength and direc-tionality, is the most frequent interaction responsible of thecrystal packing.3 However, the cooperative effect of severalweaker interactions can be useful for the design of organic solids.Such weaker interactions include: (a) non-classical hydrogenbond;4 (b) hydrophobic;5 and (c) electrostatic interactions ofhigher-order multipole (quadrupolar, octupolar, etc.).6

The aromatic compounds are versatile building blocks forcrystal engineering,7 since they can participate in a variety ofweak interactions,8 whose strength can be modulated by thetype of the interaction and the relative orientation of theinteracting chemical species.9 The class of interaction can becontrolled by the proper choice of the substitution. On theother hand, the peptides—compounds formed by amidelinkages—possess groups (CO and NH) able to form stronghydrogen bonds that, frequently, are the dominant force intheir structure; being responsible for the secondary structure

(helix, sheet, etc.)10 and the crystal packing.11 However, theuse of modified peptides and appropriate choice of substituentat the a-position can partially override the influence of theCO…HN hydrogen bond.

Our group has been involved in the research on some peptideconjugates where one or several amino acid or peptide chainsare linked to a non-peptide platform. In connection with thistopic, we have reported the synthesis, structure and biologicalactivity of peptide–carbohydrate hybrids,12 peptide–heterocyclehybrids,13 and peptide–biphenyl hybrids,14 where the non-peptide platforms are derivative of hexenopyranosides, poly-annular aromatic heterocycles and biphenyl, respectively. Thepeptide–biphenyl hybrids (A, Fig. 1) are acyclic derivatives of1,1’-biphenyl having amino acid or peptide chains at thepositions C(2) and C(2’). Since both biphenyl derivatives andpeptides possess useful properties, the combination of the twofunctionalities can provide interesting compounds. Peptidesand related compounds have sparked an intense researchactivity. This kind of compounds has been used as a tool toelucidate biochemical processes (including protein folding),15

technological materials,16 chiral catalysts,17 and biologicalactive compounds.18 On the other hand, the biphenyl fragmentis present in potential pharmaceuticals,19 chiral ligands forasymmetric synthesis,20 and mesogenic materials.21 Moreover,both peptides22 and biaryls23 have been used as building blocksfor crystal engineering.

Besides the potential practical applications indicated above,our initial objective in studying these compounds was to deepenour knowledge of their structural features, especially of themutual influence of the peptide and biphenyl fragments. As aresult of the free rotation around the aryl–aryl bond, bothpeptide chains might interact through intramolecular hydrogenbonds, to give structures of type B’ (Fig. 2). This fact mightcontrol the conformation around the phenyl–phenyl bond:both the thermodynamic stability of any rotamer (atropisomer)as well as the kinetics of their interconversion.

Our preliminary results on peptide–biphenyl hybrids haveshown that these compounds possess remarkable structuralproperties14b that are summarized as follows:

{ Dedicated to Prof. Elguero, a teacher in the field of structural organicchemistry, on the occasion of his election as Fellow of the SpanishRoyal Academy of Science. Taken partly from the PhD Thesis of A.M.,Universidad Autonoma, Madrid, 2004.{ Based on the presentation given at CrystEngComm Discussion: NewTrends in Crystal Engineering, 8–10 September 2004, Nottingham,UK.

512 CrystEngComm, 2004, 6(83), 512–521 DOI: 10.1039/b406652a

This journal is # The Royal Society of Chemistry 2004

(i) Due to the presence of the stereogenic centres in thesubstituents at the positions C(2) and C(2’), the aryl–aryl bondis a diastereogenic element, giving two atropisomers (aS) and(aR) (Fig. 3). It is well known that the conformational equili-brium around the phenyl–phenyl ring in 2,2’-disubstituted-1,1’-biphenyl is very fast. However, we have observed that theconformational equilibria in the peptide–biphenyl hybrids aremuch slower than in other 2,2’-disubstituted biphenyls,24 andthe atropisomers are detected by 1H NMR, finding that one ofthem predominates in solution.25

(ii) 1H NMR and IR data have shown that the NH and COgroups (in each atropisomer) participate in intramolecularhydrogen bonds, resulting in structures similar to B’ (Fig. 2).26

Therefore, the biphenyl scaffold causes a change in the pro-pagation sense of the peptide chains and, in a formal sense, itmimics a b-turn. It is likely that these intramolecular hydrogenbonds are responsible for the slow interconversion betweenatropisomers.

