Proton uptake upon quinone reduction in bacterial reaction centers: IR signature and possible participation of a highly polarizable hydrogen bond network

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The light-induced QA-/QA FTIR difference spectra of Rb. sphaeroides and Rp. viridis show very broad positive bands of small amplitude peaking around 2750 cm-1. Upon ¹H/²H exchange these bands shift to about 2150 cm-1. Similarly, the QB-/QB spectra exhibit broad continuum bands at ≈2600 and 2800 cm-1 shifting to ≈2100 and 2200 cm-1 in ²H2O for Rb. sphaeroides and Rp. viridis, respectively. These continuum bands are tentatively interpreted in terms of highly polarizable hydrogen bonds in a large web of polar bonds involving cofactors, amino acid residues, and structured water molecules. As a working hypothesis, we propose that the protons participating in this web redistribute upon quinone reduction, increasing their concentration around the newly formed charged species, and leading to net proton uptake. Assuming that the precise localization of the mobile protons is dependent on the local electrostatic, this model can explain the apparent discrepancies between some results of FTIR experiments and of electrostatic calculations. Notably, it could help rationalize the observation that mobile protons tend to localize on Glu L212 upon QB reduction in Rb. sphaeroides, while for QB reduction in Rp. viridis and for QA reduction in both Rb. sphaeroides and Rp. viridis, proton uptake by a small number of carboxylic residues is not supported by the FTIR data.

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