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Vibrational circular dichroism in amino acids and peptides. 7. Amide stretching vibrations in polypeptides
Author(s) -
Lal Brij B.,
Nafie Laurence A.
Publication year - 1982
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360211106
Subject(s) - amide , chemistry , vibrational circular dichroism , circular dichroism , crystallography , spectral line , stereochemistry , helix (gastropod) , organic chemistry , physics , ecology , astronomy , snail , biology
Vibrational circular dichroism (VCD) spectra for the principal amide stretching vibrations, amide A (NH stretch) and amide I (predominantly CO stretch), are presented and analyzed for a variety of polypeptides dissolved in chloroform, as well as for two examples in D 2 O. Our results for poly(γ‐benzyl‐ L ‐glutamate) confirm the first and only previous report of VCD in polypeptides carried out by Singh and Keiderling [(1981) Biopolymers 20 , 237–240]. Collectively, our spectra show that the sense of the bisignate VCD in these two regions depends on the sense of α‐helicity and not on the absolute configuration of the constituent amino acids. This conclusion is established by obtaining VCD for the two polypeptides, poly(β‐benzyl‐ L ‐asparate) and poly(im‐benzyl‐ L ‐histidine), that form left‐handed as opposed to right‐handed α‐helices. A new amide band having significant VCD intensity owing to its Fermi resonance interaction with the NH stretching mode has been identified as a weak shoulder on the low‐frequency side of the amide A band near 3200 cm −1 and is assigned as a combination band of the amide I and amide II vibrations. VCD spectra of polypeptides in D 2 O solution, although weak, have been successfully measured in the amide I region, where spectra appear to be more complicated due to the presence of solvated and internally hydrogen‐bonded amide groups. Strong monosignate contributions to the VCD in the amide A and amide I regions for some of the polypeptides indicate coupling of an electronic nature between these two regions and is deduced by an application of the concept of local sum rules of rotational strength. It appears that a detailed understanding of the VCD obtained for polypeptides will not only be diagnostic of secondary structure, but also of more subtle structural and vibrational effects that give rise to local, intrinsic chirality in the amide vibrations.