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 141, with cell parameters at 295 K of a= 15.307(2), c= 18.937(10)Å, V = 4437.1 Å3, 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>=3sigma(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 phi1, psi1 of the Val and phi2, and psi2 of the (S)-alphaMeSer 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.