(iii) The equilibrium between atropisomers is accelerated inpolar solvents with the capacity to form hydrogen bonds

(DMSO or methanol).25a This characteristics can be used toperform dynamic atropselective resolutions and to obtainsingle crystals of peptide–biphenyl hybrids. In a previouspublication,14b we reported the crystal structures of the hybrids1 and 2 (Fig. 1) showing that the peptide–biphenyl hybridscrystallized as single isomers: all the molecules in the unit cell ofeach 1 and 2 have the same sense of chirality at the stereogenicaryl–aryl bond. In these cases, the configurations are (S,aR,S)for 1 and (S,aS,S) for 2. Since the crystallization went with

Fig. 1 Structures of peptide–biphenyl hybrids.

Fig. 2 Interaction between peptide chains in peptide–biphenylhybrids.

Fig. 3 Equilibrium between atropisomers in peptide–biphenylhybrids.

CrystEngComm, 2004, 6(83), 512–521 513

higher than 95% yield, the process is a crystallization-inducedasymmetric transformation.27 From a structural point of view,the solid-state molecular structure is maintained by intra-molecular H-bonds between the NH and the CO groups fromopposite chains, the number of the intramolecular H-bondsdepends on the length of the peptide chains. The CO and NHgroups that do not participate in the intramolecular hydrogenbond are engaged in intermolecular hydrogen bonds, resultingin helical supramolecular structures. The molecular andsupramolecular structures of 1 are depicted in Fig. 4. Thismolecule is formed by (S)-amino acids, the chiral aryl–aryl axisis (aR) (Fig. 4a), and the supramolecular helix has theM-configuration (Fig. 4b).

In this paper, we report the crystal structures of the peptide–biphenyl hybrids 3–5 having different substitution at the sidechain as well as at the terminal position. These results provide

additional data on the process of crystallization as well as thesupramolecular motif in the crystal.

Experimental

General methods

1H NMR and 13C NMR spectra were measured in VarianINOVA 400 or Varian INOVA 300 spectrometers; chemicalshifts (d) are reported in parts per million and the couplingconstants are indicated in Hz. 1H NMR spectra were referencedto the chemical shift of either TMS (d ~ 0.00 ppm) or theresidual proton in the deuterated solvent. 13C NMR spectrawere referenced to the chemical shift of the deuterated solvent.The IR spectra were measured in a Perkin-Elmer-SpectrumOne FT IR spectrometer; the units in the IR spectra areindicated in cm21. MS analyses were recorded in a HewlettPackard 5973 MSD (EI) or in a Hewlett Packard 1100 MSD(ESI) spectrometers. Combustion analyses were realized in aCarlo Erba EA 1180-Elemental Analyzer. Optical rotationswere determined in a Perkin-Elmer 241 MC polarimeter atroom temperature (ca. 295 K). Melting points were measuredon a Kofler hot-stage apparatus. All the preparative chromato-graphies were done with silica gel (40–63 nm) using the flashchromatography technique.

General procedure for the synthesis of peptide–biphenyl hybrids3–5

A solution of the corresponding amino derivative (1.1 molarequiv.) and Et3N (1.5 molar equiv.) in dry CH2Cl2 (0.1 M) wasadded dropwise to a solution of the dichloride 6 (1.0 molarequiv.) in dry CH2Cl2 (0.1 M) at 0 uC. The reaction mixture wasallowed to slowly warm-up to room temperature while stirringwas continued overnight. Solvent was removed under vacuumand the residue was taken up with EtOAC. The resultingsolution was sequentially washed with aqueous 5% HCl, H2O,saturated aqueous NaHCO3 and brine. After drying (MgSO4)and solvent removal, the residue was purified by flash-chromatography to give the corresponding pure peptide–biphenyl hybrid.

Synthesis of methyl (R,R,R,R)-2-[2-({2’-[1-(1-methoxycarbonyl-ethylcarbamoyl)-3-methyl-butylcarbamoyl]biphenyl-2-carbonyl}-amino)-4-methylpentanoylamino]propanoate (3)

Following the general procedure from the dipeptide (R)–Leu–(R)–Ala–OMe trifluoroacetate, the hybrid 3 was obtained in65% yield after chromatography (1/1 hexane/EtOAc toEtOAc). White solid. M.p.: 196–201 uC; [a]25

