Effects of conformation and hydrogen bonding on the C[DOUBLE BOND]C and NCN stretching Raman bands of N-deuterated histidinium



Histidine is an important and versatile amino acid residue that plays a variety of structural and functional roles in proteins. Although the Raman bands of histidine are generally weak, histidine in the N-deuterated cationic form with imidazole Nπ[BOND]D and Nτ[BOND]D bonds (N-deuterated histidinium) gives two strong Raman bands assignable to the C4[DOUBLE BOND]C5 stretch (νC[DOUBLE BOND]C) and the Nπ[BOND]C2[BOND]Nτ symmetric stretch (νNCN) of the imidazole ring. We examined the Raman spectra of N-deuterated histidinium in 12 crystals with known structures. The observed νC[DOUBLE BOND]C and νNCN wavenumbers were analyzed to find empirical correlations with the conformation and hydrogen bonding. The effect of conformation on the vibrational wavenumber was expressed as a threefold cosine function of the Cα[BOND]Cβ[BOND]C4[DOUBLE BOND]C5 torsional angle. The effect of hydrogen bonding at Nπ or Nτ was assumed to be proportional to the inverse sixth power of the distance between the hydrogen and acceptor atoms. Multiple linear regression analysis clearly shows that the conformational effect on the vibrational wavenumber is comparable for νC[DOUBLE BOND]C and νNCN. The hydrogen bond at Nπ weakly lowers the νC[DOUBLE BOND]C wavenumber and substantially raises the νNCN wavenumber. On the other hand, the hydrogen bond at Nτ strongly raises the νC[DOUBLE BOND]C wavenumber but does not affect the νNCN wavenumber. These empirical correlations may be useful in Raman spectral analysis of the conformation and hydrogen bonding states of histidine residues in proteins. Copyright © 2010 John Wiley & Sons, Ltd.