Structure and kinetics of the photoproduct of carbonylmyoglobin at low temperatures; an x-ray absorption study

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Abstract
Photolysis and recombination of [horse and sperm whale] carboxymyoglobin at low temperatures have been studied by a variety of methods. Optical and structural studies of carboxymyoglobin photolysis and recombination in the temperature range 4.degree.-120.degree. K were combined. The absorbance changes indicate ablation of the characteristic optical transitions of carboxymyoglobin and formation of a photoproduct (Mb*CO) differing from deoxymyoglobin. When the X-ray absorption changes in the 7150-7200-eV region of the X-ray absorption spectrum are used as an indicator of structural change, the photoproduct at 4.degree. K as measured with respect to the unphotolyzed sample is 60% of that observed for the chemically produced deoxy form. Saturation of the change is obtained with repetitive flashes totaling several thousand J of energy from a Xe flash lamp by using a thin sample (1 mm) at 4 mM concentration as measured by optical transmission and X-ray absorption criteria. The kinetics of the reaction show the change to occur at 10.degree. K within the resolving time currently available (2 s) in the X-ray absorption measurements. The amplitude of the light-induced change decreases to half its maximal value at 40.degree. K and to 0 at 90.degree. K. Steady illumination suggests at least 2 recombination processes. Analysis of the extended X-ray absorption fine structure (EXAFS) data on Mb*CO indicates small distance changes in the 1st shell of Fe-N and Fe-C that can be attributed to lengthening of the pyrrole nitrogen bonds and proximal histidine motion, together with a small displacement of the CO molecule on photolysis, a form here designated Mb*CO. This structure of the geminate state, Mb*CO, may elucidate the nature of elementary steps in chemical reactions and in tunneling processes.