Crystal and molecular structure of glycyl-L-ieucine

摘要

1722 J.C.S. Perkin I1Crystal and Molecular Structure of Glycyl-L-leucineBy Vasantha Pattabhi, Centre of Advanced Study in Physics, University of Madras, Guindy Campus, MadrasK. Venkatesan," Department of Organic Chemistry, Indian Institute of Science, Bangafore 56001 2, IndiaS. R. Hall, Department of Energy, Mines and Resources, 555 Booth Street, Ottawa, CanadaThe crystal structure of the title compound has been determined by direct methods from diffractometer data.Crystals are monoclinic, with Z = 2 in a unit cell of dimensions: a = 6.369(5), b = 5.565(5), c = 15.350(10) A.p = 102-77(4)", space group P2,. The structure was refined by least-squares to R 0.044 for 823 observed reflec-tions. The four protons available for hydrogen-bond formation are a three-dimensional network which stabilizesthe structure.There is significant non-planarity of the peptide linkage, the torsion angle about the peptide bondbeing -1 1-4'. The nitrogen atom of the peptide group is significantly pyramidal. The molecular conformation isdiscussed.600025, IndiaTHE investigation of the structure of glycyl-1;-leucine the peptide linkage, which was until recently assumed(I) was undertaken as part of a project to determine to be planar.the structures of simple peptides. The study of simple For example, significant non-planarity of the peptidepeptides throws light on the nature of the distortion of linkage has been observed in crystal structures ofglyc yl-I,-alanine hydrochloride ,l a-glyc ylglycine J2 anda A. B. Biswas, E. W.Hughes, B. D. Sharma, and J. N. Wil-son, Acta Cryst., 1968, B24, 40.P. S. Naganathan and K. Venkatesan, Ada C~yst., 1972, B28,5521974 1723~-alanylglycine.~ A preliminary account of this workhas been given elsewhere.*H H H n131 I I 1H3Cf6) C 181 H3Thin needle-shaped crystals elongated along b, wereCrystal Data.-C,H,,N,O,, M = 188. Monoclinic, a =U = 530.6 A3, D, = 1.18 (by flotation) g ~ m - ~ , 2 = 2,0, = 1-176 g ~ m - ~ . Space group P2, (Cg No. 4) fromsystematic absences: Oh0 when k is odd. These datawere obtained on the diffractometer with Mo-K, radiation,Initially intensity data (k0-4Z) were obtained by use ofCu-K, radiation and the multiple-film equi-inclinationWeissenberg technique, with the crystal mounted about b.The intensities were estimated visually by use of a cali-brated strip.Lorentz, polarization, and spot-shape 5corrections were applied.Although these data were used in the solution of thestructure they later proved insufficient for the reliablelocation of hydrogen atoms. A second data set wastherefore collected by use of a four-circle Picker diffracto-meter and graphite-monochromated Mo-K, radiation. The8-28 scanwidth (24--2-3deg;) was varied according to dis-persion, out to 28 maximum 50'. The scan rate was 2"min-1 and background counts were measured each side ofthe peak for 30 s. The intensities of 3 reference reflectionsmonitored every 25 measurements showed no significantvariations during data collection.Lorentz and polarization factors were applied to the1069 net intensities and of these 823 were consideredobserved I,,t 1.650(1). The remaining unobservedreflections were set a t the threshold value of 1.65a(l) forthe purposes of refinement.The structure was solved by a