X-ray analysis of renin substrates. Phenyloxyacetyl-leucyl-valyl-phenylalanine methyl ester: an analogue of angiotensinogen-(10-13) sequence

The crystal structure of the title compound, an analogue of the angiotensinogen-(10-13) peptide in which the N-terminal leucine and the C-terminal tyrosine are respectively replaced by the phenyloxy-acetic group and by phenylalanine, has been determined by X-ray diffraction. The peptide crystallizes in the space group P2(1)2(1)2(1) with a = 4.866(1), b = 22.311(3), c = 27.213(4) A and Z = 4. The crystal structure was solved by direct methods and refined to an R value of 0.056. The molecules adopt a pleated sheet conformation with the hydrophobic residues alternatively situated on the right and left of the main chain. In the crystallographic "a" direction, the molecules are linked by hydrogen bonds and form parallel pleated sheet-type structures.     

Structure of inclusion complexes of cyclomaltoheptaose (cycloheptaamylose): crystal structure of the 1-adamantanemethanol adduct

Cyclomaltoheptaose (cycloheptaamylose) has been crystallized with 1-adamantanemethanol as the guest molecule. The complex crystallized in space group C222(1), with unit-cell dimensions a = 19.162 (13), b = 23.965 (17), and c = 32.597 (27) A. The structure was solved by rotation-translation search-methods. The cyclomaltoheptaose exists as a dimer in the crystal by means of extensive hydrogen-bonding across the secondary hydroxyl ends of two cyclomaltoheptaose molecules. The two halves of the dimer are related by a crystallographic two-fold axis. The primary hydroxyl ends of two adjacent cyclomaltoheptaose molecules are also related by a crystallographic two-fold axis, but do not directly hydrogen bond to one another. Instead, they are held in place by a strong hydrogen bond from the hydroxyl group of the 1-adamantanemethanol to a primary hydroxyl group on an adjacent cyclomaltoheptaose molecule. Other stabilizing hydrogen bonds are formed via three water molecules which are situated at the primary hydroxyl interface, and others that form parallel columns stabilizing the crystal structure. A unique feature of this complex is the presence of trapped water in the cavity at the secondary hydroxyl interface. This water is distributed over 3 disordered sites. Its presence blocks one possible site for the 1-adamantanemethanol, which, instead, binds near the primary hydroxyl end, with its hydroxyl group and part of the adamantane moiety protruding from the cyclomaltoheptaose.     

Synthesis, crystal structure and molecular conformation of the tBuCO-D,L-Ala-delta Z-Phe-NHiPr alpha,beta-unsaturated dipeptide

The crystal structure of the tBuCO-D,L-Ala-delta Z-Phe-NHiPr dipeptide has been solved by X-ray diffraction. The peptide crystallizes in monoclinic space group P2(1)/c with a = 13.445 (3) A, b = 35.088 (4) A, c = 14.755 (3) A, beta = 116.73 (1) degree, Z = 12 and dc = 1.151 g.cm-3. The three independent molecules per asymmetric unit accommodate a beta II-folded conformation, but only one of them contains the typical i + 3----i interaction characterizing a beta-turn. In the other two molecules, the N...O distance exceeds 3.2 A, a value generally considered the upper limit for hydrogen bonds in peptides. In solution, the beta II-turn conformation is largely predominant.     

Crystal and molecular structure of L-valyl-L-lysine hydrochloride.

L-Valyl-L-lysine hydrochloride, C11N3O3H23 HCl, crystallizes in the monoclinic space group P2(1) with a = 5.438(5), b = 14.188(5), c = 9.521(5) A, beta = 95.38(2) degrees and Z = 2. The crystal structure, solved by direct methods, refined to R = 0.036, using full matrix least-squares method. The peptide exists in a zwitterionic form, with the N atom of the lysine side-chain protonated. The two gamma-carbons of the valine side-chain have positional disorder, giving rise to two conformations, chi 1(11) = -67.3 and 65.9 degrees, one of which (65.9 degrees) is sterically less favourable and has been found to be less popular amongst residues branching at beta-C. The lysine side-chain has the geometry of g- tgt, not seen in crystal structures of the dipeptides reported so far. Interestingly, chi 2(3) (63.6 degrees) of lysine side-chain has a gauche+ conformation unlike in most of the other structures, where it is trans. The neighbouring peptide molecules are hydrogen bonded in a head-to-tail fashion, a rather uncommon interaction in lysine peptide structures. The structure shows considerable similarity with that of L-Lys-L-Val HCl in conformational angles and H-bond interactions [4].     

