Ligand photolysis and recombination of Fe(II)protoporphyrin IX complexes in tetramethylene sulfoxide

In this report the steady-state optical absorption and ligand rebinding kin etics of Fe(II)protoporphyrin IX (Fe(II)PPIX) and (CO)Fe(IT)PPIX in tetrame thylene sulfoxide (TMSO) are examined. The equilibrium optical absorption s pectrum of Fe(II)PPIX in neat TMSO is characteristic of heme iron that is s ix-coordinate and low-spin suggesting the formation of a (TMSO)(2)Fe(II)PPI X complex. Absorption maxima are observed at 425 nm (Soret), 557 nm (alpha -band), and 529 nm (beta -band). Addition of CO to a solution of (TMSO)(2)F e(II)PPIX in neat TMSO results in a complex with absorption maxima at 415 n m (Soret), 566 nm (alpha -band), and 535 nm (beta -band) consistent with th e formation of a (CO)(TMSO)Fe(II)PPIX complex. The transient absorption dif ference spectrum subsequent to photolysis of the (TMSO)(2)Fe(II)PPIX comple x displays an absorption maximum at 433 nm and a minimum at 423 nm that dec ays monophasically with a rate constant of (1.47 +/- 0.02) x 10(6) s(-1) co nsistent with the formation of a five-coordinate and high-spin transient. T he corresponding transient absorption data obtained at 434 nm for the (CO)( TMSO)Fe(II)PPIX complex displays biphasic kinetics with a fast phase rate c onstant that is identical to the one obtained for photolysis of the (TMSO)( 2)Fe(II)PPIX complex and a slower phase with a rate constant of (1.91 +/- 0 .01) x 10(3) s(-1). The data suggest that the decay of this transient speci es involves rebinding of a TMSO to the heme iron followed by substitution o f a bound TMSO with CO. Using these results the equilibrium constant for TM SO binding to the five coordinate (TMSO)Fe(II)PPIX complex is found to be 2 .6 x 10(3) M-1. This value is roughly a factor of ten smaller than that of the DMSO complex. Molecular modeling studies suggest that geometrical disto rtions of the TMSO upon binding to the heme are responsible for the lower b inding affinity. (C) 2001 Elsevier Science B.V. All rights reserved.