D ~ 1 60 (c ~0.55, MeOH); 1H NMR (400 MHz, DMSO-d6, 303 K): d 8.48(d, JNH,CHa ~ 8.3, 2H, 2–NH–Leu), 8.17 (broad s, 2H, 2–NH–Ala), 7.44–7.32 (m, 6H, Harom.), 7.00 (d, 2H, Jortho ~ 7.3,Harom.), 4.25 (m, 2H, CHa–Leu), 4.17 (broad s, 2H, CHa–Ala),3.59 (s, 6H, CO2CH3), 1.29 (m, 4H, CH2–Leu; 2H, CH(CH3)2–Leu), 1.21 (d, 6H, JCH3,CHa ~ 7.3, CH3–Ala), 0.77 (d, 6H,JCH3,CH ~ 5.8, CH(CH3)(CH3)–Leu), 0.70 (d, 6H, JCH3,CH ~5.8, CH(CH3)(CH3)–Leu); 13C NMR (75 MHz, DMSO–d6): d172.8 (s, 2C, CO), 171.5 (s, 2C, CO), 168.7 (s, 2C, CO), 138.8 (s,2C, Carom), 135.9 (s, 2C, Carom), 129.2 (d, 2C, Carom), 129.0 (d,2C, Carom), 127.3 (d, 2C, Carom), 127.1 (d, 2C, Carom), 51.7 (q,2C, CO2CH3), 50.8 (d, 2C, CHa–Leu), 47.4 (d, 2C, CHa –Ala),40.4 (t, 2C, CH2–Leu), 23.8 (d, 2C, CH(CH3)2–Leu), 22.9 (t,2C, CH(CH3)(CH3)–Leu), 21.4 (t, 2C, CH(CH3)(CH3)–Leu),16.7 (q, 2C, CHa(CH3)–Ala); IR (KBr): n 3435, 3061, 2958,1753, 1638, 1550, 1453, 1322, 1453, 1385, 1206, 1162, 1056, 759;MS (ES1) m/z ~ 639 ([MH]1, 100%), 661 ([MNa]1, 69%);Anal. calcd. for C34H46N4O8: C, 63.93; H, 7.26; N, 8.77; found:C, 63.58; H, 7.51; N, 8.47.

Fig. 4 (a) Molecular solid-state structure of (aR)-1; the dotted linesindicate the intramolecular H-bond and the atoms participating in theintermolecular H-bonds. (b) Left-handed helix (M-configuration) inthe crystal packing of (aR)-1.

514 CrystEngComm, 2004, 6(83), 512–521

Methyl (S,S)-2-{[2’-(1-methoxycarbonyl-2-phenylethylcarbamoyl)-biphenyl-2-carbonyl]amino}-3-phenylpropanoate (4)

Following the general procedure from (S)–Phe–OMe hydro-chloride, it was obtained in 81% yield after chromatography(hexane/EtOAc 7/3 to EtOAc gradient). White solid; m.p. 170–172 uC; [a]25

D ~ 1 34 (c ~ 1.0, MeOH); 1H NMR (300 MHz,CDCl3, 303 K, 1.2:1 mixture of conformers A1a,): d 7.48 (d,1.1H, JNH,CHa ~ 7.5, NH, A), 7.40–7.28 (m, 6H, Harom., A1a;0.9H, NH, a), 7.24 (m, 6H, Harom, A1a), 7.11 (m, 4H, Harom,A1a), 7.02 (m, 2H, Harom, A1a), 4.76 (X of ABX, 2H, CHa–Phe, A1a), 3.61 (s, 2.8H, CO2CH3, a), 3.54 (s, 3.2H, CO2CH3,A), 3.01 (A of ABX, 1.1H, JHa,Hb ~ 13.9, JHa,CHa ~ 8.1,C(HaHb)–Phe, A), 2.90 (B of ABX, 1.1H, JHb,Ha ~ 13.9,JHb,CHa ~ 5.2, C(HaHb)–Phe, A), 2.83 (A of ABX, 0.9H,JHa,Hb ~ 13.8, JHa,CHa ~ 7.8, C(HaHb)–Phe, a), 2.70 (B ofABX, 0.9H, JHb,Ha ~ 13.8, JHb,CHa ~ 5.2, C(HaHb)–Phe, a);1H NMR (300 MHz, DMSO–d6, 313 K) d: 8.76 (d, 2H,JNH,CHa ~ 7.8, 2–NH), 7.34 (m, 6H, Harom.), 7.19 (m, 6H,Harom.), 7.08 (m, 4H, Harom.), 6.91 (m, 2H, Harom.), 4.39 (m, 2H,CHa–Phe), 3.48 (s, 6H, CO2CH3), 2.81 (m, 2H, C(HaHb)–Phe),2.65 (m, 2H, C(HaHb)–Phe) 13C NMR (75 MHz, CDCl3, 303K, A 1 a mixture of conformers): d 172.1 (s, CO, A), 171.7 (s,CO, a), 169.3 (s, 2C, CO, A 1 a), 139.0 (s, Carom, a), 138.5 (s,Carom, A), 136.1 (s, Carom, 2C, A 1 a), 135.7 (s, Carom, A), 135.3(s, Carom, a), 130.1 (d, Carom, a), 129.7 (d, Carom, A), 129.3 (d,Carom), 129.2 (d, Carom), 129.0 (d, Carom), 128.5 (d, Carom),127.8 (d, Carom), 127.5 (d, Carom), 127.2 (d, Carom), 126.9 (d,Carom), 53.6 (d, 2C, CHa –Phe, A 1 a), 52.2 (q, 2C, CO2CH3,A1a), 37.9 (t, CH2–Phe, A), 37.7 (t, CH2–Phe a); IR (KBr) n3435, 3293, 1737, 1641, 1557, 1435, 1335, 1266, 1218, 757, 700;MS (ES1) m/z: 565 ([MH]1, 100%); Anal. calcd. forC34H32N2O6: C, 72.32; H, 5.71; N, 4.96; found: C, 72.37; H,5.52; N, 4.96.