Conformational studies of retro–inverso peptides: The crystal and molecular structure of the hydantoin from H-Ala-G-Ala-mGly-OBzl

The crystal structure of the hydantoin 1-[(S)-1′–aminoethylmalonyl benzyl ester]-(S)-4-methylimidazolidin-2.5-dione (1) derived from the peptide H-Ala-gAla-mGly-OBzl, Having the retro-inverso modification of the Ala-Gly bond, has been determined by x-ray diffraction analysis. The crystals are orthorhombic, space group P2₁2₁2₁ with a = 6.539. b = 14.721, c = 17.101 Å, z = 4. The structure was solved by direct methods and refined with anisotropic thermal factors to a final R value of 0.067 for the 947 observed reflections. Reversal of the Ala-Gly amide bond perturbs the folding tendency of the backbone shown by the parent peptide t-BuCO-Ala-Gly-NHiPr. The gem-diamino residue, gAla, and the malonyl moieties are found in the helical and the extended conformations, respectively. Intramolecular hydrogen bonding is not observed. The molecules in the crystal are held together by the formation of two intermolecular hydrogen bonds of the N? H?O?C type with N?O distances of 2.86 and 3.17 Å respectively. 1995 John Wiley&Sons. Inc. © 1995 John Wiley & Sons, Inc.     

Crystal structure and conformation of polypeptides: L-leucylglycylglycylglycine

Crystals of L-leucylglycylglycylglycine, LGGG (C12H22N4O5), grown from an ethanol-water solution, are orthorhombic, space groups P2(1)2(1)2(1), with unit cell dimensions (at 22 +/- 3 degrees) a = 9.337(1), b = 10.995(1), c = 15.235(1)A, v = 1563.4 A3, Z = 4 with a density of Dobs = 1.29 g.cm-3 and Dcalc = 1.279 g.cm-3. The crystal structure was solved by the application of direct methods and refined to an R value of 0.029 for 1018 reflections with I greater than or equal to 2 sigma. The molecule exists as a zwitterion in the crystal. The trans peptide backbone takes up a folded conformation at the middle glycylglycyl link accompanied by a significant nonplanarity up to delta omega of 8 degrees at the middle peptide and is relatively more extended at the two ends. The molecules are linked together intermolecularly in an infinite sequence of head to tail 1-4' hydrogen bonds, as is typical of charged peptides. It is interesting to note that while glycylglycylglycine takes up an extended beta-sheet conformation, addition of Leu to the N-terminal results in a bent conformation.     

Conformational investigation of alpha, beta-dehydropeptides. Part III. Molecular and crystal structure of acetyl-L-prolyl-alpha, beta-dehydrovaline methylamide.

The crystal structure of Ac-Pro-delta Val-NHCH3 was examined to determine the influence of the alpha,beta-dehydrovaline residue on the nature of peptide conformation. The peptide crystallizes from methanol-diethyl ether solution at 4 degrees in needle-shaped form in orthorhombic space group P2(1)2(1)2(1) with a = 11.384(2) A, b = 13.277(2) A, c = 9.942(1) A, V = 1502.7(4) A3, Z = 4, Dm = 1.17 g.cm-3 and Dc = 1.18 g.cm-3. The structure was solved by direct methods using SHELXS-86 and refined to an R value of 0.057 for 1922 observed reflections. The peptide is found to adopt a beta-bend between the type I and the type III conformation with phi 1 = -68.3(4) degrees, psi 1 = -20.1(4) degrees, phi 2 = -73.5(4) degrees and psi 2 = -14.1(4) degrees. An intramolecular hydrogen bond between the carbonyl oxygen of ith residue and the NH of (i + 3)th residue stabilizes the beta-bend. An additional intermolecular N...O hydrogen bond joins molecules into infinite chains. In the literature described crystal structures of peptides having a single alpha,beta-dehydroamino acid residue in the (i + 2) position and forming a beta-bend reveal a type II conformation.     

