Laboratory of Photobiology
Photosynthesis and vision are products of biological evolution. Our aim is to understand the photochemistry and the origin of these processes.
The pulsed, time-resolved photoacoustic methodology that we have developed allows the measurement of the thermodynamic enthalpy of individual steps of photosynthetic and other photochemical reactions. The heat given off at a particular time step following photoexcitation causes a thermal expansion of the fluid. This generates a pressure wave which is measured by a suitable detector. The time resolution is in nanoseconds and the measurement is also sensitive to volume changes at each reaction step. One can distinguish between cooling by a positive enthalpy or a true volume contraction in the magic solvent water. At the temperature of maximum density, about 4°C, the thermal expansion is zero and only volume changes are observed. On applying these measurements to the reaction center of the photosynthetic bacterium Rhodopseudomonas spheroides, we observed a rapid contraction of about 20 Å3; in less than 20 nanoseconds. This is only 0.02% of the reaction center volume, yet a change of 0.1 Å3 can be detected. The sign and rapidity of the effect point to electrostriction as its cause. This is the contraction of a dielectric in an electric field gradient, in this case arising by the sudden formation of separated positive and negative charges which is the very heart of photosynthetic energy storage. It is known to occur within 300 picoseconds in the reaction center. The thermodynamic description of electrostriction was given just 100 years ago by Drude and Nernst in the then young Zeitschrift für Physikalische Chemie. The fit of this theory to our observations indicates that the effective dielectric coefficient inside the reaction center is about 15, considerably larger than often assumed. The measurement of electrostriction is a good method to determine the value of the dielectric coefficient inside a protein, a subject of much current interest. This work was carried out in collaboration with Dr. Marilyn Gunner of the City University of New York and Jingwen Zhang. In collaboration with Professor Jehuda Feitelson of the Hebrew University, we have carried out similar measurements of photoinduced electron transfer reactions in free solution and have identified the electrostriction caused by the resulting ions. By combining the known free energies of the reaction with the measured enthalpies we also arrive at the previously unknown entropy of the ionization reaction. We have even observed the volume contraction on forming the excited triplet site of a zinc porphyrin, ~ -0.8 Å3. We are assembling a far more complete understanding of the electron transfer process so critical to photosynthesis and much of chemistry.
We have found that the photoformation of porphyrin cations inside a lipid bilayer greatly increases the current of hydrophobic anions across the membrane. Studies of this photogating effect by Dr. Kai Sun have shown that increases of over a hundredfold in current can be obtained with relatively small changes in the structure of the hydrophobic ions or of the porphyrin sensitizers. These observations have supplied strong evidence that the effect is caused by increased mobility of the ions in chains or aggregates of oppositely charged ions in the bilayer. They have also led to the concept and observation of electrostatic ion pumping: photochemical charge transfer across a single bilayer-water interface creates an electric field that can pump mobile hydrophobic ions completely across the lipid bilayer. In continuous light a current is seen that is opposite in sign to that of the photoinduced electron current. An efficiency of at least 15% can be estimated based on the amount of photoformed charge. The cause of this surprisingly large efficiency is the nonlinear photogating effect and the thinness of the bilayer. The hydrophobic ions whose mobilities have been enhanced by the photogating effects are pumped across the membrane by the photogenerated fields. As the porphyrin cations decay by reverse reaction, the enhanced mobility is rapidly lost, trapping the ions pumped across the membrane. In the steady state the flux of hydrophobic ions exceeds the decay rate of the porphyrin cation, resulting in an efficient pump. These effects are an example of the riches hidden in a "simple" lipid bilayer. They all occur in a self-assembled system less than 5 nanometers thick. These results also suggest precursors to the ion pumps so crucial to biological systems that could have been formed from simple molecules in prebiogenic times.
We have also published work showing that contrary to our previous claim, ferrous ion is not able to photochemically reduce carbon dioxide. This reaction would have ameliorated the presently difficult problem of the source of the reduced carbon required for the chemical origin of life on earth. Work on energy storage in photosynthetic systems, on the possible interactions among oxygen forming units in a cyanobacterium, and on the efficient transfer of charge across lipid bilayers by fullerene anions is continuing.