Vibrational CD of polypeptides. XII. Reevaluation of the fourier transform vibrational CD of poly(γ‐benzyl‐L‐glutamate)

Abstract

SynopsisUsing a newly constructed Fourier transform ir (FTIR) based vibrational CD (VCD) instrument, we have found that elimination of the ellipsoidal collection mirror before the detector and its replacement by a lens leads to a significant improvement in the absorption artifact problem often seen previously in FTIR-VCD. Reduction in artifact level brings the FTIR-VCD band shape of the amide 1 band in poly( y-benzyl-L-glutamate) into better agreement with that found with dispersive instruments. These results indicate that the difference between the spectra obtained with the two methods is not primarily a factor of resolution but is more dependent on artifact level. Thus the previously reported deconvolution of the amide I VCD results and their subsequent interpretation in terms of the helical vs. exciton mechanisms overemphasize weak features in the spectrum. Results based on deconvolution of the absorption spectrum remain valid. Further new data on the amide I1 and several lower energy transitions that encompass the amide I11 region show broad single-signed features-but of the opposite sense-in the two regions, implying that their VCD arises through mutual coupling.

Discriminating 310‐ from α‐helices: Vibrational and electronic CD and IR absorption study of related Aib‐containing oligopeptides

“…The 3 10 ‐helical amide II VCD is further relatively sharp and correlated with its corresponding IR absorption maximum. For the α‐helix, the amide I is always dominant (with its negative component alone having roughly twice the amide II intensity) and the amide II is broad and shifted down from its IR absorption maximum 36, 40, 54, 56, 57, 88, 89. The 3 10 ‐helical conformation appears to be the dominant contributor for moderate peptide main‐chain lengths (hepta‐ to decapeptides) of each series, i.e., (UA) 4 , (UA) 5 for the (UA) n series, and A(UA) 3 , A(UA) 4 for the A(UA) n series.…”

“…The 3 10 ‐helical amide A, I, and II bands studied to date have VCD sign patterns similar to those of α‐helices, with a negative couplet for the amide A, a positive couplet for the amide I, and a single negative band for the amide II 12, 28, 29, 36, 39, 40, 54–57. The relative intensities of the amide I and II bands are opposite for these related helical types, as illustrated in Figure 1 using data for a 3 10 ‐helical Aib/Leu octapeptide28 and an α‐helical [Met 2 Leu] 18‐residue oligopeptide,57 both in CDCl 3 solution.…”

Transfer of molecular property tensors in cartesian coordinates: A new algorithm for simulation of vibrational spectra

“…In these computations, the net intensity is positive as seen in some model helices. 36 Thus, we see that fundamental and transferred calculations both give the type of spectral interpretation we desire for a Ž tripeptide assuming it maps onto a larger peptide . of defined structure , but that, in the fine details, the transfer has an effect on the FF that causes a change in the detailed mode character.…”

Simulations of oligopeptide vibrational CD: Effects of isotopic labeling

“…We have promoted the use of vibrational CD (VCD) as an improved sensitivity diagnostic for secondary structure type because it is a naturally differential technique where analysis is dependent on band shape and has only a secondary dependence on frequency shifts of the component bands 7, 16–19. Examples are known where a common secondary structure type, such as an α‐helix, gives rise to a conserved VCD bandshape but one shifted to different frequencies in different peptides 12, 20–22. In analogy to band shape based ir studies23–26 and to well‐established electronic CD studies,5, 6, 27–30 we have in the past developed empirical correlations of amide I and II31–33 and even amide III34 VCD band shapes with secondary structure.…”