The electron exchange kinetics of horse heart cytochrome c (Cyt c) at 4,4′-bipyridyl-and 4,4′-bipyridyldisulfide-modified Au electrodes has been studied for the first time under variable pressure conditions (up to 150 MPa), by using fast scanning cyclic voltammetry. A positive activation volume of +6.1 ( 0.5 cm 3 mol -1 was determined from the pressure dependence of the heterogeneous standard rate constant in both cases. This value is similar to that for the homogeneous Cyt c self-exchange process, predicted from the cross-reaction treatment. A careful analysis based on an extended version of the contemporary charge-transfer theory indicates that the process most probably takes place through an adiabatic ("protein friction") charge-transfer mechanism in which the positive volume of activation results from the pressure-induced increase of the protein's intrinsic viscosity (decrease of the characteristic relaxation mobility), which is also in remarkable agreement with earlier results from studies in which the viscosity was varied directly. This approach allows for variation of the internal protein viscosity without significant alteration of the properties (viscosity, diffusion coefficients) of the aqueous medium.

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the Cl( 2 P J ) + C 2 Cl 4 association reaction as a function of temperature (231-390 K) and pressure (3-700 Torr) in nitrogen buffer gas. The reaction is found to be in the falloff regime between third and second order over the range of conditions investigated, although the second-order limit is approached at the highest pressures and lowest temperatures. At temperatures below 300 K, the association reaction is found to be irreversible on the experimental time scale of ∼20 ms. The kinetic data at T < 300 K have been employed to obtain falloff parameters in a convenient format for atmospheric modeling. At temperatures above 330 K, reversible addition is observed, thus allowing equilibrium constants for C 2 Cl 5 formation and dissociation to be determined. Second-and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the association reaction: ∆H°2 98 ) -18.1 ( 1.3 kcal mol -1 , ∆H°0 ) -17.6 ( 1.3 kcal mol -1 , and ∆S°2 98 ) -27.7 ( 3.0 cal mol -1 K -1 . In conjunction with the well-known heats of formation of Cl( 2 P J ) and C 2 Cl 4 , the above ∆H values lead to the following heats of formation for C 2 Cl 5 at 298 and 0 K: ∆H°f ,298 ) 8.0 ( 1.3 kcal mol -1 and ∆H°f ,0 ) 8.1 ( 1.5 kcal mol -1 . The kinetic and thermochemical parameters reported above are compared with other reported values, and the significance of reported association rate coefficients for understanding tropospheric chlorine chemistry is discussed. † Present address: Cl 2 + hν (355 nm) f nCl( 2 P 3/2 ) + (2n)Cl( 2 P 1/2 ) (2) ln{[Cl( 2 P J )] 0 /[Cl( 2 P J )] t } ) (k 1 [C 2 Cl 4 ] + k 3 )t ) k′t (I)