PUMP-PROBE SPECTROSCOPY OF PROTON-COUPLED ELECTRON TRANSFER IN PROTEINS

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The primary goal of this research project is to exploit the unique information that ultrafast vibrational spectroscopy provides to develop fundamental insights into ultrafast processes relevant to biology. We thereby intend to (1) identify key vibrational signatures of intermediate states involved in electron transfer, (2) ascertain the extent to which charge migrates in strongly coupled donor-acceptor systems, (3) examine the dynamics of reactions such as ultrafast proton-coupled electron transfer in which fast bond-breaking and bond-forming events occur on time scales similar to charge migration, (4) understand the nature of excited-state chromophore nuclear dynamics, and (5) probe the nature of electronically excited states in delocalized, multi-pigment systems. The types of dynamics-correlated ET experiments that we intend to pursue will utilize state-of-the-art experimental methodologies and instrumentation to study molecular vibrations on fast, sub-nanosecond time scales. Optical pump-probe, optical-pump/infrared-probe and dynamics Stokes shift experiments will be correlated with experiments such as transient optical pump-probe experiments. This work will delineate structure-function relationships important to protein-mediated, proton-coupled electron-transfer (PCET) reactions. These investigations will rely on the design of de novo tetra-alpha-helical proteins engineered to bind donor - spacer - acceptor (D-Sp-A) supermolecules and covalently linked multi-cofactor assemblies. While these systems possess reduced structural complexity with respect to biological proteins, in the sense that they exploit mainly helical bundle motifs, they enable independent modulation of the local electrostatic forces that surround D, A, and chromophore; furthermore, design features of these tetra-alpha-helical proteins allow control over the nature of cofactor first- and second-sphere H-bonding interactions. We will examine photoinduced PCET reactions in such systems using optical pump / optical probe and visible pump / IR probe spectroscopic methods, and correlate the observed dynamics with specific responses of the peptide matrix. Such data provide key information that will enable us to fully exploit powerful computational approaches to elaborate protein design to effect desired function.