Dynamics of structure and energy of horse carboxymyoglobin after photodissociation of carbon monoxide.

The study explores the changes in structure and energy of horse carboxymyoglobin following the photodissociation of carbon monoxide. The researchers used various techniques to study the energetic and volume alterations following this process, and uncovered new kinetic information not previously observed.

Research Methodology

• The researchers used laser-induced transient grating (TG) and photoacoustic calorimetric techniques to investigate the energetic and structural volume changes post carboxymyoglobin photodissociation.
• The analysis helped distinguish between phase grating signals attributable to temperature changes, absorption spectrum changes, and volume changes.

Key Findings

• The researchers identified new kinetics of about 700 ns at room temperature, not previously observed by conventional flash photolysis techniques. This evidence suggests the existence of an intermediate between the geminate pair (molecular entities formed together) and the complete dissociation state.
• The study estimates the enthalpy of the intermediate species to be 61 +/- 10 kJ/mol, a significantly smaller value than the expected iron-carbon monoxide bond energy.
• During this process, the volume of myoglobin-carbon monoxide compound (MbCO) slightly contracts (5 +/- 3 cm(3)/mol).
• Carbon monoxide (CO) is entirely released from the protein through an exponential kinetics from 25 to -2 degrees Celsius. There is a corresponding volume expansion by 14.7 +/- 2 cm(3)/mol, and the release of 14 +/- 10 kJ/mol of energy, believed to represent protein relaxation and CO solvation (interaction of CO with its surrounding environment).
• The final state enthalpy is calculated to be 47 +/- 10 kJ/mol.
• The study found potential barriers between the identified intermediate and the fully dissociated state, indexed with DeltaH(*) = 41.3 kJ/mol and DeltaS(*) = 13.6 J mol(-1) K(-1).
• High wavenumber TG experiments showed that volume expansion rates fluctuate with temperature from 25 to -2 degrees Celsius.
• The discussion also focuses on changes in volume and energy levels of the intermediate species.