Role of water molecules in the crystal structure of gly-L-ala-L-phe: A possible sequence preference for nucleation of α-helix?

The synthetic peptide Gly-L-Ala-L-Phe (C14H19N3O4.2H2O; GAF) crystallizes in the monoclinic space group P2I1), with a = 5.879(1), b = 7.966(1), c = 17.754(2) A, beta = 95.14(2) degrees, Dx = 1.321 g cm-3, and Z = 2. The crystal structure was solved by direct methods using the program SHELXS-86 and refined to an R value of 0.031 for 1425 reflections (greater than 3 sigma). The tripeptide exists as a zwitterion in the crystal and assumes a near alpha-helical backbone conformation with the following torsion angles: psi 1 = -147.8 degrees; phi 2, psi 2 = -71.2 degrees, 33.4 degrees; phi 3, psi 3 = -78.3 degrees, -43.3 degrees. In this structure, one water molecule bridges the COO- and NH3+ terminii to complete a turn of an alpha-helix and another water molecule participates in head-to-tail intermolecular hydrogen bonding, so that the end result is a column of molecules that looks like an alpha-helix. Thus, the two water molecules of crystallization play a major role in stabilizing the near alpha-helical conformation of each tripeptide molecule and in elongating the helix throughout the crystal. An analysis of all protein sequences around regions containing a GAF fragment by Chou-Fasman's secondary structure prediction method showed that those regions are likely to assume an alpha-helical conformation with twice the probability they are likely to adopt a beta-sheet conformation. It is conceivable that a GAF fragment may be a good part of the nucleation site for forming alpha-helical fragments in a polypeptide, with the aqueous medium playing a crucial role in maintaining such transient species.     

Conformation and hydrogen bonding of N-formylpeptides: crystal and molecular structure of N-formyl-L-alanyl-L-aspartic acid.

Crystals of N-formyl-L-alanyl-L-aspartic acid (C8H11N2O6) grown from aqueous methanol solution are orthorhombic, space group, P2(1)2(1)2(1) with cell parameters at 294K of a = 13.619(2), b = 8.567(2), c = 9.583(3)A, V = 1118.1A3, M.W. = 232.2, Z = 4, Dm = 1.38 g/cm3 and Dx = 1.378 g/cm3. The crystal structure was solved by the application of direct methods and refined to an R value of 0.075 for 1244 reflections with I greater than or equal to 3 sigma collected on a CAD-4 diffractometer. The structure contains two short intermolecular hydrogen bonds: (i) between the C-terminal carboxyl OH and the N-acyl oxygen (2.624(3)A), a characteristic feature found in many N-acyl peptides and (ii) between the aspartic carboxyl OH. and the peptide oxygen OP1 (2.623(3)A). The peptide is nonplanar (omega = 165.5(6) degrees). The molecule takes up a folded conformation in contrast to N-formyl peptides which form extended beta-sheets; the values of phi 1, psi 1, phi 2, psi 2(1), and psi 2(2) are, respectively -65.7(6), 152.0(5), -107.2(5), 30.9(5), and -150.3(6). The aspartic acid side chain conformation is g- with chi 1 = 73.1(5). The formyl group, as expected, is transplanar [OF-CF-N1-CA1 = -4.0(8) degrees]. The presence of the short O-H ... O hydrogen bond emerges as a structural feature common to this peptide and several other N-formyl peptides. There are no C-H ... O hydrogen bonds in this structure.     

