Insights into heme structure from Soret excitation Raman spectroscopy

Laser lines in resonance with the Soret band optical transitions of several heme proteins and heme model compounds have been 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 iron spin and coordination geometry have been developed. The position of this band is also a function of the pattern of porphyrin pyrrole ring beta-carbon substitution, and therefore 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 three commonly occurring classes of heme compounds: (a) protoheme derivatives, (b) iron porphyrins in which all ring positions are saturated, and (c) heme alpha species. A polarized mode in the 1470-1510-cm-1 region is also 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 also responds to changes in porphyrin geometry. However, 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 [Abe, M., Kitagawa, T., & Kyogoku, Y. (1978) J Chem. Phys. 69, 4526-4534] is used to rationalize the dependence of the various modes on porphyrin geometry and peripheral substitution.