Insights into heme structure from soret excitation Raman spectroscopy

Abstract

Laser lines in resonance with the Soret band optical transitions of several heme proteins and heme model compounds were used to obtain Raman spectra of these species. Correlations between the observed frequency of a polarized mode in the 1560-1600 cm-1 region and the heme Fe spin and coordination geometry were developed. The position of this band is also a function of the pattern of porphyrin pyrrole ring .beta.-carbon substitution, and structural information can be extracted from the Raman data only after this dependence has been taken into account. Quantitative correlations between the frequency of this band and the porphyrin core size are presented for 3 commonly occurring classes of heme compounds: protoheme derivatives, Fe porphyrins in which all ring positions are saturated and heme a species. A polarized mode in the 1470-1510 cm-1 region is consistently enhanced upon Soret excitation of these compounds, but is relatively insensitive to peripheral substituents and can be used in conjunction with the polarized mode described above to assign heme geometries. In the frequency region above 1600 cm-1, a vibration is observed which responds to changes in porphyrin geometry. This band is sometimes obscured by vibrations of unsaturated .beta.-carbon substituents, particularly in the case of protoheme derivatives. The normal coordinate analysis developed by Abe and co-workers is used to rationalize the dependence of the various modes on porphyrin geometry and peripheral substitution.