Gregory S. Engel
Shedding New Light on Photobiology: How Protein Structure Steers Energy Transfer and Photochemistry
Understanding photoreactivity dependent on internal conversion through conical intersections: In recent years, electronic structure calculations have located numerous photochemical reactions in the condensed phase that are predicted to proceed through conical intersections; while some kinetic data exists to bolster theses claims, direct evidence for the presence of conical intersections in the condensed phase is scarce. Our research focuses on leveraging precise theoretical predictions of the conical intersections structures to predict, identify, and characterize these structures with the ultimate goal of rationally controlling photoreactivity. We intend to employ laser spectroscopy to watch the reactions, then to invoke theoretical modeling to understand the data and to locate new substrates. As we move into the spectroscopy of branching reactions, we intend to manipulate macromolecular chaperones to attempt to steer and control the photochemistry.
Ultrafast Nonlinear Spectroscopy:Two key aspects of the photochemistry at conical intersections define our experimental efforts: First, the timescales of these reactions are approximately 100 femtoseconds (1/10 trillionth of a second), and such a fast timescale demands ultrafast laser pulses to follow the reaction in the time domain. Second, near the conical intersection singularity, the electronic and nuclear modes couple very strongly; we have developed a new two dimensional, fifth order spectroscopy that is uniquely sensitive to these electronic-vibrational couplings. Using this ultrafast, 2D spectroscopy, we seek to identify which vibrational modes of the system control the reactivity and how the excited state approaches, navigates, and ultimately traverses the conical intersection to produce reaction products. The experimental apparatus will use a diffractive optics based two dimensional optical spectroscopy system to carefully control both linear electronic and resonant raman interactions with the system. Leveraging rephasing pathways to eliminate inhomogeneity in the spectrum and spectral interferometry to resolve the full electric field, we can then map the evolution of the excited state toward the conical intersection.
Theory of Conical Intersections and Nonlinear Response:Our theoretical efforts are very closely tied to the experimental efforts; neither can be successful without the other. The Hamiltonian, being a Hermitian operator, can only support state crossings in N-2 dimensions; therefore, the conical intersection must have a two dimensional branching space. Further, the conical intersection necessarily permits access to multiple product wells. In conjunction with our experimental effort, we intend to map this branching space and observe which nuclear modes are most active in steering the reaction through the branching space. To read this information from the experimental data, we must model the nonlinear response of this system including evolution along the reaction coordinate during the coherence and rephrasing times. To accomplish this task, we will use a combination of high order perturbative models and an auxiliary density matrix non-perturbative approach to model the nonlinear polarization.
Chemical Biology: Manipulating Macromolecules to Control Photoreactivity:As our experimental efforts advance, we intend to employ macromolecular chaperones to steer the reaction through the branching space that determines products. Such development of photoenzymatic activity falls into a different class completely from evolution of novel enzymes using ground state reactivity. Rather than requiring wholesale changes to the ground state potentials to stabilize transition states, we only require slight steric effects to steer a reaction that already progresses under kinetic control. Using simple selection techniques, we hope to evolve novel photoenzymes to control the yields and products of reactions.
Recipent of a 2010 Presidential Early Career Award for Science and Engineering (PECASE) from the Executive Office of the President of the United States
2009 AFOSR Young Investigator Program Participant