Photodissociation of cytochrome oxidase-nitric oxide at low temperatures.

Spectrophotometric studies revealed the irreversible photodissociation of reduced cytochrome oxidase-nitric oxide (NO) at 5 K. The dissociated NO recombined as the sample temperature was raised, and the half-recombination temperature was 65 K. The photodissociation at 5K was also confirmed by a change in the EPR spectrum; that is, ferroheme a-NO signals at gx=2.09 and gm=2.006 were replaced by a new signal at gm=2.03, and this change was reversed at the temperature of liquid nitrogen. Comparison of such behavior with that of cytochrome oxidase-carbon monoxide led us to propose that on photodissociation of NO from heme iron, the NO was trapped specifically at a site near the heme iron producing a new paramagnetic species. Its identification will require further studies.