Modest enhancement is observed for Raman modes of the bound imidazole (ImH) in [(ImH)Fe(CN)5]2- upon laser excitation in a broad, weak absorption band at ~480 nm, which is assignable, on the basis of excitation profiles, to overlapping charge-transfer transitions from the two highest ImH π Orbitals to the FeIII dπ vacancy. Similar enhancement is seen for Im- modes in [(Im)Fe(CN)5]3- upon excitation in a weak band at ~650 nm and a strong band at ~440 nm, which are suggested to be the two π CT transitions, the π orbitals being split by the bonding alterations accompanying deprotonation of the bound ImH. The Fe-imidazole stretching vibration is weak or absent in these spectra, consistent with the nonbonding character of the Fe terminal orbital. Detection of ring modes of ImH or Im- bound to low-spin FeIII in heme proteins may require a concentration of ~25 mM if similar CT transitions can be located and excited. Detection limits might be lower if the CT dipole strengths are increased in the proteins via favorable orientations of the imidazole rings.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry