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 ligan d 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 a ssociated with CO-photolysis and recombination. The analysis demonstrates t hat ligand recombination occurs with a pseudo first order rate constant of (2.5 +/-0.2)X10(4) s(-1) (at 25 degreesC) with a corresponding volume decre ase of 6 +/-1 ml/mol. The activation enthalpy (DeltaH(double dagger)) and v olume (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 2O)Fe(II)4SP complex that recombines in a single step to form the low spin (CO)(H2O)Fe(II)4SP complex. Base elimination, often associated with CO phot olysis from hemes, is not observed in this system. The overall volume chang es suggest a transition state with significant high spin character. Further more, these results demonstrate the utility of coupling photothermal techni ques with variable pressure/temperature transient absorption spectroscopy t o probe heme reaction dynamics. (C) 2001 Elsevier Science B.V. All rights r eserved.