Flavin is the cofactor for the large and diverse family of proteins called flavoproteins. Flavin is present in flavoproteins as flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). At physiological conditions, flavin is usually encountered in one of five different redox/protonation states. Because of this chemical versatility, flavoproteins include oxidoreductases, transferases, lyases, isomerases, and ligases involved in a wide range of biological activities. In addition, flavin is prevalent in photoreceptors; out of six of the largest classes of photoreceptor proteins, three classes are flavoproteins (LOV, BLUF, and CRY domains). Computationally investigating the mechanisms of these receptors requires excited-state methods capable of accurately describing the relative energetics between ground and excited states of flavin for a range of relevant geometries and for multiple redox and protonation states. One approach widely used in modeling photoreceptors is the complete active-space self-consistent field (CASSCF) method followed by second order perturbative energy correction (CASPT2). However, there is a limit to the size of the CASSCF active space because the computational cost increases factorially with the size of the active space. Considering the expense and the accuracy of the result, choosing a suitable active space is very important. Therefore, we performed benchmark studies for vertical excitation energies of flavin in different redox states, and along ground and excited-state potential energy scans. The benchmark study targets several methods capable of treating multi-configurational character, including multi-reference methods (SS-CASPT2, MS-CASPT2, XMS-CASPT2, XDW-CASPT2, and MC-PDFT) with varying active spaces and states used for state averaging, as well as single-reference methods.

Excited-state benchmark studies for flavin in different redox states

ACS Spring 2022

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