title: Crystal-state structural analysis of two gamma-lactam-restricted analogs of Pro-Leu-Gly-NH2.
abstract: The crystal structures of two analogs of Pro-Leu-Gly-NH2 (1), containing a gamma-lactam conformational constraint in place of the -Leu-Gly- sequences, are described. The highly biologically active (S,R)-diastereomer 2a is semi-extended at the C-terminus, with the N-terminal Pro residue in the unusual "C5" conformation [psi 1 = -0.8(15) degrees] stabilized by a (peptide)N-H...N(amino) intramolecular H-bond [the N(3)...N(4) separation is 2.687(11)A]. Conversely, the N,N'-isopropylidene aminal trihydrate of the (S,S)-diastereomer 2b, compound 3, adopts a beta-bend conformation at the C-terminus, as already reported for 1. However, the backbone torsion angles [phi 2 = 57.4(4), psi 2 = -129.9(3) degrees; psi 3 = -92.3(4), phi 3 = 6.4(5) degrees] lie close to the values expected for the corner residues of an ideal type-II' beta-bend. A weak intramolecular 4----1 H-bond is seen between the Gly carboxyamide anti-NH and Pro C = O groups. In the newly formed 2,2,3,4-tetraalkyl-5-oxo-imidazolidin-1-yl moiety the psi 1 torsion angle is 12.9(4) degrees and the intramolecular N(3)...N(4) separation is 2.321(4)A.
authors.affiliation: Department of Organic Chemistry, University of Padova, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Valle
authors.forename: M
authors.initials: M
authors.lastname: Crisma
authors.forename: C
authors.initials: C
authors.lastname: Toniolo
authors.forename: K L
authors.initials: KL
authors.lastname: Yu
authors.forename: R L
authors.initials: RL
authors.lastname: Johnson
mesh_terms: D007769:Lactams; D009075:MSH Release-Inhibiting Hormone; D015394:Molecular Structure; D011487:Protein Conformation; D014961:X-Ray Diffraction
chemical_list: D007769:Lactams; D009075:MSH Release-Inhibiting Hormone