repeated iteration pro-cedure6 of the tangent formula7 on 295 phases withE 2 1-00.Three reflections (3,0,9, 1,1,3, and 4,0,-3)were assigned phases of zero to specify the origin and thephase of another (3,3, - 11) was permuted through values of5x/4, 3x/2, and 7x/4 to establish the enantiomorph. Twoother reflections (3,0,7 and 1,0,2) were permuted throughphases of 0 and TC and included in the starting set of aseries of separate tangent iteration calculations. Thephasing process converged most rapidly for the permutedphase values of 5x/4, x, and 0, and the resulting E maprevealed twelve of the thirteen non-hydrogen atom positions.The position of the missing atom, a terminal carbon of theleucyl side-chain, was fixed from stereochemical consider-3 M.H. J. Koch and G. Germain, Acta Cryst., 1970, B26, 410.4 V. Pattabhi, K. Venkatesan, and S. R. Hall, Cryst. Struct.5 D. C. Phillips, Acta Cryst., 1954, '7, 746.6 S. R. Hall, ' Crystallographic Computing,' ed. F. R. Ahmed,7 J. Karle and H. Hauptman, Acta Cryst., 1966, 9, 635.8 ' International Tables for X-Ray Crystallography,' vol. 111,EXPERIMENTALobtained by slow evaporation of an aqueous solution.6*369(5), b 7 5.565(5), G = 15.350(10) A, p = 102-77(4)O,h = 0.7107 A.Comnz., 1973, 2, 223.Munksgaard, Denmark, 1970, p. 66.Kynoch Press, Birmingham, 1965, p.202.ations. Structure factors calculated with these atomsgave I? 0.42. In this, and subsequent calculations, atomicscattering factors for carbon, nitrogen, and oxygen weretaken from ref. 8, and for hydrogen from ref. 9. Refine-ment was performed with the visually estimated data butthe lowest R obtained was only 0-095. Because of theimportance in reliably locating hydrogen atoms, and toobtain more accurate information about the stereo-chemistry of the amide group, further refinement was carriedout with the diffractometer data set. Full-matrix aniso-tropic least-squares, and all other crystallographic calcu-lations, were performed by use of the ' X-Ray ' system ofcomputer programs.1deg;TABLE 1Positional and thermal parameters, with standarddeviations in parentheses(a) Positional parameters ( x lo4) of non-hydrogen atomsla Y / b zlc Atom7 388 ( 8) 4801(9) 6 22 9 (4)6795(7) 2500( 0) 6631(3)4301 (6) 606 (9) 7404(3)2898(8) 1441(9) 8053(3)2 962 (6) - 768(8) 6 605 (3)292(10) 704(6) 9007(4)181 9 (9) - 529( 11) 8487( 3)3 3 9 6 ( 1 1) - 2 1 94( 14) 9083(4)7640( 5 ) 585(6) 66 15 (2)2128(5) 407 (7) 5931 (2)9 26 1 (5) 4396(7) 5825 ( 2)2724( 7) 7115(2) 5 3 1 O( 5 )-3002(6) 6708(2) 2 7 60 (5)C( 1)C(2)(73)(74)C( 5 )C(6)C(7)C(8)O(1)O(2)NU)N(2)O(3)(b) Anisotropic thermal parameters ( x lo4) of non-hydrogenatomsull u22 u33 u12 u13 u23482 266 579 -63 76 -54396 254 501 26 164 39C(l) 613 233 789 24 391 72C(2) 358 247 520 -14 41 2C(3) 325 290 389 -13 66 63C(4) 553 347 398 -23 134 -26C(5) 274 265 459 29 127 -44Cf6) 949 1040 739 -137 538 -13C(7) 744 631 441 -243 248 -911251 624 607 102 255 128 :bsol;;bsol; 603 308 822 29 337 93O(2) 533 454 361 107 29 14N(1) 371 272 377 -23 63 23* In the form: T = e ~ p - 2 ~ ( h ~ u * ~ U ~ ~ $.k2b*2U22 $- Pc*U,, + 2hka*h* U,, +- 2hZa*c* U,, + 2kbh*c* UJ.