This paper reports the chiroptical properties of thionated N-acyl amino acid and N-acyl
dipeptide N'-methylamide models. It was found that the optical activity of the thioamide
chromophore is dominated by the chiral contribution of perturbants attached to C-alpha
at the N-H side of the thioamide group. The appearance of a strong negative pi-pi*
band near 270 nm is indicative of the semiextended conformation of this residue. The
phi approximately -70-degrees, psi greater-than-or-equal-to 120-degrees set of torsion
angles is compatible with a type II beta-t-turn or a gamma-t-turn conformation with
the perturbing N-H side residue in the i + 1 position of the turn. (The subscript
t or tt denotes that one or both of the H-bonded moieties is thioamide.) Earlier data
show that both beta-t- and gamma-t-turns may be fixed by C = S ... H-N(CO) intramolecular
H bonds. The appearance of one or two weak n-pi* bands and a positive pi-pi* band
at about 270 nm is characteristic of type II beta-t-turns containing the H-bonded
thioamide group attached to the glycine residue in position i + 2. The extended conformation
(phi approximately -140-degrees, psi approximately 140-degrees) of a residue after
the thioamide group gives rise to a negative n-pi* and a positive pi-pi* band of comparable
magnitude. Peptide sequences with alternating thioamide-amide-thioamide backbone tend
to adopt 1t <-- 4t H-bonded beta-tt conformations. CD studies show that type II beta-tt-turns
have unique chiroptical properties: the pi-pi* region is dominated by an exceedingly
strong negative band near 260 nm (DELTA-epsilon = 19-24) accompanied by a weaker band
at higher wavelength values.
