1. 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
S Chaturvedi, K Go, R Parthasarathy Biopolymers. 1991 Mar;31(4):397-407. doi: 10.1002/bip.360310405.
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.
2. Histidyl conformations and short N-H...N hydrogen bonds: structure of D,L-histidyl-L,D-histidine pentahydrate
J A Krause, P W Baures, D S Eggleston Acta Crystallogr B. 1991 Aug 1;47 ( Pt 4):506-11. doi: 10.1107/s0108768191000915.
D,L-Histidyl-L,D-histidine pentahydrate, C12H16-N6O3.5H2O, Mr = 382.38, F(000) = 408, crystallizes in the monoclinic space group Pc with the cell dimensions a = 9.971 (2), b = 4.745 (2), c = 19.572 (3) A and beta = 96.08 (1) degree, V = 920.6 A3, Z = 2, D chi = 1.379 g cm-3. mu = 1.083 cm-1, T = 295 K, Mo K alpha, lambda = 0.71073 A. Final R (on F) = 0.040 for 1658 observed reflections with I greater than or equal to 3 sigma (I). This dipeptide crystallizes in a zwitterionic form with protonation of the C-terminal imidazole ring. Both histidine units exist in the g+ or 'closed' conformation with C alpha-C beta torsion angles of 67.2 (3) and 63.6 (3) degrees. Principal torsion angles, omega = 176.8 (2). psi 1 = 161.8 (3) and phi 2 = -152.1 (3) degrees, are indicative of a highly extended trans conformation. Intramolecular hydrogen bonding occurs between the imidazole rings [N2D-H2D1...N1D = 2.724 (4) A]. Intermolecular hydrogen bonding occurs between symmetry-related histidine molecules forming chains along the gamma axis and includes another short [2.764 (4) A] N-H...N interaction. The five water molecules occupy channels between adjacent histidine layers.
3. Linear tripeptide conformation. Crystal structures of Cbz-glycylglycyltyrosine methyl ester and Cbz-glycyl(D,L)tyrosylglycine ethyl ester
J A Krause, D S Eggleston Int J Pept Protein Res. 1993 Feb;41(2):133-40.
The structures of two tripeptides, Cbz-glycylglycyltyrosine methyl ester (ZGGYOMe) and Cbz-glycyl(D,L)tyrosylglycine ethyl ester (ZGYGOEt) have been determined from single-crystal X-ray diffraction data. Crystals of ZGGYOMe are monoclinic, space group P2(1), with a = 12.427(3), b = 4.999(3), c = 17.401(6) A, beta = 99.98(2) degree and Z = 2. The final R-index is 0.049 for 1698 reflections with I > or = to 2 sigma (I). Crystals of ZGYGOEt are monoclinic, space group P2(1)/n with a = 12.134(8), b = 14.614(3), c = 26.154(9) A, beta = 98.78(4) degrees, Z = 8. The final R-index is 0.067 for 4457 reflections with I > or = to 2 sigma (I). Both peptides adopt highly extended structures; principal torsion angles are omega 0 = 175.0(4) degrees, phi 1 = 69.2(5) degrees, psi 1 = -154.9(4) degrees, omega 1 = -175.8(4) degrees, phi 2 = 165.4(4) degrees, psi 2 = 154.2(3) degrees, omega 2 = 169.6(3) degrees, phi 3 = -94.8(5) degrees, psi 3 = -47.6(5) degrees for ZGGYOMe and, for the two independent molecules of ZGYGOEt, omega 0 = 177.9(4) degrees, 178.9(4) degrees, phi 1 = -172.0(4) degrees, 169.7(4) degrees; psi 1 = 174.4(4) degrees, -162.5(4) degrees; omega 1 = -170.1(4) degrees, 176.7(4) degrees; phi 2 = -130.8(4) degrees, 130.3(5) degrees; psi 2 = 162.8(4) degrees, -163.3(4) degrees; omega 2 = -177.6(4) degrees, 176.2(4) degrees; phi 3 = -169.9(4) degrees, 172.9(4) degrees; psi 3 = -168.2(4) degrees, 160.9(4) degrees. The structures are of interest since the first one adopts a conformation unlike those of related GGX sequences and the latter shows an antiparallel hydrogen-bonding pattern.