(c) Positional and isotropic thermal parameters ( x lo3) of thehydrogen atomsX Y z vim615(8) 566612) 679(3) 99463(6) 406 ( 8) 710(3) 12554(5) -4S(7) 770(2) 10395(6) 254(10) 85363) 59177(5) 255(8) 773(2) 35786(6) 608(8) 672(2) 40 H(1) W)l(3) " (2) IW4) W ) lW5) C(4)1H(71 C t 711H(8)C(6)1H(WCC(6)H(11 C(S)lH(12) C(S)lH I ~ ) E C ( ~ ) I~ ( 6 ) ~ ( 4 ) 1~ ( 9 ) ~ ( 6 ) 1H f 1 3) c (8) 1~ ( 1 4 ) "( 111~ ~ 5 ) " ( 1 ) 1 105.3(6)081(7) - 143( 10) SOl(3) 66- 103(14) 192(22) 860(6) 237XlO(9) 179(13) 948(4) 128-57(8) -53(11) 913(3) 87243(11) -367(16) 941(6) 186448(10) -302(15) 888(4) 135428(9) -118(13) 956(4) 126888(7) 447(10) 530(3) 64993(6) 279(10) 587(3) 47532(9) 606(3) 43 H (1 6) "( 1)1R.F. Stewart, E. R. Davidson, and W. T. Simpson, J . Chem.Phys., 1965, 42, 3175.10 J. M. Stewart, G. J. Kruger, H. L. Ammon, C. Dickinson,and S. K. Hall, X-Ray system of Crystallographic Programs forany Computer, Technical Report TR 192 of the Computer ScienceCentre, Maryl.and, June 19721724 J.C.S. Perkin I1Unit n-eights were used throughout the least-squaresrefinement.The hydrogen atoms were located from adifference-Fourier map and included with isotropic thermalparameters in the refinement. Refinement was terminatedwhen the calculated shifts were a. The final R for 823observed reflections was 0.044.Figure 1 Final atomic parameters are listed in Table 1.0 BAFIGURE 1 Anisotropic thermal ellipsoidsshows the thermal ellipsoids of the molecule. Observedand calculated structure factors are listed in SupplementaryPublication No. SUP 21093 (2 pp.).*DISCUSSIONBond lengths and angles corresponding to the finalco-ordinates are listed in Table 2 (mean Q in bondlengths is 0.006 and in angles 0.4") ; on the whole theydo not deviate significantly from expected values.Thevalue of 1.225(4) A for the C(2)-0(1) (C'=O) bond israther short compared to the weighted mean distance1-240(4) A.11 The deviation is as large as 40. Theshort C(2)-0(1) distance could be due partly to thelarge thermal vibration of O(1) along the c axis. Incyclotetrasarcosyl l 2 one of the C'-0 distances is as shortas 1-209(3) A.C(3)-N(2) would be expected to be shorter thanC(1)-'N(l), because N(2) is sfi2 whereas N(l) is sfi3hybridized. Thus the weighted mean value forC(S$~)-N(S+~) is 1-487 and for C(S$~)-N(S+~) is 1.455 A.l1N(2)-C(3) and C(1)-"(1) are in good agreement withexpected values. The angle at CS C(3)-C(4)-C(7)116*5(5)" is very much larger than the regular tetra-TABLE 2Molecular geometry(a) Bond lengths (A)1 ~225 (4) H (5)-C (4) 1 -07 (4) Ei;lgbsol;i 1.331(4) H(6)-C(4) l.OO(4)C(2)-C(1) 1-505(6) H(7)-C(7) l.OO(5)1-1 5 (9) g:lE$ 1 .OO( 6)N( 1)-C( 1) 1*478( 6)N( 2)-C( 3) 1.458 (5)C(3)-C(5) 1*534(6) H(1O)-C(6) 0.92(5)C(3)-C(4) 1.549(6) H(ll)-C(8) 1.20(8)C(4)-C(7) 1.523(7) H(12)-C(8) 0.93(7)O( 2)-C( 5 ) M40( 5 ) H(13)-C(8) 0.99(6)O( 3)-C( 5) 1-263 (5) H(14)-N(1) 0*79(5)C( 6)-C ( 7) 1.548 ( 8) H(15)-N(1) 0*99(4)C(7)-C(8) 1-516(9) H(lG)-N(l) 0*96(4)H(1)-C(l) 1-03(4) H(2)-C(1) 1.04(6)H ( 3)-N ( 2) 0.89 (4) H( 4)-C( 3) 1 -02( 3)( b ) Bond angles (deg.)0(1)-C(2)-N(2) 123*5(3) C(l)-N(l)-H(14) 109*0(6)O( l)-C(2)-C(l) 121.5(3) C(l)-N(l)-H(l5) 112*9(4)N(2)-C(2)-C(l) 115*0(3) C ( 1)-N ( 1)-H (1 6) 1 16.9 (4)N( 1)-C(1)-C(2) 109*9(5) C( 2)-C( 1)-H( 1) 1 10*3( 6)C(2)-N(2)-C 3) 120*6(4) C (2)-C( 1)-H (2) 1 15.9 (5)N ( 2)-C( 3)-Ct4) 1 08*0( 4) C (5)-C( 3)-H (4) 107-5 (5)N (2)-C( 3)-C( 5) 1 13- 1 ( 5 )C( 4)-C( 3)-C( 6) 1 1 1.6 (4) C( 3)-C( 4)-H( 5) 104.4( 5)C( 7)-C( 4)-C( 3) 116.6 (5) C( 3)-C( 4)-H( 6) 