Rotameric Heterogeneity of Conserved Tryptophan Is Responsible for Reduced Photochemical Quantum Yield in Cyanobacteriochrome Slr1393g3.

The red/green cyanobacteriochrome (CBCR) slr1393g3 exhibits a quantum yield of only 8 % for its forward photoconversion, significantly lower than other species from the same CBCR subfamily. The cause for this reduced photoconversion is not yet clear, although in the related NpR6012g4 dark-state structural heterogeneity of a paramount Trp residue has been proposed to cause the formation of nonproductive subpopulation. However, there is no such information on the equivalent residue in slr1393g3, W496. Here we use solid-state NMR to explore all possible sidechain rotamers of this Trp residue and their local interactions at the atomic level. The indole nitrogen (Nϵ1) is used as an NMR probe, achieved by site-specific ¹⁵N-indole labeling of a quadruply Trp-deleted variant and trehalose vitrification technique. The data reveal a set of seven indole rotamers of W496 with four distinct environments for the Nϵ1-H group. Only a minority population of 20 % is found to retain the π-stacking and hydrogen-bonding interactions with the chromophore in the dark state that has been assigned to account for complete forward photoconversion. Our results demonstrate the direct role of W496 in modulating the forward quantum yield of slr1393g3 via rearrangement of its sidechain rotameric conformations.