Pressure catalyzed bond dissociation in an anthracene cyclophane photodimer.

The anthracene cyclophane bis-anthracene (BA) can undergo a [4 + 4] photocycloaddition reaction that results in a photodimer with two cyclobutane rings. We find that the subsequent dissociation of the dimer, which involves the rupture of two carbon-carbon bonds, is strongly accelerated by the application of mild pressures. The reaction kinetics of the dimer dissociation in a Zeonex (polycycloolefin) polymer matrix were measured at various pressures and temperatures. Biexponential reaction kinetics were observed for all pressures, consistent with the presence of two different isomers of bis(anthracene). One of the rates showed a strong dependence on pressure, yielding a negative activation volume for the dissociation reaction of ΔV(++) = -16 Å(3). The 93 kJ/mol activation energy for the dissociation reaction at ambient pressure is lowered by more than an order of magnitude from 93 to 7 kJ/mol with the application of modest pressure (0.9 GPa). Both observations are consistent with a transition state that is stabilized at higher pressures, and a mechanism for this is proposed in terms of a two-step process where a flattening of the anthracene rings precedes rupture of the cyclobutane rings. The ability to catalyze covalent bond breakage in isolated small molecules using compressive forces may present opportunities for the development of materials that can be activated by acoustic shock or stress.

Charge recombination time distributions in photosynthetic reaction centers exposed to alternating intervals of photoexcitation and dark relaxation.

The charge recombination lifetime of photosynthetic reaction centers (RCs) increases significantly upon lengthy illumination, revealing nonequilibrium structural transitions in the protein-cofactor system. This paper analyzes the charge recombination kinetics measured in isolated RCs following a systematic variation of actinic illumination times (pulses) from 0.1 s to hundreds of seconds. The maximum entropy method (MEM) was utilized for optimizing the fitting procedure to retrieve the relaxation spectrum from the experimental recombination kinetics curves. The MEM-assisted analysis reveals that each relaxation curve contains at least three peaks in the relaxation time-distribution domain. Two peaks are always observed, one near 0.1 s and the other near 1 s recombination times. A third peak appears after prolonged photoexcitation with a relaxation time significantly greater than 1 s, and the time of this peak increases further in recombination time as the photoexcitation pulse duration is increased. In addition to the shifts of the time constant distributions, the amplitudes of the distributions in the time domain spectrum demonstrate a variation in the quinone occupancy of the RCs. The results reported here support our previous claim that accumulation of slow conformational changes, triggered by charge separation events in the RCs, controls system dynamics and favors stabilization of more efficient functioning regimes of the RCs.

Equilibration kinetics in isolated and membrane-bound photosynthetic reaction centers upon illumination: a method to determine the photoexcitation rate.

Kinetics of electron transfer, following variation of actinic light intensity, for photosynthetic reaction centers (RCs) of purple bacteria (isolated and membrane-bound) were analyzed by measuring absorbance changes in the primary photoelectron donor absorption band at 865 nm. The bleaching of the primary photoelectron donor absorption band in RCs, following a sudden increase of illumination from the dark to an actinic light intensity of I(exp), obeys a simple exponential law with the rate constant alphaI(exp) + k(rec), in which alpha is a parameter relating the light intensity, measured in mW/cm(2), to a corresponding theoretical rate in units of reciprocal seconds, and k(rec) is the effective rate constant of the charge recombination in the photosynthetic RCs. In this work, a method for determining the alpha parameter value is developed and experimentally verified for isolated and membrane-bound RCs, allowing for rigorous modeling of RC macromolecule dynamics under varied photoexcitation conditions. Such modeling is necessary for RCs due to alterations of the forward photoexcitation rates and relaxation rates caused by illumination history and intramolecular structural dynamics effects. It is demonstrated that the classical Bouguer-Lambert-Beer formalism can be applied for the samples with relatively low scattering, which is not necessarily the case with strongly scattering media or high light intensity excitation.

Self-regulation phenomena applied to bacterial reaction centers: 2. Nonequilibrium adiabatic potential: dark and light conformations revisited.

Experimental and theoretical results in support of nonlinear dynamic behavior of photosynthetic reaction centers under light-activated conditions are presented. Different conditions of light adaptation allow for preparation of reaction centers in either of two different conformational states. These states were detected both by short actinic flashes and by the switching of the actinic illumination level between different stationary state values. In the second method, the equilibration kinetics of reaction centers isolated from Rhodobacter sphaeroides were shown to be inherently biphasic. The fast and slow equilibration kinetics are shown to correspond to electron transfer (charge separation) at a fixed structure and to combined electron-conformational transitions governed by the bounded diffusion along the potential surface, respectively. The primary donor recovery kinetics after an actinic flash revealed a pronounced dependence on the time interval (deltat) between cessation of a lengthy preillumination of a sample and the actinic flash. A pronounced slow relaxation component with a decay half time of more than 50 s was measured for deltat > 10 s. This component corresponds to charge recombination in reaction centers for which light-induced structural changes have not relaxed completely before the flash. The amplitude of this component depended on the conditions of the sample preparation, specifically on the type of detergent used in the preparation. The redox potential parameters as well as the structural diffusion constants were estimated for samples prepared in different ways.