Raman Marker Bands of Metal Coordination Sites of Histidine Side Chains in Peptides and Proteins

Abstract

Raman spectra were measured for metal complexes of amino acid histidine and histidine-containing peptides. The C4=C5 stretching band of the histidyl imidazole ring appears in the 1606-1594 cm-1 region when the Nτ atom of the imidazole ring acts as a ligand of the metal coordination and the Nπ atom is protonated, whereas it appears in a lower wavenumber region, 1588-1573 cm-1, when the metal coordination takes place via Nπ with Nτ being protonated. These wavenumbers are generally higher than those of the corresponding Nπ- and Nτ-protonated tautomeric forms of metal-free histidine. Upon N-deuteration, the C4=C5 stretching mode shows downshifts of about 25 and 10 cm-1 for Nτ- and Nπ-metal complexes, respectively. Raman bands at ca. 1430 and ca. 1275 cm-1 (ca. 1350 and ca. 1265 cm-1 for N-deuterated histidine) gain intensity upon binding of metal to Nτ and Nπ, respectively, and they may also serve as markers of the metal coordination site. In the Raman spectrum of a 27mer zinc finger peptide that contains two histidine residues, the C4=C5 stretching band is identified at ca. 1606 cm-1 in H2O solution and an intense band is observed at 1342 cm-1 in D2O solution. These observations are consistent with the zinc coordination by the histidyl Nτ atom in the zinc finger. The histidyl Raman marker bands of metal coordination found here may be useful in analyzing metal-histidine interactions in peptides and proteins. © 1998 John Wiley & Sons, Ltd.