Tailored vibrational structure theory for difference infrared spectra of photoactive flavoproteins

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Infrared spectroscopy has the potential to provide detailed structural information on biomolecular systems. However, computational methods are indispensable for extracting this information. For example, the subtle changes in the hydrogen bonding network of the blue light sensor using flavin (BLUF) protein upon illumination can be monitored by difference infrared spectroscopy. Yet, there is no consensus on the associated structural changes and current computational methods are either too inefficient or too inaccurate to provide a reliable interpretation of the difference infrared spectra for a system as large as the BLUF protein. Computational methods can be expected to provide the most beneficial accuracy-to-cost-ratio for a computational scheme that complies with the experimental setup: In difference spectroscopies, change is monitored rather than full spectra. That means, for the interpretation of these spectra only vibrations that contribute to this change have to be modelled. For the BLUF protein, this change amounts to only a few reciprocal centimetres - an accuracy that for hydrogen-bonded systems requires anharmonic quantum treatments. To achieve this accuracy, we will establish multi-level anharmonic vibrational structure methodologies. Our methods will exploit fragmentation and multi-scale approaches in the potential energy surface as well as embedding schemes for the vibrational wave function including vibrational coupled cluster response methodologies.