The hydrogen bonding in the crystal structure of raffinose pentahydrate

The crystal structure of raffinose pentahydrate, O-alpha-D-galactopyranosyl-(1----6)-O-alpha-D-glucopyranosyl-(1----2)- beta-D- fructofuranose pentahydrate, C18H32O16.5H2O, has been redetermined using low-temperature, 119 K, CuK alpha X-ray data. All hydrogen atoms were unambiguously located on difference syntheses. The final R-factor is 0.036 for 2423 observed structure amplitudes. The hydrogen bonding is composed of infinite chains, which are linked through the water molecules to form a three-dimensional network containing a chain of five linked water molecules. Three of the infinite chains extend in the directions of the crystallographic axis of the space group P2(1)2(1)2(1). Four of the water molecules accept two hydrogen bonds and one accepts one. All the hydroxyls and the ring and glycosidic oxygen atoms are involved in the hydrogen bonding. With one exception, the ring and glycosidic oxygens are hydrogen-bonded by means of the minor components of unsymmetrical three-center bonds.     

Crystal structure of a cyclomaltoheptaose-4-hydroxybiphenyl inclusion complex

The crystal structure of the inclusion complex of cyclomaltoheptaose (beta-cyclodextrin) with 4-hydroxybiphenyl was determined by single-crystal X-ray diffraction at 150K. The complex contains two cyclomaltoheptaose molecules, two 4-hydroxybiphenyl molecules, one ethanol molecule and fifteen water molecules in the asymmetric unit, and could be formulated as [2(C(42)H(70)O(35)).2(C(12)H(10)O).(C(2)H(6)O).15(H(2)O)]. It crystallized in the triclinic space group P1 with unit cell constants a=15.257(3), b=15.564(3), c=15.592(2)A, alpha=104.485(15) degrees , beta=101.066(14) degrees , gamma=104.330(17) degrees , V=3,343.6(10)A(3). In the crystal lattice, two beta-cyclodextrins form a head-to-head dimer jointed through hydrogen bonds. Two 4-hydroxybiphenyls were included in the dimer cavity with their hydroxyl groups protruding from two primary hydroxyl sides of the cyclodextrin molecules. The guest 4-hydroxybiphenyl molecules linked into a chain via a combination of an O-Hcdots, three dots, centeredO hydrogen bond and face-to-face pi-pi stacking of the phenyl rings. The crystal structure supports the calculation results indicating that the 2:2 inclusion complex formed by beta-cyclodextrin and 4-hydroxybiphenyl is the energetically favored structure.     

A sequence preference for nucleation of alpha-helix--crystal structure of Gly-L-Ala-L-Val and Gly-L-Ala-L-Leu: some comments on the geometry of leucine zippers

The synthetic peptide Gly-L-Ala-L-Val (C10H19N3O4.3H2O; GAV) crystallizes in the monoclinic space group P21, with a = 8.052(2), b = 6.032(2), c = 15.779(7) A, beta = 98.520(1) degree, V = 757.8 A3, Dx = 1.312 g cm-3, and Z = 2. The peptide Gly-L-Ala-L-Leu (C11H21N3O4.3H2O; GAL) crystallizes in the orthorhombic space group P212121, with a = 6.024(1), b = 8.171(1), c = 32.791(1) A, V = 1614 A3, Dx = 1.289 g cm-3, and Z = 4. Their crystal structures were solved by direct methods using the program SHELXS-86, and refined to an R index of 0.05 for 1489 reflections for GAV and to an R index of 0.05 for 1563 reflections for GAL. The tripeptides exist as a zwitterion in the crystal and assume a near alpha-helical backbone conformation with the following torsion angles: psi 1 = -150.7 degrees; phi 2, psi 2 = -68.7 degrees, -38.1 degrees; phi 3, psi 32 = -74.8 degrees, -44.9 degrees, 135.9 degrees for GAV; psi 1 = -150.3 degrees; phi 2, psi 2 = -67.7 degrees, -38.9 degrees; phi 3, psi 31, psi 32 = -72.2 degrees, -45.3 degrees, 137.5 degrees for GAL. Both the peptide units in both of the tripeptides show significant deviation from planarity [omega 1 = -171.3(6) degrees and omega 2 = -172.0(6) degrees for GAV; omega 1 = -171.9(5) degrees and omega 2 = -173.2(6) degrees for GAL]. The side-chain conformational angles chi 21 and chi 22 are -61.7(5) degrees and 175.7(5) degrees, respectively, for valine, and the side-chain conformations chi 12 and chi 23's are -68.5(5) degrees and (-78.4(6) degrees, 159.10(5) degrees) respectively, for leucine. Each of the tripeptide molecule is held in a near helical conformation by a water molecule that bridges the NH3+ and COO- groups, and acts as the fourth residue needed to complete the turn by forming two hydrogen bonds. Two other water molecules form intermolecular hydrogen bonds in stabilizing the helical structure so that the end result is a column of molecules that looks like an alpha-helix.     

