Photoacoustic calorimetry studies of CO photo-dissociation from chloramine-T modified horse heart cytochrome-c.

This research explores the kinetics, enthalpy and molar volume changes resulting from the photo-dissociation of CO from a complex, involving horse heart Cytochrome-c and Chloramine-T, using photoacoustic calorimetry. The study discovers two additional kinetic phases not previously identified in this context.

Study Overview and Methodology

• The study begins with chemically treating horse heart Cytochrome-c (Cc) with N-chloro-4-toluosulfonamide (also known as Chloramine-T or CT). This treatment results in the oxidation of methionine residues, including a specific one known as Met80, to form a related compound called sulfoxide.
• Upon formation, the sulfoxide coordinates with an iron atom (Fe) and forms a relatively labile bond. This weak bond is easily replaced or displaced by other units like carbon monoxide (CO).
• A complex of CO-CT-Cc is then subjected to photolysis, a process that uses light to induce chemical reactions. This sets the stage to study the dynamics of ligand binding and their relevance to a distal heme pocket, a specific section of the protein structure.

The Use of Photoacoustic Calorimetry (PAC)

• The method of examination used in this research is Photoacoustic Calorimetry (PAC). PAC is a technique valuable for investigating a wide range of chemical reactions and physical transformations, allowing scientists to measure the heat changes involved in chemical reactions.
• Key parameters like kinetics, enthalpy (A measure of heat energy) and molar volume changes were obtained using PAC after CO photo-dissociation from the CO-CT-Cc complex.

Discovering New Kinetic Phases

• The CO-CT-Cc complex presents a model for ligand recombination, where the Met80-sulfoxide and CO re-unite after photolysis. The recombination displays slow timescales (50 μs and ∼500 μs, respectively).
• Notably, the PAC data in this study reveals two additional kinetic phases with lifetimes of <20 ns and 534 ± 75 ns that were previously undocumented in the context of CO-CT-Cc reactions.
• The faster phase (<20 ns) demonstrates an enthalpy change (ΔH) of 44 ± 5 kcal mol and molar volume change (ΔV) of −0.5 ± 0.5 mL mol. The slower phase (534 ns) is associated with a smaller ΔH of 2 ± 3 kcal mol and ΔV of 1 ± 0.5 mL mol.