A. Santini et al., MOLECULAR AND CRYSTAL-STRUCTURES OF 2 TERMINALLY-BLOCKED AC(5)C HOMO-OLIGOPEPTIDES, Zeitschrift fur Kristallographie, 211(9), 1996, pp. 616-621
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.