Raman spectroscopic study of poly (β‐benzyl‐L‐aspartate) and sequential polypeptides

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Abstract
Poly‐β‐benzyl‐L‐aspartate (poly[Asp(OBzl)]) forms either a lefthanded α‐helix, β‐sheet, ω‐helix, or random coil under appropriate conditions. In this paper the Raman spectra of the above poly[Asp(OBzl)] conformations are compared. The Raman active amide I line shifts from 1663 cm−1 to 1679 cm−1 upon thermal conversion of poly[Asp(OBzl)] from the α‐helical to β‐sheet conformation while an intense line appearing at 890 cm−1 in the spectrum of the α‐helix decreases in intensity. The 890 cm−1 line also displays weak intensity when the polymer is dissolved in chloroform–dichloroacetic acid solution and therefore is converted to the random coil. This line probably arises from a skeletal vibration and is expected to be conformationally sensitive. Similar behavior in the intensity of skeletal vibrations is discussed for other polypeptides undergoing conformational transitions.The Raman spectra of two cross‐β‐sheet copolypeptides, poly(Ala‐Gly) and poly(Ser‐Gly), are examined. These sequential polypeptides are model compounds for the crystalline regions of Bombyx mori silk fibroin which forms an extensive β‐sheet structure. The amide I, III, and skeletal vibrations appeared in the Raman spectra of these polypeptides at the frequencies and intensities associated with β‐sheet homopolypeptides. Since the sequential copolypeptides are intermediate in complexity between the homopolypeptides and the proteins, these results indicate that Raman structure–frequency correlations obtained from homopolypeptide studies can now be applied to protein spectra with greater confidence.The perturbation scheme developed by Krimm and Abe for explaining the frequency splitting of the amide I vibrations in β‐sheet polyglycine is applied to poly(L‐valine), poly‐(Ala‐Gly), poly(Ser‐Gly), and poly[Asp(OBzl)]. The value of the “unperturbed” frequency, V0, for poly[Asp(OBzl)] was significantly greater than the corresponding values for the other polypeptides. A structural origin for this difference may be displacement of adjacent hydrogen‐bonded chains relative to the standard β‐sheet conformation.