Most of the chemical, physical, and biological processes of interest to humans occur in the solution or liquid phase. However, a detailed molecular picture of the role of the environment in these processes has not been obtained due to a lack of structural probes of solute-solvent interactions. A main goal of our research is the elucidation of these interactions and their effect on ground and excited state properties in a structurally-specific manner. We are pursuing this goal in a number of specific research areas:
A. Charge Transfer in Proteins.
B. Intramolecular Charge Transfer-molecules with twisted intramolecular charge transfer states.
Our technique of choice is resonance Raman spectroscopy, although fluorescence spectroscopy, absorption spectroscopy, and time-resolved spectroscopy are also used.
A. Charge Transfer in Proteins.
Our research program in proteins is designed to probe environmental effects on structure and dynamics in a fairly rigorous way. One of the major problems in defining the structural and dynamic role of the solvent has been the lack of structurally-specific probes of solution dynamics and the large range of timescales of solvent-solute interactions. By probing charge transfer in chromophores buried inside a structurally well-defined protein environment we can start to partition the various effects of the protein environment among the different processes.
We have had some success in this by looking at the charge transfer dynamics of plastocyanin, a blue copper protein involved in photosynthesis in plants, and azurin, a blue copper protein involved in bacterial respiration. Both of these processes are electron transfer processes, in which the electron is transported through the particular blue copper protein.
Blue copper proteins provide good models for our purposes for two reasons: 1) many blue copper proteins have had both their oxidized and reduced structures determined from x-ray crystallography, and 2) NMR and x-ray crystallography suggest that the copper site is inaccessible to bulk solvent, i.e. the protein plays the role of the solvent environment for the copper site in these proteins.
Plastocyanin contains a copper ion bound to the side-chains of 4 amino acid residues. The copper site is located near the edge of the protein. In contrast, the structure of azurin demonstrates that the copper ion is bound to the side-chains of 4 amino acid residues and one side-chain oxygen atom. These structural differences are important in determining the redox potential, a significant characteristic for its function.
By measuring the resonance Raman excitation profiles of plastocyanin and analyzing them using a time-dependent formalism, we have determined that the protein has two effects on the dynamics of the charge transfer between the sulfur of Cys84 and the copper ion: dynamics along specific normal modes that contain contributions from protein internal coordinates and a stochastic effect due to random reorientation of the protein backbone and amino acid side-chains. The observation of specific protein internal coordinates involved in the charge transfer dynamics on a 30 femtosecond timescale suggests that plastocyanin is engineered by Nature to optimize the rate of charge transfer and electron transfer (J. Phys. Chem. 1996, 100, 3278) (Abstract).
We have examined the charge transfer dynamics in plastocyanin from several species using this technique to determine the effects of small changes in protein composition. The species are chosen to ensure the geometry at the copper site is conserved. We find that the dynamics along specific normal modes change as a function of changing the composition of the protein far (>10 A) from the copper site. However, the stochastic effect is unchanged (J. Am. Chem. Soc. 1997, 119, 896)(Abstract).
Recently we have begun work on azurin, a blue copper protein involved in bacterial respiration. Azurin has a higher reorganization energy than plastocyanin, qualitatively and quantitatively consistent with the greater number of nominal ligands at the copper site in azurin. The bulk solvent-like reorganization energy component from the protein is also greater in azurin than in plastocyanin, although the molecular cause of this is still unclear. (J. Phys. Chem. 1997, 101, 5062) (Abstract).
B. Intramolecular Charge Transfer-Molecules with Twisted Intramolecular Charge Transfer (TICT) states.
Molecules in which part of the molecule acts as an electron acceptor and another part acts as an electron donor may exhibit TICT states. Upon photoexcitation, charge is transferred from the donor to the acceptor part of the molecule. However, the transfer of charge is dependent on the ability of the bond connecting the donor and acceptor moieties to twist, usually from an all-planar geometry to a 90 degree twisted geometry.
The unique coupling of electronic and nuclear coordinates in these systems should provide an interesting system to test the effects of environment. We are currently studying the effect of changes in environment on the ground state structure of a simple molecule that exhibits a TICT state. We have shown that the Raman spectrum is sensitive to the hydrogen-bonding ability of the solvent and that this solute-solvent interaction occurs at the ester end of the molecule exclusively. (Chem. Phys. Lett. 1996, 261, 691) (Abstract).
The interaction of light with biological molecules is one of the most important reactions for sustaining and optimizing life on this planet. In addition to causing tanning, the interaction of light and biological molecules is responsible for vision and photosynthesis. Some evidence suggests that light is also responsible for behavior in animals and humans through some kind of circadian clock, perhaps through the interaction of light with a photoactive molecule in the pineal gland of the brain.
Our research interests in this area overlap somewhat with the research effort in elucidating the role of the environment in various processes. Specifically, research is directed in two areas
A. Photosynthesis
Nature has created a photosynthetic apparatus in higher plants that is optimized for its function. The quantum yield of electron transfer is nearly unity, suggesting that one electron is transferred for every photon absorbed. The amazing thing is that nature has accomplished this with a complex array of proteins, essentially insulating material. To understand better how to optimize artificial energy conversion schemes, it is necessary to understand the natural system. Towards that end, we are working to elucidate the role of the protein environment in photosynthetic proteins, of which plastocyanin is the first (see #1 above). As described above, significant progress has been made in determining what the effects on the charge transfer dynamics are of this structured and complex environment.
B. Vision
The interaction of light with the photoreceptor protein rhodopsin is also a highly efficient process-the quantum yield is 2/3 and the eye can see single photons (~11 photons at the cornea, or surface of the eye). Absorption of a photon by rhodopsin initiates a complex series of biochemical reactions. The nature of the signalling between rhodopsin and the first protein in the cascade, transducin, is not well understood. We are currently exploring the structural dynamics on the nanosecond to second timescales to better understand the signalling mechanism in this system.
One applied direction of our research has been the development of sensors based on advanced optical technologies. Currently we are developing an optically-based detector of inhomogeneities in polymer solutions. This is of interest in determining polymer reactor characteristics.
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