Effect of point mutations on the ultrafast photo-isomerization of Anabaena Sensory Rhodopsin

초록

We report experimental results on the ultrafast photo-isomerization of the ASRs-PSBR where point mutations lead to an up to 5-fold increase of the photo-isomerization speed, due to electrostatic modifications of the retinal binding pocket. Anabaena sensory rhodopsin (ASR) is a microbial retinal protein that binds the protonated Schiff base of retinal (PSBR) in two ground state conformations: all-trans, 15-anti (AT) and 13-cis, 15-syn (13C). The isomer ratio depends on external light conditions, leading to the hypothesis that ASR is used as a color sensor, based on two distinct isomer-dependent photo-cycles [1,2]. In the context of the long-standing question as to how the protein environment controls the photo-isomerization reaction speed and quantum yield (QY), ASR offers the intriguing opportunity to study the isomerization of both PSBRs in the same environment. Indeed, recent investigations of the light- and dark-adapted proteins by ultrafast transient absorption allowed to single out the dynamics of each isomer [3,4], and showed that their excited state lifetimes (ESL) are very different (13C: 110 fs, AT: 0.75 ps). This study allowed to unambiguously confirm that the ESL and the reaction QY are two independent quantities, since they are determined by distinct regions of the reactive potential energy surfaces (PES), the Si Franck-Condon (FC) region and the Si P-So conical intersection, respectively. In addition, ASR was shown to have an outstandingly low QY (< 30%) [4]. Our ongoing extensive study, on a series of point-mutated proteins, reveals many important new features, which we are trying to rationalize by QM/MM calculations at the CASPT2/CASSCF (12,12)/6-31G*/AMBER level. Interestingly, some mutations (V112N and L83Q) cancel the color sensor effect: Dark- and light-adapted proteins have the same absorption spectra, and both isomers turn out to display the same excited state lifetime. Notably, and unlike mutations of other retinal proteins, the ESL of the AT isomer is significantly reduced (0.2 ps). We will systematically compare the reaction dynamics (ESL, QY, and vibrational coherences) in the context of an optimized biological function of the wild-type protein.