MOLECULAR AND CRYSTAL-STRUCTURES OF 2 TERMINALLY-BLOCKED AC(5)C HOMO-OLIGOPEPTIDES

The molecular and crystal structures of the terminally blocked homo-tr i- and homo-pentapeptides from 1-amino-1-cyclopentane-carboxylic acid, Z-(Ac(5)c)(3)-OtBu (1) and Z-(Ac(5)c)(5)-OtBu (2), were determined by X-ray diffraction. The two compounds exhibit the following parameters : (1) orthorhombic, P2(1)2(1)2(1), a = 11.539(7) Angstrom, b = 16.185( 2) Angstrom, c = 30.925(4) Angstrom and Z = 8; (2) triclinic, <P(1)ove r bar>, a = 10.835(5) Angstrom, b = 11.631(9) Angstrom, c = 18.212(5) Angstrom, a = 92.20(7)degrees, beta = 94.79(10)degrees, gamma = 107.6( 3)degrees and Z = 2. The crystal structures were solved by direct meth ods. The least-squares refinements fed to R values of 0.080 and 0.060 for 4061 and 4018 reflections with I greater than or equal to 2 sigma( I) (according to the SHELXL-93 computer program weighting scheme) for (1) and I greater than or equal to 3 sigma(I) (Structure Determination Programs, SDP crystallographic package weighting scheme) for (2), res pectively. The two independent molecules, A and B, in the asymmetric u nit of the tripeptide (1) are folded into a type-I/III and a type-I' b eta-bend conformation, respectively, each stabilized by an intramolecu lar C=O...H-N H-bond. The pentapeptide molecules adopt a 3(10)-helical structure, stabilized by three consecutive, intramolecularly H-bonded type-III (III') beta-bend conformations.