CRYSTAL-STRUCTURE AND CONFORMATION OF 2 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(l), b = 15.625(4), c = 11.003(l) angstrom, beta = 91.28(1)-degrees, V = 844.8 angstrom3. M(r) = 348.4, d = 1.37(2) g cm-3, Z = 2, lambda(Cu Kalpha) = 1.5418 angstrom, mu = 1.99 mm-1, T = 293 K. R = 0.032 for 1451 unique reflections with I> 2a(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) angstrom, V = 1729.3 angstrom3, Z = 4. M(r) = 344.4, d = 1.32(2) g cm-3, mu(Cu Kalpha) = 1.90 mm-1. R = 0.040 for 1060 unique reflections with I > 2sigma(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. The Thr side-chain in compound B is in the sterically preferred (tg-) conformation [chi1,1 = - 179.4(4)-degrees and chi1,2 = - 62.3(5)-degrees]. The conformations were found to be in general agreement with those obtained by an energy-minimization procedure. The energy-minimized structure of N-tosyl-L-Ser-Gly-Ome (anhydrous) showed a strong hydrophobic interaction between the methyl substituents of the tosyl group and the methyl ester (C-C = 4.08 angstrom). (C) Munksgaard 1993.