Structure and conformation of peptides:N-benzyloxycarboyl-(?-ethyl)-L-glutamyl-(?-ethyl)-L-glutamic acid ethyl ester

Preferred conformation of the benzyloxycarbonyl-amino group in peptides

Structural parameters, derived from X-ray crystallographic data, have been compiled for 35 derivatives of amino acids, peptides, and related compounds, which contain the N-terminal benzyloxycarbonyl (Z) group. The geometry of the urethane moiety of this end group is closely similar to that of the tert-butoxycarbonyl (Boc) group, except for a relaxation of some bond angles because the Z group is sterically less crowded than the Boc group. For the same reason, the Z group has greater conformational flexibility. As a result, packing forces in the crystal may cause greater deformations of bond angles, resulting in larger variations of observed bond lengths and bond angles than in Boc-peptide crystals. The aromatic rings of the Z end groups tend to stack in crystals. Conformational energy calculations indicate that most conformations of Z-amino acid-N'-methylamides and of corresponding Boc derivatives have similar dihedral angles and relative energies, i.e. the nature of the N-terminal end group has little effect on the conformational preferences of the residue next to it. In particular, the computed fraction of molecules with a cis urethane (C-N) bond is similar for the two derivatives: 0.51 and 0.42 in Boc-Pro-NHCH3 and Z-Pro-NHCH3, respectively, and 0.02 in the two Ala derivatives. There exist several computed conformations of Z-Ala-NHCH3 and Z-Pro-NHCH3 in which the phenyl ring and the C-terminal methylamide group are close to each other. Because of favorable nonbonded interactions, such conformations are of low energy.

Conformation and crystal structure of L-prolyl-L-glutamic acid dihydrate

The crystal structure of the dipeptide L-prolyl-L-glutamic acid dihydrate, L-Pro-L-Glu . 2H2O, C10H20O7N2, has been determined from three-dimensional X-ray diffractometer data. The dipeptide crystallizes in the space group P21 of the monoclinic system with two formula units in a cell of dimensions a = 5.629(2), b = 11.832(5), c = 10.485(4)A, and beta = 103.06(3) degrees. The structure was solved by direct methods and refined by least squares techniques to a final value of the conventional R-factor (on F) of 0.039 based on 1798 independent intensities with I greater than or equal to 3 sigma(I). The dipeptide occurs as a zwitterion in the crystal with the pyrrolidine nitrogen atom protonated and the main chain carboxyl group deprotonated. The conformation of the peptide linkage is trans, the omega torsional angle being 173.7 degrees. The pyrrolidine ring adopts the Cs-C beta endo conformation and the conformation of the glutamyl side chain is fully extended. There is considerable intermolecular hydrogen bonding in the crystals.