QUANTUM COHERENCE IN MACROMOLECULES
I am currently investigating the mechanisms behind electron delocalization and tunneling in macromolecules, specifically light harvesting proteins like the rhodopsins and the chlorophyll-based photosynthetic unit. The rhodopsins produce a transmembrane potential which is achieved by photoisomerization of a double bond in the chromophore (retinal). This structural change lowers the pKa of its Schiff base and deforms the molecule in a way which translocates a proton to the extracellular side. A voltage results (~20 mV) which is used, for example in bacteriorhodopsin, for phosphorylation. I am currently trying to determine the degree of localization of the electron/hole states which isomerize the double bond, how localization varies within the family of rhodopsins, and how it compares to that in free retinal Schiff base. The objective is to determine how the protein environment fine tunes the excited states of the chromophore to accomplish certain tasks. Some things I wonder are (1) if the selectivity toward the 13-cis photoproduct is a result of electron/hole localization around the 13-14 double bond, and (2) if the retinal in one rhodopsin is really independent from a neighboring one, since in 2D crystals they are separated by only ~15 A. Results will also be compared to ab intio calculations in order to test fundamental models of photoisomerization which incorporate protein and solvent environments and other effects, the long term hope being rational design of optomechanical devices which convert light energy directly into mechanical work.
The Photosynthetic Unit [above image, borrowed from Hu, PNAS, 95, 5935 (1998)], which consists of a reaction center surrounded by many light harvesting complexes (LH-II, LH-III, etc.) is rumored to be a quantum-mechanically coherent object. This property, which comes from the poor screening which permits long range dipole-dipole coupling, allows the PSU to transport energy efficiently over large distances. I plan to measure this coherence length quantitatively to shed light on this issue, which is important not only for photosyntheisis but also for the design of synthetic dendritic antenna systems which supposedly function by the same mechanism.
This project is
based on the technique of inelastic x-ray scattering (IXS), which is a
momentum-resolved form of Raman scattering from which, among other things,
tunneling can be probed in a model-independent fashion.
Experiments are performed at
synchrotron x-ray facilities, specifically C line at CHESS, X21 at Brookhaven, and
later this year the CMC-CAT at the Advanced Photon Source.
As a demonstration,
a real time movie showing the
disturbance created in liquid water by a delta function in space and time is available
here. In this figure the x axis is in angstroms, the y axis
is the electron density (relative units) and the running clock is in femtoseconds. This is
experimental data from CHESS.
As a demonstration, a real time movie showing the disturbance created in liquid water by a delta function in space and time is available here. In this figure the x axis is in angstroms, the y axis is the electron density (relative units) and the running clock is in femtoseconds. This is experimental data from CHESS.
MESOPHASES IN THIN FILMS, SURFACE SELF-ASSEMBLIES, DEVICES
I recently co-invented a new type of x-ray diffraction technique which employs spectroscopy effects in the soft x-ray regime to probe more diverse phenomena than just the electron density. Some examples are oxygen hole states in superconductors (for stripe phases and other forms of inhomogeneity), lipid species in surface self-assemblies (cholesterol-induced mesophases and rafting phenomena), magnetic moments in devices containing transition metals (quantum wires and dots), polymer species (diblock copolymer films), etc. The contrast to oxygen hole states has been used, for example, to search for a so-called “stripe phase” in the doped cuprates, and to investigate whether there is a modulation of the carrier density associated with the inhomogeneity discovered in Bi2Sr2Ca2CuO8+x by the J. C. Davis group at Berkeley [image at left, lifted from Pan, Nature, 413, 282 (2001)]. This work was recently published in Science and the abstract and full text are publicly available. This project is currently on hold but I will return to it soon enough.
If you still haven’t had enough, here is my C.V.
Last modified: August 7, 2002