Volume and enthalpy profiles of CO binding to Fe(II) tetrakis-(4-sulfonatophenyl)porphyrin

The focus of this study is to examine volume and enthalpy profiles of ligand binding associated with CO-Fe(II) tetrakis-(4-sulfonato phenyl)-porphyrin (COFe(II)4SP) in aqueous solution. Temperature dependent photothermal beam deflection was employed to probe the overall enthalpy and volume changes associated with CO-photolysis and recombination. The analysis demonstrates that ligand recombination occurs with a pseudo first order rate constant of (2.5+/-0.2)x10(4) s(-1) (at 25 degrees C) with a corresponding volume decrease of 6+/-1 ml/mol. The activation enthalpy (DeltaH(double dagger)) and volume (DeltaV(double dagger)) change for CO recombination (determined from temperature/pressure dependent transient absorption spectroscopy) are found to be 3.9 kcal/mol and 8.2 ml/mol, respectively. These data are consistent with a mechanism in which photolysis yields a five-coordinate high spin (H(2)O)Fe(II)4SP complex that recombines in a single step to form the low spin (CO)(H(2)O)Fe(II)4SP complex. Base elimination, often associated with CO photolysis from hemes, is not observed in this system. The overall volume changes suggest a transition state with significant high spin character. Furthermore, these results demonstrate the utility of coupling photothermal techniques with variable pressure/temperature transient absorption spectroscopy to probe heme reaction dynamics.