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Paper
Photochem. Photobiol. Sci., 2006, 5, 603 - 610, DOI: 10.1039/b516977a
Time-resolved thermodynamic profiles for CO photolsysis from the mixed valence form of bovine heart cytochrome c oxidase
Randy W. Larsen
Photoacoustic calorimetry has been utilized to probe the thermodynamics accompanying photodissociation of the CO mixed valence form of bovine heart cytochrome c oxidase (COMV CcO). At pH
s below 9 photolysis of the COMV CcO results in three kinetic phases with the first phase occurring faster than the time resolution of the instrument (i.e., < 
50 ns), a second phase occurring with a lifetime of 100 ns and a third phase occurring with a lifetime of 2 µs. The corresponding volume and enthalpy changes for these processes are: 
H1, V1 = +79 ± 10 kcal mol–1, +9 ± 1 mL mol–1; H2, V2 = –79 ± 5 kcal mol–1, –9 ± 2 mL mol–1; H3, V3 = +54 ± 7 kcal mol–1, +8 ± 1 mL mol–1. At pHs above 9 only one phase is observed, a prompt phase occurring in < 50 ns. The overall volume change is negligible above pH 9 and the enthalpy change is +29 ± 5 kcal mol–1. The data are consistent with the prompt phase being associated with CO–Fea3 bond cleavage, CO–CuB+ bond formation, Fea3 low-spin to high-spin transition and fast electron transfer (ET) from heme a3 to heme a followed by proton transfer from Glu242 to Arg38 on an 100 ns timescale. The slow phase is likely a combination of CO themal dissociation from CuB and additional ET between heme a3 to heme a. Interestingly, this phase is not evident above pH 9 suggesting linkage between CO dissociation/ET and the protonation state of a group or groups near the binuclear center.
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