Studying hydrogen bonding and dynamics of the acetylate groups of the Special Pair of Rhodobacter sphaeroides WT

Daniel Gräsing,Katarzyna M Dziubińska-Kühn,Jörg Matysik,A Alia,Stefan Zahn

doi:10.1038/s41598-019-46903-4

Daniel Gräsing, Katarzyna M Dziubińska-Kühn + Show 3 more

Open Access

https://doi.org/10.1038/s41598-019-46903-4

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Abstract

Although the cofactors in the bacterial reaction centre of Rhodobacter sphaeroides wild type (WT) are arranged almost symmetrically in two branches, the light-induced electron transfer occurs selectively in one branch. As origin of this functional symmetry break, a hydrogen bond between the acetyl group of PL in the primary donor and His-L168 has been discussed. In this study, we investigate the existence and rigidity of this hydrogen bond with solid-state photo-CIDNP MAS NMR methods offering information on the local electronic structure due to highly sensitive and selective NMR experiments. On the time scale of the experiment, the hydrogen bond between PL and His-L168 appears to be stable and not to be affected by illumination confirming a structural asymmetry within the Special Pair.

Highlights

The reaction centre (RC) of the purple bacterium Rhodobacter (R.) sphaeroides is a membrane protein in which the primary charge separation, the first step of photosynthesis, is taking place
Raman spectroscopic and QM/MM studies on mutated RC showed that the orientation of the acetyl group of PL depends on the protonation state of His-L168 as it is either involved in a hydrogen bond to His-L168 or, if no hydrogen
It was suggested that a re-orientation of the acetyl group of PL acts as a valve to block the electron back-transfer upon cleavage of the hydrogen bond to His-L168 and thereby re-tuning of the electronic properties of the Special Pair[16]

Summary

Introduction

The reaction centre (RC) of the purple bacterium Rhodobacter (R.) sphaeroides is a membrane protein in which the primary charge separation, the first step of photosynthesis, is taking place. The localization of the LUMO mainly on cofactor PM, from which the electron transfer occurs, has been explored by the photo-CIDNP MAS NMR analysis of the donor triplet state 3P8. This asymmetry is already present in the electronic ground-state of the supermolecule P, as demonstrated by differences in chemical shifts[9,10,11,12,13]. It was suggested that a re-orientation of the acetyl group of PL acts as a valve to block the electron back-transfer upon cleavage of the hydrogen bond to His-L168 and thereby re-tuning of the electronic properties of the Special Pair[16]. No experimental evidence on the cleavage or the dynamics of the hydrogen bond is known, since no appropriate method with enough sensitivity was available

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