Linear tripeptide conformation. Crystal structures of Cbz-glycylglycyltyrosine methyl ester and Cbz-glycyl(D,L)tyrosylglycine ethyl ester.

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.