The photolysis of carbonyl horseradish peroxidase studied by laser-induced optoacoustic spectroscopy

Abstract

Several spectroscopic methods — both time-resolved and steady-state — have been applied to the study of the CO adducts of heme proteins [1–3]. The never-ceasing interest for this topic is due to the usefulness of CO as a probe for the heme protein active site. We have studied the photolysis of CO from a prototypical heme protein — horseradish peroxidase isozyme C (HRPC) — by laser-induced optoacoustic spectroscopy (LIOAS). This technique allows to measure both enthalpy and volume changes in photochemical reactions and to resolve them in a time window of 10-8 — 10-5 s [4]. Structural volume changes △V R , which originate in the molecular rearrangement following photoexcitation, can be separated from thermal volume changes by varying the thermoelastic parameter ratio β / (c P ρ) of the solvent and measuring the signals of the sample, H s , and of a calorimetric reference, H R , under identical conditions. Then the relation holds: where α is the energy released by the sample as “prompt” heat, E λ the photon energy, cp the heat capacity at constant pressure, ρ the mass density, and β the thermal expansion coefficient. Since the thermoelastic parameters of aqueous solutions strongly vary with temperature, △V R can be determined by temperature-dependent LIOAS