Non coded C alpha, alpha-disubstituted amino acids. X-ray diffraction analysis of a dipeptide containing (S)-alpha-methylserine.

The crystal and molecular structure of the fully protected dipeptide Boc-Val-(S)-alpha-MeSer-OMe has been determined by X-ray diffraction techniques. Crystals grown from ethyl acetate/n-pentane mixtures are tetragonal, space group I4(1), with cell parameters at 295 K of a = 15.307(2), c = 18.937(10)A, V = 4437.1A3, M.W. = 332.40, Z = 8, Dm = 0.99 g/cm3 and Dx = 0.995 g/cm3. The structure was solved by application of direct methods and refined to an R value of 0.028 for 1773 reflections with I > or = 3 sigma (I) collected on a CAD-4 diffractometer. Both chiral centers have the (S) configuration. The dipeptide assumes in the solid state an S shape. The urethane moiety is in the cis conformation, while the amide bond is in the common trans conformation. The conformational angles phi 1, psi 1 of the Val and phi 2, and psi 2 of the (S)-alpha MeSer fall in the F region of the phi-psi map. The isopropyl side chain of the Val residue has the (t, g-) conformation, while the Ser side chain has a g+ conformation. The hydrogen bond donor groups are all involved in intermolecular H-bond interactions. Along the quaternary axis the dipeptide molecules are linked to each other with the formation of infinite rows.