title: Non coded C alpha, alpha-disubstituted amino acids. X-ray diffraction analysis of a dipeptide containing (S)-alpha-methylserine.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Naples, Italy.
authors.forename: V
authors.initials: V
authors.lastname: Pavone
authors.forename: B
authors.initials: B
authors.lastname: Di Blasio
authors.forename: A
authors.initials: A
authors.lastname: Lombardi
authors.forename: O
authors.initials: O
authors.lastname: Maglio
authors.forename: C
authors.initials: C
authors.lastname: Isernia
authors.forename: C
authors.initials: C
authors.lastname: Pedone
authors.forename: E
authors.initials: E
authors.lastname: Benedetti
authors.forename: E
authors.initials: E
authors.lastname: Altmann
authors.forename: M
authors.initials: M
authors.lastname: Mutter
mesh_terms: D003460:Crystallization; D004151:Dipeptides / Q000737:chemistry*; D006860:Hydrogen Bonding; D007536:Isomerism; D008958:Models, Molecular; D008968:Molecular Conformation; D012694:Serine / Q000031:analogs & derivatives*; D014961:X-Ray Diffraction
chemical_list: D004151:Dipeptides; C079735:tert-butyloxycarbonyl-valyl-alpha-methylserine methyl ester; D012694:Serine