Methyl (R,R)-2-{[2’-(1-methoxycarbonyl-2-phenylethylcarbamoyl)-biphenyl-2-carbonyl]amino}-3-phenylpropanoate (5)

Following the general procedure from (R)–Phe–OMe hydro-chloride, the hybrid 5 was obtained in 71% yield after

chromatography (7/3 hexane/EtOAc to EtOAc gradient),that was identical to the enantiomer 4, but on the signof the optical rotation. White solid. M.p. 168–170 uC;[a]25

D ~ 2 36 (c ~ 0.25, MeOH); Anal. Calcd. forC34H32N2O6: C, 72.32; H, 5.71; N, 4.96; found: C, 72.15; H,5.94; N, 4.78.

Crystal structure analysis of peptide–biphenyl hybrids 3–5

Data collection were carried out on a Bruker SMART-CCDarea diffractometer with graphite monochromated Mo Karadiation (l ~ 0.71073 A) operating at 50 kV and 30 mA.Suitable single crystals were obtained by slow evaporationat room temperature of methanolic solutions of the corre-sponding peptide–biphenyl hybrid. A total of 1271 framesof intensity data were collected over a hemisphere of thereciprocal space by combination of three exposure sets.Each frame covered 0.3u in v and the first 50 frameswere recollected at the end of data collection to monitorcrystal decay. Absorption corrections were applied using theSADABS program.28 The structures were solved using theBruker SHELXTL-PC software29 by direct methods andrefined by full-matrix least-squares methods on F2. Hydrogenatoms were included in calculated positions and refined inthe riding mode, less those bonded to nitrogen atoms thatwere located on residual density maps, then fixed theirpositions and refined in the riding mode. All non-hydrogenatoms were refined anisotropically. The structures ofcompounds 4 and 5 present disorder in each O(5). The detailsof the data collection and refinement are summarized inTable 1. The structure drawings were prepared usingPLATON30 and MERCURY31 programs. The absolute con-figurations at the stereogenic centres and axes were assigned tothe synthetic schemes that start from amino acids of knownstereochemistry.

CCDC reference numbers 235195–235197.See http://www.rsc.org/suppdata/ce/b4/b406652a/ for crys-

tallographic data in CIF or other formats.

Table 1 Crystal data and structure refinement for 3–5

3 4 5

Empirical formula C34H46N4O8 C34H32N2O6 C34H32N2O6

Mw 637.74 564.62 564.62T (K) 173(2) 298(2) 298(2)Crystal size 0.41 6 0.18 6 0.17 mm3 0.45 6 0.25 6 0.20 mm3 0.32 6 0.28 6 0.13 mm3

Crystal system Orthorhombic Orthorhombic OrthorhombicSpace group P212121 P212121 P212121

Z 4 4 4a/A 10.0753(8) 10.3013(6) 10.3024(9)b/A 16.4505(13) 16.6933(11) 16.6796(14)c/A 20.8507(16) 17.3165(11) 17.3322(15)V/A3 3455.9(5) 2977.8(3) 2978.4(4)rcalc/g cm23 1.226 1.259 1.259mcalc/mm21 0.088 0.087 0.087F(000) 1364 1192 1192Transmission factors (max; min) 0.9852, 0.9649 1.000000, 0.505795 0.9888, 0.9728Scan type v v vH range for data collection 1.95–26.37u. 1.69–26.38u. 2.30–26.38u.Total reflections measured 20 277 18 530 17 457Independent reflections [R(int)] 7044 [0.0803] 6090 [0.0658] 6060 [0.0372]Completeness to Hmax 99.7% 100.0% 99.9%Absorption correction Multi-scan Multi-scan Multi-scanRefinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2

