High-resolution emission spectra of mesoporphyrin IX (MP) were determined under site-selective conditions for MP horseradish peroxidase (HRP) at pH 5.1 and 8.0, and for MP in diethyl ether (DEE) and polycrystalline n-octane in the 5-40 K temperature range. Spectra corresponding to MP tautomeric forms were identified, and the range of 0 ← 0 emissions was used for determining the site distribution functions. The distribution of 0 ← 0 energies was found to be narrow, of 60-70 cm-1 in HRP and n-octane, and resolvable for the tautomeric components. A downward shift in 0 ← 0 energies was found parallel with the strength of crystal field effect as HRP > octane > DEE. The photoconversion kinetics of MP-HRP at pH 5.1 was found to be non-single-exponential. The transition probability was temperature independent below 15-20 K; the temperature dependence found at higher temperatures was mainly accounted for by a ground-state, phonon-assisted phenomenon. Deactivation from the triplet state was studied by phosphorescence lifetime measurements. The decay was non-single-exponential, and the slight temperature dependence of the lifetime components was in accordance with the kinetic results for the tautomerization reaction. MP in HRP was found to be photochemically stable at pH 8.0; only one of the tautomeric forms is populated.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry