Phylloquinone (PhQ) is an important cofactor in the electron transport chain of photosystem I (PSI), where local interactions with the protein environment tune its properties and make PhQ one of nature’s most reducing quinones. Studying more directly how the PSI protein scaffold is able to tune the reactivity of its cofactors could yield valuable insight into design principles for optimizing reactivity through the local environment. PhQ is one candidate for such a study, due to the asymmetric hydrogen bond to one of its carbonyl groups—however, little is known about how hydrogen bonding fundamentally impacts the spectral and dynamical properties of PhQ on the ultrafast timescales. We apply two-dimensional infrared spectroscopy (2DIR) to PhQ in hexane and hexanol to characterize how hydrogen bonding presents in the spectra of PhQ, and to determine the potential utility of using PhQ as a probe of the protein binding pocket. While the 2DIR spectrum of PhQ in hexane shows a single round ground state bleach, PhQ in hexanol exhibits a round ground state bleach as well as an elongated shoulder. To interpret the differences in the 2DIR spectra we applied MD simulations, harmonic and anharmonic DFT frequency calculations, and spectral modeling. Through the MD simulations and DFT calculations, we quantified the different hydrogen bonding species and assigned the vibrational frequencies associated with each. We found that the hydrogen bonding interaction completely localizes and decouples the carbonyl modes, leading to the appearance of the elongated shoulder peak in the 2DIR spectra. We further extracted anharmonicities from anharmonic DFT calculations. Our interpretation of the data was confirmed through spectral modeling using third order response functions. With this study, we have gained fundamental insight as to how hydrogen bonding presents in the 2DIR spectra of PhQ and further evidence of the potential to use PhQ as a probe of the protein environment of PSI.

Characterizing hydrogen bonding to phylloquinone with ultrafast 2DIR spectroscopy

ACS Spring 2022

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