Myoglobin recombination at low temperature. Two phases revealed by Fourier transform infrared spectroscopy

The low-temperature recombination of CO with myoglobin was studied using Fourier transform infrared spectroscopy. The bound state of carbon monoxide myoglobin has two distinct conformers observed at 1926 and 1945 cm-1 with an intensity ratio of 1 to 8. The recombination of these bands after complete photolysis at 10 K followed by a temperature jump shows distinct kinetics for the two bands. Although both bands apparently follow the nonexponential kinetics originally described by Frauenfelder and co-workers (Austin, R., Beeson, K., Eisenstein, L., Frauenfelder, H., and Gunsalus, I. (1975) Biochemistry 14, 5355-5373), the 1926 cm-1 band does not appear appreciably below 70 K. In fact, after 20 min of recombination at 70 K the 1945 cm-1 band is fully recovered, while no detectable amount of the 1926 cm-1 band is present. This is the first association of a spectroscopic marker of protein substructure with reaction kinetics.