Wm. Wolf et al., DESTABILIZATION OF THE 3(10)-HELIX IN PEPTIDES BASED ON C-ALPHA-TETRASUBSTITUTED ALPHA-AMINO-ACIDS BY MAIN-CHAIN TO SIDE-CHAIN HYDROGEN-BONDS, Journal of the American Chemical Society, 120(45), 1998, pp. 11558-11566
The homooligopeptide series based on O,O-isopropylidene-alpha-hydroxym
ethylserine from dimer through pentamer has been synthesized to examin
e the conformational preferences of this new C-alpha-tetrasubstituted
cr-amino acid characterized by concomitant C-i(alpha <---->) C-i(alpha
) cyclization and presence of two ether oxygen atoms in the gamma-posi
tions of the six-membered ring 1,3-dioxane system. To this aim we have
exploited X-ray diffraction in the crystal state and FTIR absorption
and H-1 NMR techniques in solution; The results obtained are compared
with those of the homooligopeptides based on the related cyclohexane-c
ontaining C-alpha-tetrasubstituted residue. We conclude that in the fo
rmer peptides a competition takes place between the classical intramol
ecular (peptide) C=O...H-N (peptide) H-bonds, stabilizing the beta-ben
d/3(10)-helical structures, and the newly discovered (peptide) Ni+1l-H
...O(i)gamma (side-chain ether) intramolecular H-bonds. The extent of
regular (incipient) 3(10)-helix formation, where this latter type of I
I-bond is absent, tends to increase as peptide main-chain length incre
ases. As a result of this intramolecular N-H...O-gamma interaction, th
e critical main-chain length for 3(10)-helix formation in the crystal
state shifts from the shortest possible oligomer, the terminally prote
cted trimer, in the cyclohexane series to the pentamer in the 1,3-diox
ane series. Interestingly, a Strict correlation has been found between
the observed (peptide) Ni+1-H...O-i(gamma) (side-chain ether) intramo
lecular H-bond and (i) the backbone psi torsion angle of the i residue
(extended), and (ii) the disposition of the a-amino substituent in th
e 1,3-dioxane ring of the i + 1 residue (axial).