Crystal Structures of N-(Benzyloxycarbonyl)prolylleucine Ethyl Ester and N-(t-Butoxycarbonyl)prolylleucine Benzyl Ester

Abstract

Abstract The molecular structures of N-(benzyloxycarbonyl)prolylleucine ethyl ester and N-(t-butoxycarbonyl)prolylleucine benzyl ester, were determined by the X-ray method. The former belongs to the orthorhombic system P212121 with Z=4, a=9.917(3), b=21.960(3), and c=9.956(1) Å; the latter belongs to the monoclinic system P21 with Z=2, a=10.475(1), b=17.910(1), c=6.303(1) Å, and β=90.213(7)°. Both structures were determined by the direct method; the final R indexes are 0.062 for the former and 0.072 for the latter. An extended conformation found in both dipeptides shows that in terms of the Pro–Leu unit alone this conformation is preferable to the folded β-turn conformation such as is found in longer peptides. The leucyl side chain conformations found in peptides or amino acids were also studied. For the residues occurring in the peptides the C′–Cα–Cβ–Cγ–Cδ1 transzigzag conformation was found overwhelmingly, without any obvious relationships to the main chain torsion angles. On the other hand, for the C-termini leucyl residues and leucines the N–Cα–Cβ–Cγ–Cδ2 trans-zigzag conformation was rather preferred and the ψ values were restricted to a narrow range.