木棉素的分离与晶体结构

从木棉科植物木棉(Gossmpinus malabarica(DC.)Mer)的叶中首次分离纯化得到木棉素(Gossampinusxanthone)。经元素分析、波谱数据和X-射线单晶衍射分析,确认该化合物为1,3,6,7-四羟基-2-β-D-吡喃葡萄糖基口山酮(1,3,6,7-terahydroxy-2-β-D-glucopyanosyl-xanthone)。C19H18O11,M=422.34。晶体属斜方晶系,空间群P21212。晶胞参数a=7.265(5),b=30.086(4),c=8.342(2),V=1822(2)3,Z=4,Dc=1.54g/cm3,F(000)=880。木棉素分子由平面口山酮基和椅式葡萄糖基通过C(2)-C(1′)键连接而成。两基团的两个最小二乘平面间的夹角约为76.5°。存在分子内氢键O┄H…O的同时,也有分子间O┄H…O氢键。     

Helix packing of leucine-rich peptides: a parallel leucine ladder in the structure of Boc-Aib-Leu-Aib-Aib-Leu-Leu-Leu-Aib-Leu-Aib-OMe.

The packing of peptide helices in crystals of the leucine-rich decapeptide Boc-Aib-Leu-Aib-Aib-Leu-Leu-Leu-Aib-Leu-Aib-OMe provides an example of ladder-like leucylleucyl interactions between neighboring molecules. The peptide molecule forms a helix with five 5----1 hydrogen bonds and two 4----1 hydrogen bonds near the C terminus. Three head-to-tail NH ... O = C hydrogen bonds between helices form continuous columns of helices in the crystal. The helicial columns associate in an antiparallel fashion, except for the association of Leu ... Leu side chains, which occurs along the diagonal of the cell where the peptide helices are parallel. The peptide, with formula C56H102N10O13, crystallizes in space group P2(1)2(1)2(1) with Z = 4 and cell parameters a = 16.774(3) A, b = 20.032(3) A and c = 20.117(3) A; overall agreement factor R = 10.7% for 2014 data with magnitude of F(obs) greater than 3 sigma (F); resolution 1.0 A.     

Crystal structure of the alpha, beta, gamma-tridentate manganese complex of adenosine 5'-triphosphate cocrystallized with 2,2'-dipyridylamine

The 1:1:1 complex of Mn2+, ATP, and 2,2'-dipyridylamine (DPA) crystallizes as Mn-(HATP)2.Mn(H2O)6.(HDPA)2.12H2O in the orthorhombic space group C222(1) with unit cell dimensions a = 10.234 (3) A, b = 22.699 (3) A, and c = 31.351 (4) A. The structure was solved by the multisolution technique and refined by the least-squares method to a final R index of 0.072 using 3516 intensities. The structure is composed of two ATP molecules sharing a common manganese atom. The metal exhibits alpha, beta, gamma coordination to the triphosphate chains of two dyad-related ATP molecules, resulting in a hexacoordinated Mn2+ ion surrounded by six phosphate groups. The metal to oxygen distances are 2.205 (6), 2.156 (4), and 2.144 (5) A for the alpha-, beta-, and gamma-phosphate groups, respectively. No metal-base interactions are observed. There is a second hexaaqua-coordinated Mn2+ ion that is also located on a dyad axis. The hydrated manganese ions sandwich the phosphate-coordinated manganese ions in the crystal with a metal-metal distance of 5.322 A. The ATP molecule is protonated on the N(1) site of the adenine base and exhibits the anti conformation (chi = 66.0 degrees). The ribofuranose ring is in the 2/3 T conformation with pseudorotation parameters P = 179 (1) degrees and tau m = 34.1 (6) degrees. The adenine bases form hydrogen-bonded self-pairs across a crystallographic dyad axis and stack with both DPA molecules to form a column along the dyad. The structure of the metal-ATP complex provides information about the possible metal coordination, conformation, and environment of the nucleoside triphosphate substrate in the enzyme.     

