MOLECULAR HYDROGEN PRODUCTION BY UROPORPHYRIN AND COPROPORPHYRIN: A MODEL FOR THE ORIGIN OF PHOTOSYNTHETIC FUNCTION

Abstract— Since life originated when the earth's atmosphere was still chemically reducing, the photooxidation of the prevalent reduced organic compounds and the emission of molecular hydrogen would have been a useful form of photosynthesis. If the biosynthetic pathway to chlorophyll recapitulates the evolutionary history of photosynthesis, then uroporphyrin served an early photosynthetic function. In the present study, ethylenediamine tetraacetic acid was oxidized by photoexcited uroporphyrin and coproporphyrin to produce molecular hydrogen in aqueous solution in the presence of colloidal platinum catalyst. In the presence of methyl viologen, a one-electron carrier, the reaction is cyclic and hydrogen gas is produced at a constant rate. The evidence suggests that porphyrin radical intermediates rather than hydroporphyrin are active in the formation of molecular hydrogen. A coproporphyrin-polyvinyl alcohol-colloidal platinum polymer was used as a model for the evolving biological system for photosynthesis in which reactants are held in close proximity.     

The A-Fx to F(A/B) step in synechocystis 6803 photosystem I is entropy driven

We have previously reported the enthalpy and volume changes of charge separation in photosystem I from Synechocystis 6803 using pulsed photoacoustics on the microsecond time scale, assigned to the electron-transfer reaction from excited-state P(700) to F(A/B) iron sulfur clusters. In the present work, we focus on the thermodynamics of two steps in photosystem I: (1) P(700) --> A(1)(-)F(X) (<10 ns) and (2) A(1)(-)F(X) --> F(A/B)(-) (20-200 ns). The fit by convolution of photoacoustic waves on the nanosecond and microsecond time scales resolved two kinetic components: (1) a prompt component (<10 ns) with large negative enthalpy (-0.8 +/- 0.1 eV) and large volume change (-23 +/- 2 A(3)), which are assigned to the P(700) --> A(1)(-)F(X) step, and (2) a component with approximately 200 ns lifetime, which has a positive enthalpy (+0.4 +/- 0.2 eV) and a small volume change (-3 +/- 2 A(3)) that are attributed to the A(1)(-)F(X) --> F(A/B)(-) step. For the fast reaction using the redox potentials of A(1)F(X) (-0.67 V) and P(700) (+0.45 V) and the energy of P(700) (1.77 eV), the free energy change for the P(700) --> A(1)(-)F(X) step is -0.63 eV, and thus the entropy change (TDeltaS, T = 25 degrees C) is -0.2 +/- 0.3 eV. For the slow reaction, A(1)(-)F(X) --> F(A/B)(-), taking the free energy of -0.14 eV [Santabara, S.; Heathcote, P; Evans, C. W. Biochim. Biophys. Acta 2005, 1708, 283-310], the entropy change (TDeltaS) is positive, +0.54 +/- 0.3 eV. The positive entropy contribution is larger than the positive enthalpy, which indicates that the A(-)F(X) to F(A/B)(-) step in photosystem I is entropy driven. Other possible contributions to the measured values are discussed.     

MULTIPLE EXCITATIONS AND REACTION YIELDS IN PHOTOSYNTHETIC SYSTEMS

Alternative explanations of the limited yield of product when bacterial reaction centers are excited by intense ps pulses (Moskowitz and Malley, Photochem. Photohiol. 27 55, 1978) are given. These alternatives, based on photoreversal or multitrap effects, can be experimentally distinguished by a predicted wavelength variation or by the effect of a second flash.     

SYNTHESIS OF AMPHIPATHIC PORPHYRINS AND THEIR PHOTOINDUCED ELECTRON TRANSFER REACTIONS AT THE LIPID BILAYER-WATER INTERFACE

A one flask synthesis of cis -substituted amphipathic porphyrins is reported. These porphyrins were used to study electrostatic effects on photoinduced electron transfer across the lipid bilayer-water interface. A neutral porphyrin undergoes only dynamic interfacial electron transfer reactions irrespective of charge of the acceptor, although ionic strength effects indicate a negative charge on the porphyrin donor species. A dianionic porphyrin forms an interfacial static complex with a dicationic electron acceptor, methyl viologen, at low ionic strength. The electron transfer rate within the complex is slow, 10⁵∼ 10⁶ s^-1, which is attributed to a near orthogonal orientation between the donor and the acceptor ∼ orbitals.     

MULTIPLE EXCITATIONS AND THE YIELD OF CHLOROPHYLL a FLUORESCENCE IN PHOTOSYNTHETIC SYSTEMS

Abstract. Analysis of the effect of multiple excitations on chlorophyll a fluorescence yields in the green alga Chlorella reveals several distinct reactions. The first excitation in dark-z-adapted units produces photochemistry and the high yield state with a rise time of 35 ns. It is ascribed to a change in coupling between the antenna pigments and the photochemical trap. The second hit produces with the same quantum yield a quenched state which changes to the high yield state with a rise time of 4 μ s. This is ascribed to the formation and the decay of a particular carotenoid triplet state near the funnel or antenna-trap junction. Further hits produce enhanced quenching assigned to mobile triplets with lifetimes in the order of 100 ns. The fluorescence yield decreases monotonically with increasing excitations during the 7 ns pulse. This effect can be adequately ascribed to annihilation of excitations with lifetimes longer than the trapping time, or by a unique model of a multi-trapped unit. The latter model is favored by arguments based both on the absence of a local maximum in the graph of fluorescence yield vs excitation energy and on the fact that the high yield state shows a different behaviour on multiple excitation, fit by a single-trapped unit. This analysis is related to that used in experiments with ps flashes and is applied to the qualitatively different bacterial system.     

Listening to PS II: enthalpy, entropy, and volume changes

Photosystem II, located in the thylakoid membranes of green plants, algae, and cyanobacteria, uses sunlight to split water into protons, electrons, and a dioxygen molecule. The mechanism of its electron transfers and oxygen evolution including the structure of the protein and rates of the S-state cycle has been extensively investigated. Substantial progress has been made; however, the thermodynamics of PS II electron transfer and of the oxygen cycle are poorly understood. Recent progress in thermodynamic measurements in photosynthesis provides novel insights on the enthalpic and entropic contribution to electron transfer in proteins. In this review the thermodynamic parameters including quantum yield, enthalpy, entropy, and volume changes of PS II photochemistry determined by photoacoustics and other laser techniques are summarized and evaluated. Light-driven volume changes via electrostriction are directly related to the photoreaction in PS II and thus can be a useful measurement of PS II activity and function. The enthalpy changes of the reactions observed can be directly measured by photoacoustics. The apparent reaction entropy can also be estimated when the free energy is known. Dissecting the free energy of a photoreaction into enthalpic and entropic components provides critical information about mechanisms of PS II function. Potential limitations and future direction of the study of the thermodynamics of PS II electron transfer and oxygen evolution are presented.