108.8( 6)0 (2)-C (5)-0 (3) 126.9 (6) C( 7)-C( 4)-H (5) 1 12.8 (6)O( 2)-C( 5)-C( 3) 11 7.7 (4) C( 7)-C( 4)-H( 6) 108*6( 5)0 (3)-C( 5)-C( 3) 11 6-4( 4) C( 7)-C( 6)-H (8) 1 16*6( 6)C( 4)-C( 7)-C( 6) 107.6 ( 5 ) C( 7)-C (6)-H (9) 1 1 1-2( 6)C(4)-C(7)-C(8) 113.6(5) C(7)-C(6)-H( 10) 104.2(6)C(S)-C(7)-C(S) 11 1.6(5) C( 4)-C( 7)-H( 7) 108.7 (6)N(1)-C(1)-H(1) 106*2(5) C( 6)-C( 7)-H( 7) 103*0( 6)N ( 1)-C( 1)-H (2) 1 1 1 -7( 6)C( 2)-N (2)-H (3) 12 1.6 (4) C( 7)-C(8)-H( 11) 109-8(7)C( 3)-N( 2)-H( 3) 1 13.7 (4) C(7)-C(8)-H( 12) 122.5(8)N (2)-C f 3)-H (4) 106.4 (4) C( 7)-C( 8)-H ( 13) 106.8 (7)H( 8)-C( 6)-H( 9) 104( 5) H(ll)-C(S)-H(12) 107(4)H( 14)-N( 1)-H (16) 11 3 (3) H(l4)-N(l)-H(l5) 107(3)H(l5)-N(l)-H(16) 97(2) H ( 1 1)-C (8)-H ( 13) 1 10( 4)H(S)-C(6)-H(lO) 99(5) H(1)-C(1)-H(2) 102(2)H( 5)-C( 4)-H( 6) 105( 2) H(9)-C(6)-H( 10) 122(4)H( 12)-C( 8)-H( 13)1 1 1*4( 4) C( 4)-C( 3)-H( 4)11 1.5 (5) C (8)-C( 7)-H (7)10 1 (4)TABLE 3Equations of least-squares planes and in square bracketsdeviations (A) of relevant atoms from these planes.X,Y,Z are orthogonal (A) co-ordinates defined by thea,b,c* axesPlane (1): Pcptide group, C(1), C(2), N(2), C(3), O(1)0.5025X + 0.1400Y + 0.85312 = 9.6854C(l) -0.0568, C(2) 0.0188, 0(1) 0.0117, C(3) -0.0720, N(2)0.0963, H(3) -0,06661Plane (2): Amide group, C(1), C(2), N(2), 0(1)0.5510X + 0-1641Y + 0.81822 = 9.4922C(1) 0.0003, C(2) -0.0010, 0(1) 0.0004, N(2) 0.0003, C(3)- 0.24741Plane (3): Carboxy-group, C(3), C(5), 0(2), O(3)-0.9200X + 0.1538Y + 0.36042 = 3.8392C(3) 0.0031, C(5) -0.0109, O(2) 0.0039, O(3) 0,0039, N(2)- 0.65311hedral value, a feature common in other amino-acidsand peptides.12-15 The widening of this angle appearsto be due to intramolecular overcrowding of atoms inthis part of the molecule.The carboxy-group exists in this structure in theionized form CO,-.The two C-0 bond lengths areessentially equal, mean 1.261(5) A. Both oxygen atomsare involved in hydrogen bonding and the protonassociated with this group has been accepted by theatom "(11.The least-squares plane through the bsol; I v * See Notice to Authors No. 7 in J.C.S. Perkin 11, 1973, Index11 R. E. Marsh and J. Donohue, Ado. Pvotein Chew., 1967, 22,12 IT. C. Leung and R. E. Marsh, Acta Cvyst., 1958, 11, 17.13 D. A. Wright and R. E. Marsh, ~~t~ Cryst., 1962, 15, 54.14 P. S. Naganathan and K. Venkatesan, Acta Cryst., 1.971,15 T. Ueki, T. Ashida, M. Kakudo, V. Sasada, aiidV. Xatsube,issue.234.B27, 1079.Actn Cvyst., 1969, B25, 18401974 1725carboxy-group (Table 3) is planar. The deviation ofthe peptide nitrogen from this plane is large (0.653 A).The peptide group C(l), C(2), 0(1), N(2), C(3), andH(3) is significantly non-planar. The best planeamong its constituent atoms is that through C(1), 0(1),C(2), and N(2) (Table 3).From this plane Ca atomC(3) at the C-terminal end deviates by 0.25 A and thepeptide hydrogen by 0.17 A. Non-planarity of thedefined by atoms Gal, C', 0, and N. The non-planarityof the peptide linkage could be partly due to inter-molecular influence, in particular of the hydrogen bondinvolving the hydrogen attached to the peptide nitrogen.The angle H(3)-N(2) * O(3) is 10.9", but when thisangle is calculated after fixing the hydrogen H(3) to bein the plane defined by C q , C', 0, and N, this angle isincreased to 20*1deg;, showing that the hydrogen bondFIGURE 2 Intermolecular packing viewed down the b axisTABLE 4Hydrogen bond lengths (A) and angles (deg.)N(l)-H(14) - * * O ( 2 9 2.704 1.93 105.7 167.2 9.1N(2)-H(3) * - * O(3IV) 2.870 2-00 113.2 164.2 10.9I1 -x, g + y , -z v l - ~ , * + y , l - zC-D.