Volume and thermodynamic profiles of CO-binding to Fe(II) protoporphyrin IX in detergent micelles

Abstract

The focus of this study is to examine volume and thermodynamic profiles of ligand binding associated with CO–Fe(II) protoporphyrin IX (COFe(II)PPIX) encapsulated in cetyltrimethylammonium bromide (CTAB) micelles. Temperature-dependent photoacoustic calorimetry was employed to probe the thermodynamics and volume changes associated with CO-photolysis. The results of the acoustic analysis reveal an enthalpy associated with this process of −10.4±1.1 kcal mol −1. In addition, the analysis demonstrates that ligand photodissociation results in a volume increase of 3.1±0.3 ml mol −1 that occurs on a time scale <100 ns. The corresponding ligand recombination occurs with a pseudo-first order rate constant of (5.1±0.3)×10 4 s −1 (at 23°C). The enthalpy of activation (Δ H +) and activation volume (Δ V +) for CO recombination (determined from temperature/pressure dependent transient absorption spectroscopy) are found to be 18±1.9 kcal mol −1 and +3.7±0.7 ml mol −1, respectively. The volume changes are consistent with a high-spin heme within a ligand–porphyrin contact pair within the first 50 ns of photolysis. No additional changes are observed between <50 ns and ∼5 μs.