Biomolecules form a class of complex systems that are fundamental to the existence of life. The function of biomolecules can be controlled by very small length scale fluctuations, vibrations, and the “spin” associated with metal ions, all of which involve quantum effects and are associated with the newly developing field of “Quantum Biology”. Our lab studies the structure and dynamics of biomolecules using a variety of ultrafast laser-based techniques such as vibrational coherence spectroscopy (VCS) and broadband pump-probe kinetics that span the femtosecond to millisecond timescales. We also use more traditional techniques such as resonance Raman scattering and we are developing new methodologies, such as femtosecond stimulated Raman scattering. The latter method has excellent time and frequency resolution and it can be used as a spectroscopic probe of individual biomolecules as well as a method for imaging biological tissue and cells.
Time-resolved dynamic information helps us to probe biochemical reaction coordinates as well as unstable catalytic intermediates that involve enzyme-substrate complexes. The vibrational coherence measurements are designed to probe very low frequency motions, which have energies that can be thermally excited. Such motions are used by biomolecules to extract energy from the environment in order to do useful biochemical work. The kinetic studies characterize processes taking place over a wide dynamic range of timescales. These processes involve diatomic ligand binding, rapid (local) structural relaxations, and more global protein conformational interconversions. Much of our work involves studies of heme containing proteins, which have roles in oxygen storage, electron transport, signaling and catalysis. Specific enzymes as well as photoactive molecules, such as the green fluorescent protein, are also studied in order to better understand the fundamental aspects of proton transport in biological systems.
We often use resonance enhancement and tune the laser frequency to energies ~1-4 eV that coincide with the electronic excitations of the various biological chromophores. In the case of Raman scattering, the resonance enhancement effects are enormous and allow us to selectively interrogate specific regions within the complex biological macromolecule. Measurements of absolute scattering cross-sections and the intensity of the scattering as a function of excitation frequency are used to obtain the electron-nuclear coupling parameters as well as other information pertinent to the structure and function of these materials. Samples are studied in the solution, crystalline and frozen states.
Current studies are focusing on single biomolecule spectroscopy techniques that involve nanostructures as well as femtosecond coherence and kinetic studies, which facilitate a better understanding of functionally important low frequency biomolecular dynamics. Theoretical work is focused on disentangling the structural and dynamic information that is contained in these novel experiments.