*.A D-H...A H-D...A H - .* AN(l)-H(15) * * * 0(2:i1 2.856 1.94 131.3 152.1 18.5N(l)-H(16) - * 013 ) 2.750 1.80 110.7 169.2 7.1Hydrogen bond D . * - ARoman numeral superscripts denote the following equivalent positions relative to the reference molecule a t x , y, 2:1 1 3 . x , y , z IV x , 1 + y ,I11 1 + x, 1 + y, 2peptide group has also been observed in several otherstructures, e.g. a-glycylglycine, L-alanylglycine, glycyl-L-alanine hydrochloride, glutathione,16 and cyclotetra-sarc0sy1.~~ In most of these structures the best planeamong the atoms of the peptide group was obtained byomitting the Ca atom at the C-terminal end.Winklerand Dunitz have observed that the nitrogen atom ofail amide group may be slightly pyramidal rather thanplanar, and our observations support this. By use ofthe CND0/2 method, it has been shown19 that inN-methylacetamide the peptide unit is non-planar withthe NH and NCa2 bonds significantly out of the planel6 F. E. Cole, 1971, personal communication.17 P. Groth, Acta Chem. Scam!., 1970, 24, 780.becomes more non-linear. This would mean a decreasein the hydrogen-bond energy contribution. Thus it isclear that the out-of-plane displacement of H(3) fromthe peptide group observed in the present structure isdictated to some extent by the formation of goodintermolecular hydrogen bonds.Crystal Packing and Hydrogepz Bonding.-Figure 2shows the packing of the molecules down the b axis.The orientation of the molecule is almost parallel to theshort a axis.The molecule exists as zwitterion withNH,+ and C0,- groups. The peptide nitrogen N(2) is18 P. K. Winkler and J. D. Dunitz, J . Mol. Biol., 1971, 59, 169.1s G. N. Ramachandran, A. V. Lakshminarayanan, and A. S.Kolaskar, Uiochim. et Biofihys. Ada, 1973, 303, 81726 J.C.S. Perkin I1hydrogen bonded to the carboxylate O(3). The hydro-gen bond distance N(2) * * * O(3) is longer than the otherthree hydrogen-bond distances between N( 1) andoxygen atoms (Table 4). This situation is consistentwith the observation that in pegtide structures thenitrogen of the peptide group forms a longer hydrogenbond than the amino-nitrogen.Three hydrogens at-tached to N(l), looking along the bond C(1)-N(l) aredisposed in eclipsed positions (Figure 3). While atomsHfFIGURE 3 Surroundings of nitrogen N( 1) viewed along theC ( 1)-N ( 1) bondO(2) and O(3) of the C0,- group are each involved intwo hydrogen bonds, the oxygen O(1) of the peptidegroup is not involved in any hydrogen bonding. Table 5TABLE 5Intermolecular contacts 4 AC(2) * * * O(2I) 3.957 C(1) * - 9 C(5IV) 3.883C(2) * * - C(6I) 3.958 C(1) * * * O(3IV) 3.418N(l) * * - O(2I) 2.856 N(2) - * * C(5IV) 3.930N( 1) * - * C(5I) 3.742 N(2) * - - O(31v) 2.870C(l) - - - O(2I) 3.989 C(3) * * - O(31V) 3.7810 ( 1 ) * C(4I) 3,677 C(4) * * - O(3") 3.709O(1) * - * O(2I) 3.181 C(4) - - * C(8IV) 3-863O(1) - * C(5I) 3.445 O(2) * * * O(3m) 3.850O(1) - * * C(61) 3.825 N ( l ) * - * C ( 5 9 3.687O(1) - - * (371) 3.615 "1) * * * O(3III) 2.750O(1) * * - O(3I) 3.779 N(l) ' ' * O(2III) 3.797side-chain are described by a