Cobalamins are a family of Vitamin B12 derivatives that serve as essential cofactors in a variety of biological reactions. The rate of their key catalytic step, cleavage of a Co - C bond, has been shown to be enhanced by up to 12 orders of magnitude following enzyme binding in certain cobalamin species. Such a staggering rate-enhancement has lead to considerable interest into the reaction pathways of these species during Co-C bond scission. A large library of spectroscopic data has been generated to identify the states relevant to the reaction coordinate surface during bond cleavage. However, several overlapping states have congested linear spectra, making interpretation difficult. Additionally, low frequency vibrations are difficult to detect above solvent background in Raman data. Finally, linear spectra cannot reveal information about relationships between states, which is key to identifying states relevant to the PES.<br/><br/>To address the challenges above, we apply fully coherent, four wave mixing (FWM) methods to investigate cobalamins. This class of methods is a unique subset of multidimensional spectroscopy utilizing several directly tunable light sources to interrogate multiple states. Individual lasers can be tuned from the mid IR to near UV, allowing us to simultaneously excite multiple vibrational and electronic states. Emission intensity is enhanced when input light is resonant with coupled electronic and vibrational states and, therefore, spectra contain detailed information about the relationship between states. This coupling requirement also reduces the number of states contributing to a single spectra, allowing for decongestion of overlapping states. Furthermore, coupling to electronic states can enhance weak low frequency vibrational modes so that they are distinct above solvent response. In this presentation, we detail how these benefits of FWM methods have been harnessed to advance our knowledge of cobalamin reaction surfaces.

Fully coherent four wave mixing spectroscopic methods to probe bond cleavage in cobalamins

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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