Parameters refined, restraints 499, 0 389, 0 389, 0Final R indicesa [I w 2s(I)] R1 ~ 0.0538 R1 ~ 0.0463 R1 ~ 0.0440Final wR2 indices (all data)a wR2 ~ 0.1077 wR2 ~ 0.1060 wR2 ~ 0.1125Weights (a, b)b 0.0415, 0.00 0.0497, 0.00 0.0580, 0.01Goodness-of-fit on F2 0.974 0.882 0.998Largest diff. peak and hole 0.306 and 20.213 e.A23 0.126 and 20.155 eA23 0.134 and 20.149 e. A23

a R1 ~ S||F0| 2 |Fc||/S|Fo| and wR2 ~ {S[w(F20 2 F2

c )]/S[w(F20)2]}½. b Weighting scheme, w ~ [s2(F2

0 1 (aP)2 1 bP]] and P ~ (|F0|2 1 2|Fc|2)/3.

CrystEngComm, 2004, 6(83), 512–521 515

Results and discussion

The target compounds A (3–5) were synthesized from 1,1’-biphenyl-2,2’-dicarbonyl dichloride (6) and the correspondingamino acid/peptide methyl ester as nucleophile, following areported method (Scheme 1).14b Suitable single crystals of 3–5were obtained by slow evaporation of the correspondingmethanolic solution; the crystallizations went in over 95%yield. All the compounds crystallized as single atropisomers atthe stereogenic aryl–aryl single bond, being a dynamic atrop-selective transformation.

Crystal structure of the peptide–biphenyl hybrid 3

This compound was prepared using the dipeptide (R)–Leu–(R)–Ala–OMe as nucleophile according to Scheme 1. Suitablesingle crystals of 3 were obtained as indicated above andanalyzed by X-ray diffraction at 173 K. The molecularstructure and crystal packing are depicted in Figs. 5 and 6,respectively. All the molecules in the unit cell have the samesense of the chirality at the stereogenic aryl–aryl bond. Sincethe configurations of all the amino acid residues are (R), weassign an (aR) configuration to the stereogenic axis. In aprevious paper we reported the crystallization-induced resolu-tion and the crystal structure (at room temperature) ofcompound 2 (enantiomer of 3). Regardless the absoluteconfiguration, both the crystallization process and the solid-state structure are identical for the two enantiomers that provethe reproducibility of the procedure. A selection of structuralparameters for compound (aR)-3 is indicated in Table 2.

The molecular structure of (aR)-3, depicted in Fig. 5, ismainly maintained by two intramolecular hydrogen bondsformed by the NH and CO groups of the leucine residue in eachchain having NH…O distances of 2.84 and 2.90 A and N–H–Oangles of 174 and 171u, respectively. As commented above, it isquite likely that these hydrogen bonds are also present in non-polar solvents.26 The existence of these two intramolecularhydrogen bonds influences the conformation of the rest of themolecule, especially the geometry around the biphenyl system.Thus, the torsion angle between both aryl rings (at thepositions substituted by the chains) is ca. 72u that is a smallerangle than the observed in other peptide–biphenyl hybridshaving only one intramolecular H-bond (see below and ref.14b). Similarly to other peptide–biphenyl hybrids, compound(aR)-3 presents a significant non-coplanarity between eachphenyl ring and its vicinal CO group, the torsion angles being

60 and 65u for each chain. The w and y dihedral angles of thepeptide chains are quite typical for peptides, laying in theregion (or nearby) of extended (b-sheet) conformation in aRamachandran plot.32 The only slight deviation from thestandard geometries in peptide conformations is the dihedralangle around the peptide bond of the terminal residue in one ofthe chains (v type, 2165u, Table 2). Therefore, the biphenylsystem does not cause any constraint to the peptide fragments.The relative orientation of the two peptide chains and the twoaromatic rings of the biphenyl system are schematicallydepicted in the Fig. 5b: each peptide chain is oriented towardone hemisphere of the biphenyl fragment, resulting in astructure with C2 symmetry (in a formal sense).

As found in other peptide–biphenyl hybrids, crystalline (aR)-3 has a highly ordered supramolecular structure (Fig. 6), wherethe NH and CO groups non-involved in the intramolecularhydrogen bonds (indicated as broken lines in Fig. 5) participatein intermolecular bonding (Table 2). Thus, any moleculeof (aR)-3 interacts with two vicinal molecules, forming an

Scheme 1

Fig. 5 (a) Molecular solid-state structure of (aR)-3 showing theintramolecular H-bonds. The dotted lines indicate the atoms linked byintermolecular H-bonds. (b) Schematic representation of the con-formation of (aR)-3; the orientation of the molecule is different from(a) to illustrate the relative orientations of the side chains and the arylrings (compare to Fig. 7).

