ISSN:
0006-3525
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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 Cα at the N-H side of the thioamide group. The appearance of a strong negative ππ* band near 270 nm is indicative of the semiextended conformation of this residue. The φ ∼ -70°, ψ ≥ 120° set of torsion angles is compatible with a type II βt-turn or a γt-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 βt- and γt-turns may be fixed by C=S⃛H-N(CO) intramolecular H bonds. The appearance of one or two weak nπ* bands and a positive ππ* band at about 270 nm is characteristic of type II βt-turns containing the H-bonded thioamide group attached to the glycine residue in position i + 2. The extended conformation (φ ∼ -140°, ψ ∼ 140°) of a residue after the thioamide group gives rise to a negative nπ* and a positive ππ* band of comparable magnitude. Peptid1e sequences with alternating thioamide-amide-thioamide backbone tend to adopt 1t ⇆ 4t H-bonded βtt conformations. CD studies show that type II βtt-turns have unique chiroptical properties: the ππ* region is dominated by an exceedingly strong negative band near 260 nm (|Δε| = 19-24) accompanied by a weaker band at higher wavelength values.
Additional Material:
4 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/bip.360301107
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