The photochemical reaction that plants and photosynthetic bacteria use to capture the energy of sunlight occurs in pigment-protein “reaction centers.” Bacterial reaction centers typically contain four molecules of bacteriochlorophyll (BChl), two bacteriopheophytins (BPh) and two quinones (QA and QB) bound to three polypeptides. Two of the four BChls are very close together. When this special pair (P) is excited with light, it passes an electron to the quinones by way of the neighboring BChl and BPh.
Dr. Parson’s research group is using fast spectroscopic techniques and computational approaches to study the initial electron-transfer steps in reaction centers. A homebuilt Ti-sapphire laser is used to excite a sample with pulses of light lasting less than 10-13 s. A second pulse of light probes the absorption spectrum of the sample as a function of time after the excitation. The spectrum changes as an electron moves from one molecule to another. The excited BChl dimer (P*) appears to transfer an electron to one of the neighboring BChls in about 3 ps, creating a radical-pair (P+BChl-). An electron moves from BChl- to a BPh in about 1 ps and from BPh- to QA in 200 ps. In addition to being very fast and efficient, these reactions have the unusual property of speeding up with decreasing temperature.
The reaction center provides an ideal system for addressing the general question of how enzymes work because the kinetics can be measured with high precision under a wide range of conditions in a protein with a well-defined structure, and because the initial reactions occur on a time scale that is amenable to molecular-dynamics simulations.
To explore the way in which pigment-protein interactions affect the energies of P+BChl- and P+BPh-, and thus control the rate, temperature dependence and specificity of electron transfer, Dr. Parson’s research group is measuring the energies and electron-transfer kinetics in bacterial mutants with amino acid substitutions near the pigments. Mutations of a particular tyrosine residue were found to change the rate and temperature dependence of the initial reaction dramatically. In a current project, the electrostatic symmetry of the protein is being modified by mutations of ionizable residues. A variety of computational approaches also are being used, including molecular dynamics simulations and electrostatics and quantum calculations. By combining accurate electrostatics and molecular dynamics calculations, it is possible to calculate the free energy surfaces of the reactant and product states as functions of the reaction coordinate and to identify vibrational motions of the protein that are coupled to the electron-transfer reactions.
Dr. Parson’s group also is using experimental and computational techniques to study antenna complexes that absorb light and transfer energy to the reaction center. These are integral membrane proteins that typically contain 24 to 36 BChls. Topics currently under investigation include the extent to which excitations are delocalized over the entire complex, the manner in which excitations move from complex to complex, and the structural organization of the antenna complexes and reaction centers in the membrane.
For more information see the U.W. Biomolecular Structure and Design Program.