516 CrystEngComm, 2004, 6(83), 512–521

extensive pattern of hydrogen bond: each amide group of (aR)-3 participates in two hydrogen bonds. Each molecule of (aR)-3is involved in six hydrogen bonds. The overall supramoleculararrangement in the crystal is parallel left-handed helices with(M)-configuration.

According to the graph-set analysis of H-bonded crystalstructures,33 each intramolecular H-bond can be classifiedas a S(12) motif and each intermolecular H-bond is classifiedas a D motif in each molecules, resulting in a first level graphset N1 ~ S(12)S(12)DD, with three binary graph sets, one ofthem with N2 ~ R2

2(14) and two of them with N2 ~ C22(12).

Thus, the crystals of (aR)-3 present three levels of chirality:The a-carbons of the amino acid residues (R-configuration),

the axis of the biphenyl system ([(aR)-configuration], and thesupramolecular helix (M-configuration).

Crystal structure of the peptide–biphenyl hybrid 4

This hybrid was prepared from (S)–Phe–OMe followingthe procedure indicated in Scheme 1. On comparing to theother peptide–biphenyl studied, compound 4 possessesadditional aromatic rings, which were expected to influencethe structure and property of the molecule. Previously, weobserved interesting differences in the biological activity: Thehybrids having aromatic rings at the side chains are calpaininhibitors, the analogues having only aliphatic amino acids areinactive.14d

Crystallization from methanol of 4, as described above, gavesingle crystals (w95% yield) that were analyzed by X-raydiffraction at room temperature. Analogously to otherpeptide–biphenyl hybrids, all the molecules in the unit cellhave the same configuration at the stereogenic aryl–aryl bond.Since the configurations at the chiral carbons are (S), we assignthe (aS) stereochemistry to the chiral axis. The molecular solidstate structure of (aS)-4 is indicated in Fig. 7. A remarkabledifference between the solid-state structure of (aS)-4 and theother peptide–biphenyl hybrids with aliphatic amino acids isthe relative orientation of the substituents on the amino acidsresidues. A schematic representation of the conformation of(aS)-4 is shown in Fig. 7b (compare with Fig. 5b) where the twophenyl rings of the phenylalanine residues are disposed to thesame hemisphere of the molecule, forming a cavity whosetriangular surface is ca. 33 A2, that is big enough to host smallmolecules. Preliminary evidence has shown that the cavity isroughly kept in solution and the compound is able to coordi-nate Ca21 and Mg21 salts.34

Since the molecular and supramolecular structures in thecrystal can influence each other,35 the dissimilarities insolid state conformations between 3 and 4 may be due todifferences in the packing of the molecules in the crystal;although another factor might be the favourable electrostaticinteractions between the three phenyl groups in the samehemisphere of 4.14c

A selection of geometrical parameters for (aS)-4 is indicatedin Table 3. The main intramolecular interaction is a nearly

Fig. 6 Crystal packing of (aR)-3 showing the supramolecular left-handed helix (M-configuration). Click here to access a 3D view ofFig. 6.

Table 2 Selected structural parameters for peptide–biphenyl hybrid (aR)-3a

D–H…A d (D–H) d (H…A) d (D…A) v(DHA) Class

N(1)–H(1)…O(6) 0.89(4) 1.96(4) 2.841(3) 174(3) IntramolecularN(3)–H(3)…O(2) 0.82(3) 2.08(3) 2.898(3) 171(3) IntramolecularN(2)–H(2)…O(5)b 0.90(3) 1.97(3) 2.871(3) 174(3) IntermolecularN(2)–H(4)…O(1)c 0.82(3) 2.13(3) 2.953(3) 173(3) Intermolecular

Torsion angle Type Angle

C(13)–N(1)–C(14)–C(15) w 64.3(3)C(15)–N(2)–C(16)–C(18) w 67.5(4)C(24)–N(3)–C(25)–C(26) w 73.8(4)C(26)–N(4)–C(27)–C(29) w 54.2(4)N(1)–C(14)–C(15)–N(2) y 2146.1(3)N(2)–C(16)–C(18)–O(4) 2157.6(3)N(3)–C(25)–C(26)–N(4) y 2157.5(3)N(4)–C(27)–C(29)–O(8) 2142.1(3)C(14)–N(1)–C(13)–C(1) v 173.7(2)C(16)–N(2)–C(15)–C(14) v 2170.8(3)C(25)–N(3)–C(24)–C(8) v 171.3(2)C(27)–N(4)–C(26)–C(25) v 2165.0(3)C(2)–C(1)–C(13)–O(1) Phenyl/carbonyl 260.1(4)C(7)–C(8)–C(24)–O(5) Phenyl/carbonyl 265.3(4)C(1)–C(2)–C(7)–C(8) Phenyl/phenyl 272.4(4)C(1)–C(2)–C(7)–C(12) Phenyl/phenyl 106.7(3)a The distances are in A, and the angles in u. Standard uncertainties are included. b Symmetry transformations: 2x, y 2 1/2, 2z 1 3/2. c Sym-metry transformations: 2x, y 1 1/2, 2z 1 3/2.

