QM/MM Photodynamics of Retinal in the Channelrhodopsin Chimera C1C2 with OM3/MRCI

Deciphering Photoreceptors Through Atomistic Modeling from Light Absorption to Conformational Response

In this review, we discuss the successes and challenges of the atomistic modeling of photoreceptors. Throughout our presentation, we integrate explanations of the primary methodological approaches, ranging from quantum mechanical descriptions to classical enhanced sampling methods, all while providing illustrative examples of their practical application to specific systems. To enhance the effectiveness of our analysis, our primary focus has been directed towards the examination of applications across three distinct photoreceptors. These include an example of Blue Light-Using Flavin (BLUF) domains, a bacteriophytochrome, and the orange carotenoid protein (OCP) employed by cyanobacteria for photoprotection. Particular emphasis will be placed on the pivotal role played by the protein matrix in fine-tuning the initial photochemical event within the embedded chromophore. Furthermore, we will investigate how this localized perturbation initiates a cascade of events propagating from the binding pocket throughout the entire protein structure, thanks to the intricate network of interactions between the chromophore and the protein.

Modeling pH-Dependent Biomolecular Photochemistry

The tuning mechanism of pH can be extremely challenging to model computationally in complex biological systems, especially with respect to the photochemical properties. This article reports a protocol aimed at modeling pH-dependent photodynamics using a combination of constant-pH molecular dynamics and semiclassical nonadiabatic molecular dynamics simulations. With retinal photoisomerization in Anabaena sensory rhodopsin (ASR) as a testbed, we show that our protocol produces pH-dependent photochemical properties, such as the isomerization quantum yield or decay rates. We decompose our results into single-titrated residue contributions, identifying some key tuning amino acids. Additionally, we assess the validity of the single protonation state picture to represent the system at a given pH and propose the most populated protein charge state as a compromise between cost and accuracy.

MoBioTools: A toolkit to setup quantum mechanics/molecular mechanics calculations

We present a toolkit that allows for the preparation of QM/MM input files from a conformational ensemble of molecular geometries. The package is currently compatible with trajectory and topology files in Amber, CHARMM, GROMACS and NAMD formats, and has the possibility to generate QM/MM input files for Gaussian (09 and 16), Orca (≥4.0), NWChem and (Open)Molcas. The toolkit can be used in command line, so that no programming experience is required, although it presents some features that can also be employed as a python application programming interface. We apply the toolkit in four situations in which different electronic-structure properties of organic molecules in the presence of a solvent or a complex biological environment are computed: the reduction potential of the nucleobases in acetonitrile, an energy decomposition analysis of tyrosine interacting with water, the absorption spectrum of an azobenzene derivative integrated into a voltage-gated ion channel, and the absorption and emission spectra of the luciferine/luciferase complex. These examples show that the toolkit can be employed in a manifold of situations for both the electronic ground state and electronically excited states. It also allows for the automatic correction of the active space in the case of CASSCF calculations on an ensemble of geometries, as it is shown for the azobenzene derivative photoswitch case.

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

Structural Factors Determining the Absorption Spectrum of Channelrhodopsins: A Case Study of the Chimera C1C2

Channelrhodopsins are photosensitive proteins that trigger flagella motion in single-cell algae and have been successfully utilized in optogenetic applications. In optogenetics, light is used to activate neural cells in living organisms, which can be achieved by exploiting the ion channel signaling of channelrhodopsins. Tailoring channelrhodopsins for such applications includes the tuning of the absorption maximum. In order to establish rational design and to obtain a desired spectral shift, a basic understanding of the absorption spectrum is required. We have studied the chimera C1C2 as a representative of this protein family and the first member with an available crystal structure. For this purpose, we sampled the conformations of C1C2 using quantum mechanical/molecular mechanical molecular dynamics and subjected the resulting snapshots of the trajectory to excitation energy calculations using ADC(2) and simplified time-dependent density functional theory. In contrast to previous reports, we found that different hydrogen-bonding networks-involving the retinal protonated Schiff base, the putative counterions E162 and D292, and water molecules-had only a small impact on the absorption spectrum. However, in the case of deprotonated E162, increasing the distance to the Schiff base hydrogen-bonding partner led to a systematic blue shift. The β-ionone ring rotation was identified as another important contributor. Yet the most important factors were found to be the bond length alternation and bond order alternation that were linearly correlated to the absorption maximum by up to 62 and 82%, respectively. We ascribe this novel insight into the structural basis of the absorption spectrum to our enhanced protein setup that includes membrane embedding as well as long and extensive sampling.

