Crystal structure and conformation of two N-tosyl-protected dipeptides containing amino acids with polar side-chains.

X-Ray diffraction studies and energy-minimization calculations were carried out on two dipeptides, N-tosyl-L-Ser-Gly-OMe monohydrate (C13H18N2O6S.H2O, compound A) and N-tosyl-L-Thr-Gly-OMe (C14H20N2O6, compound B). Compound A crystallized in the monoclinic system, space group P2(1) with unit cell parameters a = 4.915(1), b = 15.625(4), c = 11.003(1) A, beta = 91.28(1) degrees, V = 844.8 A3. M(r) = 348.4, d = 1.37(2) g cm-3, Z = 2, lambda(Cu K alpha) = 1.5418 A, mu = 1.99 mm-1, T = 293 K. R = 0.032 for 1451 unique reflections with I > 2 sigma(I). Compound B crystallized in the orthorhombic system, space group P2(1)2(1)2(1), with unit cell parameters a = 5.050(2), b = 16.483(3), c = 20.769(5) A, V = 1729.3 A3, Z = 4. M(r) = 344.4, d = 1.32(2) g cm-3, mu(Cu K alpha) = 1.90 mm-1. R = 0.040 for 1060 unique reflections with I > 2 sigma(I). The major difference in the backbone conformation of the two compounds is in their glycine residues, with the glycine residue in compound A adopting an extended conformation with phi = -132.6(3) degrees and psi = 175.3(3) degrees and that in compound B having a folded conformation with phi = -56.3(6) degrees and psi = -42.6(7) degrees. In compound A the oxygen atom of the Ser side-chain and the carbonyl oxygen atom of glycine are bridged by the water of crystallization through O--H ... O hydrogen bonds, resulting in the relatively rare trans conformation [chi = -175.7(2) degrees] for this side-chain.(ABSTRACT TRUNCATED AT 250 WORDS)