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collinear H-bond between the NH(2) and O(1) having a N–Odistance of 2.86 A (Fig. 7a). The torsion angle between botharyl rings (at the positions substituted by the chains) is ca 107uthat is much wider than the one observed for hybrids havingtwo intramolecular H-bonds [such as (aR)-3] and thisdifference must reflect the different constraints imposed byone or two intramolecular H-bonds. Analogously to otherpeptide–biphenyl hybrids, each phenyl ring and its vicinal COgroup are non-coplanar, having dihedral angles of ca. 55u. Theconformations of the two amino acid chains are quite distinctas shown by their different torsion angles (Table 3). Althoughthese values do not exactly fit into zones accessible in aRamachandran plot, they are near the regions of b-sheet[for the chain starting in C(13)] and right handed a-helix[for the residue starting in C(24)]. Regarding the conformationof the side chain (torsion angles x1 and x2), there are alsosome differences between the two chains, and these angles areslightly distorted from the ones more frequently found inthe crystal structures of peptides and proteins.36 Anotherpotential intramolecular interaction can be detected in thesolid-state conformation of (aS)-4: The distance betweenone H-atom in the methyl group C(34) and the aromaticcarbon C(4) is 2.70 A that is slightly larger than the sum ofvan der Waals radii of carbon and hydrogen.37 This interactionis of the type CH-p (hydrophobic fragment–arene inter-action)38 and has not been detected in the crystal structureof other peptide–biphenyl hybrids without aryl rings in the sidechain.14b,39

Similarly to other peptide–biphenyl hybrids, the NH and COgroups [N(1) and O(4)] that do not participate in the intra-molecular interactions are engaged in intermolecularH-bonding: [d(N…O) ~ 2.87 A, angle(NHO) ~ 171u],generating a supramolecular structure (Fig. 8) where theindividual molecules are disposed in such a way that relativelylarge cavities are formed. Overall, the molecules of (aS)-4 packin a left-handed helix (M-configuration) that propagates alongthe c-axis (Fig. 9). The H-bond pattern of (aS)-4 can beanalyzed as N1 ~ S(9)DD at the first level graph set, with abinary graph set N2 ~ C2

2(9).It is worth noting that although the higher-order motif in the

crystal packing of all the peptide–biphenyl hybrids studied upto now are identical (helical structures), some remarkabledifferences are observed between the hybrids having aliphaticor aromatic substitution at the side chains. The most significantdissimilarities are the presence of cavities in (aS)-4, which is dueto the particular packing of molecules. This arrangement is

Fig. 7 (a) Molecular solid-state structure of (aS)-4 showing theintramolecular H-bond between O(1) and N(2). The dotted lines fromN(1) and O(4) indicate intermolecular H-bonds. The molecule isdisordered in O(5). (b) Schematic representation of the conformation of(aS)-4.

Table 3 Selected structural parameters for peptide–biphenyl hybrid (aS)-4a

D–H…A d (D–H) D (H…A) d (D…A) v(DHA) Class

N(2)–H(2)…O(1) 0.86 2.00 2.857(3) 179 IntramolecularN(1)–H(1) …O(4)b 0.86 2.02 2.870(3) 171 Intermolecular

Torsion angle Type Angle

C(13)–N(1)–C(14)–C(15) w 291.3(3)C(24)–N(2)–C(25)–C(33) w 2113.7(3)N(1)–C(14)–C(15)–O(2) 7.8(5)N(2)–C(25)–C(33)–O(6) 2142.1(3)C(14)–N(1)–C(13)–C(2) v 164.1(2)C(25)–N(2)–C(24)–C(8) v 2175.0(2)C(13)–N(1)–C(14)–C(16) x1 145.8(2)N(1)–C(14)–C(16)–C(17) x2 272.5 (3)C(24)–N(2)–C(25)–C(26) x1 122.7(3)N(2)–C(25)–C(26)–C(27) x2 258.5(3)C(1)–C(2)–C(13)–O(1) Phenyl/carbonyl 256.6(3)C(7)–C(8)–C(24)–O(4) Phenyl/carbonyl 128.1(3)C(2)–C(1)–C(7)–C(8) Phenyl/phenyl 107.2(3)C(2)–C(1)–C(7)–C(12) Phenyl/phenyl 279.2(3)a The distances are in A, and the angles in u. Standard uncertainties are included. b Symmetry transformations: 2x, y 1 1/2, 2z 1 1/2.

