Three-dimensional view of ultrafast dynamics in photoexcited bacteriorhodopsin

Gabriela Nass Kovács ,Sébastien Boutet,Marie Luise Grünbein,M Stricker ,Yang Yang,J.p Colletier ,Tatiana Domratcheva,Raphael Gasper,Marco Kloos,A Batyuk ,S Haacke ,Jochen Reinstein,M Hilpert ,Matthew Seaberg,Sergio Carbajo,Robert L Shoeman,R Bruce Doak,Thomas R M Barends,Joachim Heberle,A Gorel ,Till Stensitzki,Karsten Heyne,Jason E Koglin,L Foucar ,D Ehrenberg ,Ramona Schlesinger,C.m Roome ,Ilme Schlichting

doi:10.1038/s41467-019-10758-0

Abstract

Bacteriorhodopsin (bR) is a light-driven proton pump. The primary photochemical event upon light absorption is isomerization of the retinal chromophore. Here we used time-resolved crystallography at an X-ray free-electron laser to follow the structural changes in multiphoton-excited bR from 250 femtoseconds to 10 picoseconds. Quantum chemistry and ultrafast spectroscopy were used to identify a sequential two-photon absorption process, leading to excitation of a tryptophan residue flanking the retinal chromophore, as a first manifestation of multiphoton effects. We resolve distinct stages in the structural dynamics of the all-trans retinal in photoexcited bR to a highly twisted 13-cis conformation. Other active site sub-picosecond rearrangements include correlated vibrational motions of the electronically excited retinal chromophore, the surrounding amino acids and water molecules as well as their hydrogen bonding network. These results show that this extended photo-active network forms an electronically and vibrationally coupled system in bR, and most likely in all retinal proteins.

Highlights

The all-trans→13-cis photoisomerization reaction (Fig. 1a) has been mapped out initially by ultrafast spectroscopy[5,6,7] and calculations[8]. These studies indicate that the Franck–Condon (FC) region of photoexcited all-trans retinal depopulates by relaxation along high frequency stretching modes to form the excited (S1) electronic state I intermediate with a time constant τ of ~0.2 ps
When studying reactions by spectroscopy and crystallography, one has to bear in mind that reactions in solutions and crystals can differ due to effects of crystal packing and/or differences in composition of the buffer and crystallization mother liquor
The use of intense ultrafast optical pump pulses can entail a number of undesired multiphoton effects

Summary

Introduction

Bacteriorhodopsin (bR) is a light-driven proton pump. The primary photochemical event upon light absorption is isomerization of the retinal chromophore. The all-trans→13-cis photoisomerization reaction (Fig. 1a) has been mapped out initially by ultrafast spectroscopy[5,6,7] and calculations[8] These studies indicate that the Franck–Condon (FC) region of photoexcited all-trans retinal depopulates by relaxation along high frequency stretching modes to form the excited (S1) electronic state I intermediate with a time constant τ of ~0.2 ps. Transient absorption (TA) spectroscopy and quantum chemical calculations were used to characterize both single and multiphoton effects, which are expected to dominate in the TR-SFX experiments The latter show distinct phases in the evolution of the twisting C13=C14 retinal bond and long-range correlated dynamics preceding retinal isomerization. We compare our comprehensive study with a recently published related investigation[10]

Methods

Results

Conclusion