Sub-nanosecond photolysis studies of Fe2+ protoporphyrin IX solubilized in neat dimethyl sulfoxide

The present study examines the optical properties of the sub-nanosecond photolytic transient of Fe(II)protoporphyrin IX (Fe(II)PPIX) in neat dimethyl sulfoxide (DMSO). Previous nanosecond studies have revealed that photolysis of the (DMSO)(2)Fe(II)PPIX complex in neat DMSO results in the formation of a five-coordinate high-spin (DMSO)Fe(II)PPIX complex within similar to 100 ns which decayed with a pseudo-first order rate constant of 2 x 10(6) s (1) (Larsen et al. (1995) [19]). The results presented here demonstrate that the five-coordinate (DMSO) Fe(II) PPIX species is generated in <100 ps and that no significant changes occur in the kinetic difference between 100 ps and similar to 100 ns. The 100 ps transient spectrum of the (DMSO)Fe(II)PPIX complex was also constructed from the kinetic difference spectrum and the equilibrium spectrum of the (DMSO)(2)Fe(II)PPIX. The 100 ps transient spectrum exhibits a Soret maximum at similar to 432 nm close to that of deoxyMb (435 nm, imidazole coordination) consistent with S-bonded DMSO occupying the fifth coordination site. Neither base elimination is detected on time scales down to 100 ps nor is there evidence for transient O-bonded DMSO followed by linkage isomerization to the equilibrium S-bonded form. The unusually slow rate of DMSO recombination is attributed to electrostatic interactions between DMSO and the five-coordinate heme iron as well as intermolecular interactions between solvent molecules in the bulk, as has been previously proposed. (C) 2011 Elsevier B.V. All rights reserved.