set of dihedral anglesxl, x2.. . xi which are close to 60 (position I), 180(position 11), and 300" (position 111), corresponding toC(7 f (-1.3611-2-81FIGURE 4 Backbone conformation showing the angles: (a) 4,(b) t,b for the N-terminal end, (c) t,bl, and #2 for the C-terminalleu, and (a) angle oLa 1 (b1N(2j . . . c(s1j 3.961 c(ij. - - o(iI11j 3.654 FIGURE 5 Projection of the side-group on the NCaC' planeC(2) * * - O(3IV) 3.608 N(1) - - * O(2V) 2.704C(l) * * * O(1m)preceding Table.N(l) * * - O(1N) 3.812 NU) . - C(5T 3.603 the three staggered positions of the CY,C* .. . atoms.22In the present structure the Cv atom C(7) occupiesposition I1 with x1 184.2", i.e. trans to N(2). The twoc(l) ' * * o(2v) 3'414 3.248Roman numeral superscripts are defined in footnote toTABLE 6Dihedral angles (deg.) describing the conformation of the recordspacking of the molecules in the crystal is reflected in thelow crystal density.Molecular Conformation.-The notation followed inthe description of the conformational parameters of this Gly-~-Leu 181 188 63molecule is that of Edsall et aL2* The rotations about Gly-L-Pro-L-Leu-Gly 30 1 301 180279 292 170181 185 65 4 and t,h respectively. In the present case there is one L-Leu,HBr 188 182 58angle # for the N-terminal glycyl residue, one angle $, ~-Leu-Gly,HBr 294 272 15531 1 186and two angles and amp;, for the C-terminal leucyl ~ ~ ~ ~ ~ ~ ~ ~ ~ N - m e t h y * a m i d e 303 174 176 63intermolecular contacts 4 A* The looseleucyl side-chain in different molecules. (These anglesrefer to the L-configuration of the chains}Compound X1 X Z 1 X Z 2the N-CQ and CGC' bonds are denoted by torsion angles amp; ~ : ~ ~ ~ ~ ~ y , H B rresidue.As in many peptides, the value (92.7") of 171 174 594 C(2)-N(2)-C(3)-C(5)1 is close to 90" (Figure 4).The observed values for the conformational angeuro;es fallclearly within the allowed regions of the (+,$) map ofRamachandran for peptide configuration. The nitro-gen N(1) is almost in the plane through C(1), C(2), and0 ( 1 ) , the torsion angle N(1)-C(1)-C(2)-O(1) being-8.2". Rotations about the various single bonds in theCS atoms C(8) and C(6) occupy positions I ( ~ 2 1 62.9')and 11 (x22 187.0"). The corresponding angles in otherstructures containing leucyl side-chains are given inTable 6. In spite of the differences in the mode ofpacking of the molecules in these crystals, it is sig-21 G. N. Ramachandran, 'Collagen,' ed. N. Ramanathan,Academic Press. London. 1963. D. 25.2o J. T. Edsall, P. J. Flory, J. C. Kendrew, A. M. Liquori, G.Nemethy, G. N. Ramachandran, and H. A. Scheraga, J . MoZ.Biol., 1966, 15, 399.22 A. V. Lakshminarayanaz V. Sasisekharan, and G. N.Ramachandran, ' Conformation of BiopoIymers,' vol. 1, ed. G. N.Ramachandran, Academic Pi-ess, London, 1967, p. 611974 1727nificant that only two combinations are observed forxl, xZ1, and xZ2, i.e. (1) x1 300, x21 300, and x22 180deg;, and(2) x1 180, x21 180, and x22 60". Although only sevenstructures with the leucyl side-chain are considered, itseems reasonable to conclude that the two combinationsfor the side-chain atoms are energetically more favour-able than are others. Figure 5 illustrates the side-chainprojected on the N,Ca,C' plane to show the orientationof the side-group with respect to the backbone.4/704 Received, 8th April, 1974