Dependence of NO recombination dynamics in horse myoglobin on solution glycerol content

The recombination dynamics of NO with horse heart myoglobin (Mb) following photolysis with a 570 nm excitation pulse were measured by time-resolved ab sorption with 250 fs temporal resolution. These measurements were carried o ut in room-temperature solutions in which the glycerol concentration was va ried from 0 to 90% (w/v). The recombination of NO is nonsingle exponential in all cases, but becomes faster as the glycerol concentration is increased . The interpretation of these results is aided by a maximum entropy analysi s to determine a distribution of rate processes consistent with the data. T his analysis suggests that in buffer there are two dominant rate processes for NO recombination on the subnanosecond time scale, one at approximate to 10 ps and one at approximate to 200 ps. The dominant effect of increasing glycerol content is to increase the amplitude of the fast process, with no corresponding significant change in rate, and to decrease the amplitude of the slow process, but with a corresponding increase in rate. These results are consistent with a photodissociation process in which photolyzed NO part itions immediately between two distinct populations, one of which has a rap id and the other of which has a slower recombination. The rate of the rapid recombination is independent of glycerol concentration, and therefore deco upled from any protein relaxation process influenced by glycerol, while tha t of the slower recombination increases at higher glycerol concentration. F urther, the partitioning between these two populations also depends on glyc erol content, with the relative amplitude of the faster recombination proce ss increasing as the glycerol content is increased. Interpretation of these observations in terms of ligand trajectories following photodissociation, and possible connections of these results with both infrared and crystallog raphic studies of photodissociated MbCO are discussed.