Spectroscopy, bioinorganic chemistry, Mössbauer and EPR spectroscopy, synchrotron radiation techniques, nuclear resonance vibrational spectroscopy (NRVS), synchrotron Mössbauer, metalloproteins, enzyme mechanisms, transition metal complexes, electronic structures, density functional theory.

Metalloproteins play essential roles in many key biological processes, including photosynthesis, respiration, nitrogen fixation and carbon cycle, antibiotics biosynthesis, gene regulation, post transcriptional modification, bio-signal sensing, and DNA/RNA repair. In these biological processes, an astounding array of chemical transformations with high stereo- and regio-selectivities is achieved by metal centers buried inside the protein scaffold of metalloproteins. Studies of these metal centers in both geometric and electronic level not only can help us reveal the fundamental chemical principles underpinning these biological processes, also provide valuable guidelines for designing and synthesizing efficient and inexpensive catalysts to perform some of these important yet challenging chemical transformations, such as water oxidation, dinitrogen reduction, selective C-H functionalization.

Our main research focus is iron containing metalloproteins, in particular, small molecule (N2, H2, O2 etc.) activating enzymes, such as nitrogenases, hydrogenases, and oxygenases. Various advanced spectroscopic methods are utilized to gain information regarding iron center changes, both geometric and electronic, during enzyme catalytic cycles. Spectroscopic studies are coupled with Density Functional Theory (DFT) calculations to establish structural models and further identify reaction mechanisms. The ultimate goal of our research is to impact favorably on alternative energy research and human health.

The spectroscopic methods we use include Mössbauer Spectroscopy, Electron Paramagnetic Resonance (EPR) Spectroscopy, Nuclear Resonance Vibrational Spectroscopy (NRVS), and Synchrotron Mössbauer.

Mössbauer spectroscopy arguably has the highest information content of any spectroscopic methods on studies of iron containing systems. It detects all iron in a given sample, and determines their oxidation, spin states, and precise quantification of iron in different local environments. It is a high-resolution technique, which allows one to probe the electronic structure of iron sites in great detail through the hyperfine coupling between electronic states and nuclear states. EPR spectroscopy is a high sensitivity technique; it detects paramagnetic systems and reveals their electronic structures through electronic Zeeman Effect.

Recent developments of synchrotron radiation (SR) facilities enable us to utilize SR sources for Mössbauer spectroscopy in replacement of traditional radioactive sources. The advantage of Synchrotron Mössbauer over the conventional lab-based Mössbauer is that it can have higher spectral resolution, shorter measurement time, and the capability of measuring smaller samples, which are crucial for studies on metalloproteins. The advancement of modern synchrotron radiation sources also promotes the development of a new type of vibrational spectroscopic technique, Nuclear Resonance Vibrational Spectroscopy (NRVS). NRVS detects nuclear excitation coupled molecular vibration excitation (or de-excitation), which conceptually resembles Raman scattering. Yet, compared with Raman and Infrared techniques, NRVS has much higher element selectivity and less restrictive selection rules. It allows us study structures and dynamics of iron centers in metalloproteins by selectively probing iron related vibrations in the far IR region (essentially down to 0 cm-1) without the interference of protein scaffold and solvent vibrations.

Yisong Guo

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