f-Element/crown ether complexes. 1. Synthesis and structure of [Y(OH2)8]Cl3·(15-crown-5)

The crystal and molecular structure of [Y(OH₂)₈]Cl₃·(15-crown-5) has been determined by single- crystal X-ray diffraction. The complex crystallizes in the monoclinic space group P2₁/n with Z = 4. Lattice parameters are a = 9.202(2), b = 17.247(3), c = 15.208(3) Å, and β = 92.39(2)°. The structure was solved by Patterson and Fourier techniques and refined by least-squares to a final conventional R value of 0.081. The Y(III) ion is eight coordinate, bonded to the oxygen atoms of the eight water molecules. Three of the water molecules are hydrogen bonded to crown ether molecules. The three chloride ions participate in hydrogen bonds with the remaining five water molecules. The YO(water) distances range from 2.322(6) to 2.432(7) Å and average 2.37(4) Å. The average O(water)···Cl and O(water)···O(crown) hydrogen bonded separations are 3.08(4) and 2.76(7) Å, respectively.     

Crystal structure and conformational analysis of angiotensinogen fragments

The tripeptide acetyl-L-prolyl-L-phenylalanyl-L-histidine crystallizes in the orthorhombic space group P2(1)2(1)2(1) with eight molecules in a unit cell of dimensions a = 9.028(2), b = 140.54(6) and c = 42.41(1)A. The structure has been solved by direct methods and refined to an R value of 0.056 for 2904 observed reflections. The molecule exists as a zwitterion with terminal (His)CO2- and (imidazole)H+ as charged groups. The two peptide molecules in the structure adopt a type I beta-turn with Pro and Phe as the corner residues. The main conformational difference between the two crystallographically independent molecules is seen to be in the histidine side-chain orientations. The molecules arrange themselves in sheets perpendicular to the c axis. All hydrophobic side chains lie on one side of the sheets thus generated, whereas the hydrophilic groups are located on the other side. An interesting feature of the crystal structure is the existence of a water layer between adjacent peptide sheets. The conformational study of the isolated Ac-His-Pro-Phe-His-MA using energy calculations gives a rather limited number of stable conformers. The most stable corresponds to a type I beta-turn stabilized through two hydrogen bonds, followed by a less stable type II beta-turn (delta E = 2.0 kcal) and a partly helical structure (delta E = 2.6 kcal).     

Crystal and molecular structure of Boc-D-Ala-delta Phe-Gly-delta Phe- D-Ala-OMe: a 3 10-helical dehydropeptide

The crystal and molecular structure of the pentapeptide Boc-D-Ala-delta Phe-Gly-delta Phe-D-Ala-OMe, containing two dehydrophenylalanine residues, was determined by x-ray diffraction. The molecule crystallizes in the orthorombic P2(1)2(1)2(1) space group, with a = 10.439(3), b = 15.319(3) and c = 21.099(4) A. In the solid state, the conformation of the pentapeptide is characterized by the presence of two type III' beta-turns. Thus the peptide assumes a left-handed 3(10-helical conformation, the left sense being due to the D configuration of the alanine residues. The two unsaturated residues are located in the (i + 1) position of the first beta-turn and in the (i + 2) position of the second beta-turn, respectively. In the crystal, the helical molecules are linked head to tail by hydrogen bonds. Lateral hydrogen bonds are also formed between molecules related by a twofold screw symmetry. This gives rise to a typical mode of packing characterized by infinite helical "chains,' similar to the packing found in other oligopeptides that adopt a 3(10)-helical structure.     

