Michael Ramek,
Ching-Hsing Yu, Joshua Sakon, and Lothar Schäfer,
|  |
, of the central
peptide group in N-formyl-L-alanyl-L-alanine amide (ALA-ALA) was
investigated using
a database of 11 664 RHF/4-21G ab initio
gradient optimized structures. The data base was generated by optimizing the
geometries of ALA-ALA at grid points in its four-dimensional
(
,
,
,
) conformational
space defined by 40° increments along the outer torsions
and
and by 30° increments
along the inner torsions
and
.
Using cubic spline functions, the grid structures were then used to
construct analytical representations of complete surfaces of in
(,
,
,
)-space.
Analyses of the conformational surfaces of reveal that the
peptide N-C torsion is a smoothly varying function of associated
and angles and that, for many conformational regions,
deviations from planarity are the rule rather than the exception.
Comparisons with protein crystallographic data show that, in contrast
to peptide torsional angles calculated for an entire protein, the angles of smaller model
peptides, such as ALA-ALA, cannot be used to model peptide groups
in proteins, because of long-range effects present in the latter
but not in the former. This finding indicates the general difficulty
of predicting the exact positions of the backbone torsional angles in
proteins from smaller model peptides. Furthermore, the results confirm
the directional nature of polypeptide chains. That is, conformation
transmission effects from neighboring groups differ, depending on
whether they are transmitted from right to left or from left to right
in the peptide chain.