Electrostatic Control of Photoisomerization in Channelrhodopsin 2

Channelrhodopsin 2 (ChR2) is the most commonly used tool in optogenetics. Because of its faster photocycle compared to wild-type (WT) ChR2, the E123T mutant of ChR2 is a useful optogenetic tool when fast neuronal stimulation is needed. Interestingly, in spite of its faster photocycle, the initial step of the photocycle in E123T (photoisomerization of retinal protonated Schiff base or RPSB) was found experimentally to be much slower than that of WT ChR2. The E123T mutant replaces the negatively charged E123 residue with a neutral T123 residue, perturbing the electric field around the RPSB. Understanding the RPSB photoisomerization mechanism in ChR2 mutants will provide molecular-level insights into how ChR2 photochemical reactivity can be controlled, which will lay the foundation for improving the design of optogenetic tools. In this work, we combine ab initio nonadiabatic dynamics simulation, excited state free energy calculation, and reaction path search to comprehensively characterize the RPSB photoisomerization mechanism in the E123T mutant of ChR2. Our simulation agrees with previous experiments in predicting a red-shifted absorption spectrum and significant slowdown of photoisomerization in the E123T mutant. Interestingly, our simulations predict similar photoisomerization quantum yields for the mutant and WT despite the differences in excited-state lifetime and absorption maximum. Upon mutation, the neutralization of the negative charge on the E123 residue increases the isomerization barrier, alters the reaction pathway, and changes the relative stability of two fluorescent states. Our findings provide new insight into the intricate role of the electrostatic environment on the RPSB photoisomerization mechanism in microbial rhodopsins.

Non-adiabatic Excited-State Molecular Dynamics: Theory and Applications for Modeling Photophysics in Extended Molecular Materials

Optically active molecular materials, such as organic conjugated polymers and biological systems, are characterized by strong coupling between electronic and vibrational degrees of freedom. Typically, simulations must go beyond the Born-Oppenheimer approximation to account for non-adiabatic coupling between excited states. Indeed, non-adiabatic dynamics is commonly associated with exciton dynamics and photophysics involving charge and energy transfer, as well as exciton dissociation and charge recombination. Understanding the photoinduced dynamics in such materials is vital to providing an accurate description of exciton formation, evolution, and decay. This interdisciplinary field has matured significantly over the past decades. Formulation of new theoretical frameworks, development of more efficient and accurate computational algorithms, and evolution of high-performance computer hardware has extended these simulations to very large molecular systems with hundreds of atoms, including numerous studies of organic semiconductors and biomolecules. In this Review, we will describe recent theoretical advances including treatment of electronic decoherence in surface-hopping methods, the role of solvent effects, trivial unavoided crossings, analysis of data based on transition densities, and efficient computational implementations of these numerical methods. We also emphasize newly developed semiclassical approaches, based on the Gaussian approximation, which retain phase and width information to account for significant decoherence and interference effects while maintaining the high efficiency of surface-hopping approaches. The above developments have been employed to successfully describe photophysics in a variety of molecular materials.

Nonadiabatic Photodynamics of Retinal Protonated Schiff Base in Channelrhodopsin 2

Channelrhodopsin 2 (ChR2) is a light-gated ion channel and an important tool in optogenetics. Photoisomerization of retinal protonated Schiff base (RPSB) in ChR2 triggers channel activation. Despite the importance of ChR2 in optogenetics, the detailed mechanism for photoisomerization and channel activation is still not fully understood. Here, we report on computer simulations to investigate the photoisomerization mechanism and its effect on the activation of ChR2. Nonadiabatic dynamics simulation of ChR2 was carried out using the ab initio multiple spawning (AIMS) method and quantum mechanics/molecular mechanics (QM/MM) with a restricted ensemble Kohn-Sham (REKS) treatment of the QM region. Our results agree well with spectroscopic measurements and reveal that the RPSB isomerization is highly specific around the C₁₃=C₁₄ bond and follows the "aborted bicycle-pedal" mechanism. In addition, RPSB photoisomerization facilitates its deprotonation and partially increases the hydration level in the channel, which could trigger subsequent channel opening and ion conduction.