Raman spectra of polypeptides containing <scp>L</scp>‐histidine residues and tautomerism of imidazole side chain

Abstract

Abstract Raman spectra were measured for poly( L ‐histidine) in H 2 O, poly( L ‐histidine‐ d 2 and ‐ d 3 ) in D 2 O, L ‐histidine in H 2 O, L ‐histidine‐ d 3 (and d 4 ) in D 2 O, and 4‐methylimidazole in H 2 O with various pH (or pD) values. The Raman scattering peaks observed for these samples were ascribed to the neutral and positively charged imidazole groups on the basis of the spectral changes due to the pH variation and to the deuterium substitution of the imino protons. The vibrational modes of these peaks were deduced from the normal coordinate analysis made on the positively charged and neutral 4‐ethylimidazoles. The Raman scattering peaks from the imidazole groups in the neutral form clearly indicate that these imidazole groups exist in the equilibrium between the two tautomeric forms, the 1‐N protonated from (tautomer I) and the 3‐N protonated one (tautomer II). For example, the breathing vibration of the 1‐N protonated form is observed at 1282 cm −1 for L ‐histidine and at 1304 cm −1 for 4‐methylimidazole, while the breathing vibration of the 3‐N protonated form is observed at 1260 cm −1 for L ‐histidine and 4‐methylimidazole. From the temperature dependence of the relative intensities of the tautomer I peak to that of the tautomer II, it was concluded that the tautomer I is energetically more stable than the tautomer II, and the Δ H value is 1.0 ± 0.3 kcal/mol for L ‐histidine and 0.4 ± 0.1 kcal/mol for 4‐methylimidazole. Poly( L ‐histidine) with the neutral imidazole side chains shows the amide I peak at 1672 cm −1 , indicating that the sample assumes the antiparallel pleated‐sheet structure. Poly( L ‐Ala 75 L ‐His 25 ) and poly( L ‐Ala 50 L ‐His 50 ) were found to take the α‐helical and β‐form conformations, respectively.