Molecular and crystal structures of terminally protected tri- and pentapeptides of trichogin A IV

The molecular and crystal structures of N-benzyloxycarbonyl-alpha-aminoisobutyrylglycy-glycine tert-butyl ester (1) and N-benzyloxycarbonyl-L-leucyl-alpha-aminoisobutyryl-glycyl-glycyl-L-leucine methyl ester (2), terminally protected tri- and pentapeptides of trichogin A IV, have been determined by X-ray diffraction in their monohydrated form. The compounds possess the following parameters: (1) monoclinic, space group Cc, a = 21.386(2) Angstrom, b = 9.064(2) Angstrom, c = 11.845(2) Angstrom beta = 96.1(1)degrees, and Z = 4; (2) tetragonal, P4(3)2(1)2, a = b = 13.366(2) Angstrom, c = 38.642(2) Angstrom, and Z = 8. The structures were solved by direct methods. The least-squares refinements led to conventional R(1) factors of 0.052 and 0.065 for (1) and (2), respectively. The -Aib-Gly- sequence of tripeptide (1) is folded into a type-I (I') beta-bend conformation. In the pentapeptide (2) a water-mediated type-II beta-bend is followed by two consecutive, left-handed beta-bends (type-III'/type-I').