Helix-forming tendencies of amino acids depend on the restrictions of side-chain rotamer conformations: crystal structure of the tripeptide GAI in two crystalline forms.

In our attempts to design crystalline alpha-helical peptides, we synthesized and crystallized GAI (C11H21N3O4) in two crystal forms, GAI1 and GAI2. Form 1 (GAI1) Gly-L-Ala-L-Ile (C11H21N3O4.3H2O) crystals are monoclinic, space group P2(1) with a = 8.171(2), b = 6.072(4), c = 16.443(4) A, beta = 101.24(2) degrees, V = 800 A3, Dc = 1.300 g cm-3 and Z = 2, R = 0.081 for 482 reflections. Form 2 (GAI2) Gly-L-Ala-L-Ile (C11H21N3O4.1/2H2O) is triclinic, space group P1 with a = 5.830(1), b = 8.832(2), c = 15.008(2) A, alpha = 102.88(1), beta = 101.16(2), gamma = 70.72(2) degrees, V = 705 A3, Z = 2, Dc = 1.264 g cm-3, R = 0.04 for 2582 reflections. GAI1 is isomorphous with GAV and forms a helix, whereas GAI2 does not. In GAI1, the tripeptide molecule is held in a near helical conformation by a water molecule that bridges the NH3+ and COO- groups, and acts as the fourth residue needed to complete the turn by forming two hydrogen bonds. Two other water molecules form intermolecular hydrogen bonds in stabilizing the helical structure so that the end result is a column of molecules that looks like an incipient alpha-helix. GAI2 imitates a cyclic peptide and traps a water molecule. The conformation angles chi 11 and chi 12 for the side chain are (-63.7 degrees, 171.1 degrees) for the helical GAI1, and (-65.1 degrees, 58.6 degrees) and (-65.0 degrees, 58.9 degrees) for the two independent nonhelical molecules in GAI2; in GAI1, both the C gamma atoms point away from the helix, whereas in GAI2 the C gamma atom with the g+ conformation points inward to the helix and causes sterical interaction with atoms in the adjacent peptide plane. From these results, it is clear that the helix-forming tendencies of amino acids correlate with the restrictions of side-chain rotamer conformations. Both the peptide units in GAI1 are trans and show significant deviation from planarity [omega 1 = -168(1) degrees; omega 2 = -171(1) degrees] whereas both the peptide units in both the molecules A and B in GAI2 do not show significant deviation from planarity [omega 1 = 179.3(3) degrees; omega 2 = -179.3(3) degrees for molecule A and omega 1 = 179.5(3) degrees; omega 2 = -179.4(3) degrees for molecule B], indicating that the peptide planes in these incipient alpha-helical peptides are considerably bent.     

Crystal structure of sodium isosaccharate, NaC6H11O6 x H2O

Sodium isosaccharate, NaC(6)H(11)O(6).H(2)O (Na-ISA), has been synthesized, and its crystal structure solved by single-crystal X-ray diffraction methods. Na-ISA crystallizes in the monoclinic space group P2(1) (#4) with cell parameters a = 9.2267(11) A, b = 5.0765(6) A, c = 9.7435(11) A, beta = 103.304(2) degrees, V = 444.13(9) A(3), Z = 2. The structure was refined by full-matrix least-squares on F2 yielding final R-values (all data) R1 = 0.0361 and Rw2 = 0.0935. The structure of Na-ISA consists of (C(6)H(11)O(6))(-) anions arranged in layers parallel to the bc plane. An extended network of O-H...O hydrogen bonds links the (ISA)(-) anions and the crystal water molecules. Each sodium atom is coordinated by four oxygen atoms belonging to four different (ISA)(-) anions and by one water molecule. The resulting NaO(5) polyhedra are linked by sharing common corners in zig-zag chains running parallel to the b-axis.