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Time-resolved thermodynamic profiles for CO photolsysis from the mixed valence form of bovine heart cytochrome c oxidase.

Larsen RW

Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue, Tampa, FL 33620, USA.

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., < approximately 50 ns), a second phase occurring with a lifetime of approximately 100 ns and a third phase occurring with a lifetime of approximately 2 micros. The corresponding volume and enthalpy changes for these processes are: DeltaH(1), DeltaV(1) = +79 +/- 10 kcal mol(-1), +9 +/- 1 mL mol(-1); DeltaH(2), DeltaV(2) = -79 +/- 5 kcal mol(-1), -9 +/- 2 mL mol(-1); DeltaH(3), DeltaV(3) = +54 +/- 7 kcal mol(-1), +8 +/- 1 mL mol(-1). At pH's 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-Fe(a3) bond cleavage, CO-Cu(B)(+) bond formation, Fe(a3) low-spin to high-spin transition and fast electron transfer (ET) from heme a(3) to heme a followed by proton transfer from Glu242 to Arg38 on an approximately 100 ns timescale. The slow phase is likely a combination of CO thermal dissociation from Cu(B) and additional ET between heme a(3) 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.

Published 8 June 2006 in Photochem Photobiol Sci, 5(6): 603-10.
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