518 CrystEngComm, 2004, 6(83), 512–521

caused by intramolecular and intermolecular H-bonds as wellas by the presence of aromatic rings in the side chains. Onanalyzing the putative intermolecular interactions of thearomatic rings (Fig. 10), we observe the following facts:

(i) The distance between H(9) of a first molecule (green ring)and C(20) of a second molecule (purple ring in Fig. 10) is 2.86 Awith a C(9)–H(9)–C(20) angle of 141u.

(ii) The phenyl ring formed by C(7)–C(12) (green ring) isstacked to the phenyl ring comprised by C(27)–C(32) of a thirdmolecule (yellow ring): the angle between them is ca. 6u, theoffset of the ring centres is 0.45 A and the distance between thenearest atoms is 3.53 A [C(10) and C(27)].

(iii) The relative orientation of these first and third moleculessets up another potential interaction between the ring formedby C(1)–C(6) (blue ring, Fig. 10) and the C(27)–C(32) ring(yellow). These two rings are nearly perpendicular and theycan participate in C(p)–H…p interactions with the followingshortest distances and angles: d[H(6)–H(31)] ~ 2.68 A, d[H(6)–C(31)] ~ 3.16 A, d[H(31)–C(6)] ~ 3.55 A, angle[H(6)–H(31)–C(31)] ~ 113u.

All these data indicate that the arene–arene interactions mayplay a role in the crystal packing of (aS)-4.

Crystal structure of the peptide–biphenyl hybrid 5

This compound is the enantiomer of the hybrid 4. It wassynthesized from (R)–Phe–OMe and 6 (Scheme 1). It wasprepared with the objective to relate the biological activity tothe stereochemistry and, moreover, to prove the reproducibilityof the crystallization-induced asymmetric transformation.Thus, compound 5 was dissolved in methanol and the solventwas slowly evaporated at room temperature to yield crystals(95% mass balance) that were analyzed by X-ray diffraction(Fig. 11). All the molecules in the unit cell have the same type ofchirality at the aryl–aryl bond: since the chiral carbons of thephenylalanine residues are (R), we assign it an (aR) stereo-chemistry. Therefore, the crystalline compound prepared from(R)-PheOMe, (aR)-5, is enantiomer of the one obtained from(S)-PheOMe [(aS)-4] and the crystallization process is repro-ducible. All geometrical parameters, as well as the intramole-cular and intermolecular interactions, are identical in (aR)-5and in (aS)-4. The supramolecular crystal packing of (aR)-5 isenantiomer of those of (aS)-4: compound (aR)-5 forms right-handed helices (P-configuration).

Conclusions

We have reported examples on the crystallization-inducedasymmetric transformation and the crystal structure ofpeptide–biphenyl hybrids A. The shift of the dynamicequilibrium between the atropisomers of peptide–biphenylhybrids is controlled by the efficient packing of the moleculesof one of the isomers that, in turn, is determined by favour-able intermolecular interactions, that besides the hydrogenbonds of the amino acid residues are highly dependent onthe presence of additional aromatic rings. In the crystallinestate, these molecules possess three sterochemical levels:the chiral carbons at the amino acid residues, the stereogenicaryl–aryl bond, and the supramolecular helix. Overall, ourresults show the interplay between the different structuralfeatures, peptide and biaryl, that have many utilities as additivefor organic synthesis, pharmaceuticals, and technologicalmaterials, as well as building blocks for crystal engineering.Work is in progress to obtain further experimental andcomputational data on the structures of peptide–biaryl andtheir interactions.

Fig. 8 View along the a-axis of the crystal packing of (aS)-4 showing the array of H-bonds. Click here to access a 3D view of Fig. 8.

Fig. 9 View along the b-axis of the crystal packing of (aS)-4 showingthe left-handed helix. Click here to access a 3D view of Fig. 9.

CrystEngComm, 2004, 6(83), 512–521 519

Acknowledgements

Financial support from the Spanish Ministry of Science andTechnology (project # BQU2001–2270) and Fundacion ‘laCaixa’ (project # 02/162-02) is gratefully acknowledged.

References

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Fig. 10 Partial representation of the crystal packing of (aS)-4 showing the potential interactions between aromatic rings.

Fig. 11 Molecular conformation of (aR)-5 showing the intramole-cular H-bond. Click here to access a 3D view of Fig. 11.

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