Non-radiative relaxation mechanisms of electronically excited phenylalanine in model peptides

A systematic study of the non-radiative deactivation mechanisms of the three photoexcited N-acetylphenylalanylamide conformers was conducted in order to disclose the experimentally observed conformational dependent lifetime of phenyl vibrationless 1ππ* excited state. The all-atom trajectory surface hopping non-adiabatic molecular dynamics simulations, based on linear response time dependent density functional theory, were utilized for blind screening of relaxation pathways and revealed a number of excitation transfer mechanisms from 1ππ* to the 1nπ* excited states localized on each of the two amide groups. Possible pathways were further refined by obtaining conical intersection barrier energies from corresponding reaction paths constructed at the coupled cluster (CC2) level of theory. Finally, from semiclassical consideration of conical intersection accessibility with only the nuclear zero point vibrational energy and from the increased rigidity of the second amide group towards distorsion upon its methylation, it was concluded that the classically accessible part of conical intersection seam for population transfer to the 1nπ* state of the second amide group is the largest for the conformer with the shortest 1ππ* state lifetime

Neradijativni relaksacijski mehanizmi elektronski pobuđenog fenilalanina u modelnim peptidima

A systematic study of the non-radiative deactivation mechanisms of the three photoexcited N-acetylphenylalanylamide conformers was conducted in order to disclose the experimentally observed conformational dependent lifetime of phenyl vibrationless 1* excited state. The all-atom trajectory surface hopping non-adiabatic molecular dynamics simulations, based on linear response time dependent density functional theory, were utilized for blind screening of relaxation pathways and revealed a number of excitation transfer mechanisms from 1* to the 1n* excited states localized on each of the two amide groups. Possible pathways were further refined by obtaining conical intersection barrier energies from corresponding reaction paths constructed at the coupled cluster (CC2) level of theory. Finally, from semiclassical consideration of conical intersection accessibility with only the nuclear zero point vibrational energy and from the increased rigidity of the second amide group towards distorsion upon its methylation, it was concluded that the classically accessible part of conical intersection seam for population transfer to the 1n* state of the second amide group is the largest for the conformer with the shortest 1* state lifetime.Provedeno je sustavno istraživanje neradijativnih deaktivacijskih mehanizama odgovornih za eksperimentalno opaženu konformacijsku ovisnost vremena života vibracijski osnovnog pobuđenog fenilnog 1* elektronskog stanja u tri konformera N-acetilfenilalanilamida. Simulacije metodom neadijabatske molekulske dinamike, s preskokom među plohama elektronske potencijalne energije dobivenih vremenski ovisnom teorijom funkcionala gustoće, ukazale su na nekoliko mehanizama prijenosa ekscitacije iz 1* u 1n* stanja locirana na pojedinim amidnim grupama. Pronađeni mehanizmi potom su utočnjeni pripadajućim vrijednostima energijama barijera koničnih presjecišta dobivenih ih reakcijskih puteva izračunatim na razini teorije spregnutih grozdova (CC2). Konačno, iz poluklasičnog razmatranja dostupnosti koničnog presjecišta samo na temelju nuklearne vibracijske energije nulte točke te iz povećanja rigidnosti druge peptidne skupine naspram njene deformacije usljed metilacije, određeno je kako klasično dostupan dio šava koničnog presjecišta za prijenos populacije u 1n* stanje druge peptidne skupine je najveći u konformeru s najkraćim vremenom života pobuđenog 1* stanja

The ΔSCF method for non-adiabatic dynamics of systems in the liquid phase

Computational studies of ultrafast photoinduced processes give valuable insights into the photochemical mechanisms of a broad range of compounds. In order to accurately reproduce, interpret, and predict experimental results, which are typically obtained in a condensed phase, it is indispensable to include the condensed phase environment in the computational model. However, most studies are still performed in vacuum due to the high computational cost of state-of-the-art non-adiabatic molecular dynamics (NAMD) simulations. The quantum mechanical/molecular mechanical (QM/MM) solvation method has been a popular model to perform photodynamics in the liquid phase. Nevertheless, the currently used QM/MM embedding techniques cannot sufficiently capture all solute–solvent interactions. In this Perspective, we will discuss the efficient ΔSCF electronic structure method and its applications with respect to the NAMD of solvated compounds, with a particular focus on explicit quantum mechanical solvation. As more research is required for this method to reach its full potential, some challenges and possible directions for future research are presented as well

On the Possible Role of an Intermolecular Charge Transfer State in the Excitation of the Biologically Active Bond of the Retinal Chromophore-counterion Pair

In non-polar solvents the protonation of the all-trans Schiff base of retinal (SBR+) using strong acids leads to formation of retinal chromophore-counterion pairs. Previously we have shown that the main non-reactive deactivation channel of these ion pairs involves internal conversion from the initially excited ππ* state to an inter-molecular charge transfer state (inter-CT) with subsequent dissociation of the chromophore-counterion pair. In solution this leads to the abortion of isomerization. Motivated by the recent X-ray diffraction experiments showing that the disruption of the hydrogen-bonded network of counterions precedes the isomerization of all-trans SBR+ in bacteriorhodopsin we decided to take a closer look at the dynamics of the chromophore-counterion pair in the inter-CT state. Using constrained non-adiabatic dynamics simulations in which the dissociation of the chromophore-counterion pair was impeded, we show that the charge distribution in the inter-CT state leads to site-specific elongation of the biologically active C13=C14 bond. On this basis we hypothesize that an inter-molecular charge transfer state involving the retinal chromophore and the H-bonded counterions (S2) may play an active role in the photoisomerization reaction in a constraint environment. This work is licensed under a Creative Commons Attribution 4.0 International License

The photodissociation of solvated cyclopropanone and its hydrate explored via non-adiabatic molecular dynamics using ΔSCF

The decay of cyclopropanone is a typical example of a photodecomposition process. Ethylene and carbon monoxide are formed following the excitation to the first singlet excited state through a symmetrical or asymmetrical pathway. The results obtained with non-adiabatic molecular dynamics (NAMD) using the delta self-consistent field (ΔSCF) method correspond well to previous experimental and multireference theoretical studies carried out in the gas phase. Moreover, this efficient methodology allows NAMD simulations of cyclopropanone in aqueous solution to be performed, which reveal analogue deactivation mechanisms, but a shorter lifetime and reduced photodissociation as compared to the gas-phase. The excited state dynamics of cyclopropanone hydrate, an enzyme inhibitor, in an aqueous environment are reported as well. Cyclopropanone hydrate strongly interacts with the surrounding solvent via the formation of hydrogen bonds. Excitation to the first singlet excited state shows an asymmetric pathway with cyclopropanone hydrate and propionic acid as the main photoproducts

Trajectory Surface Hopping Nonadiabatic Molecular Dynamics with Kohn–Sham ΔSCF for Condensed-Phase Systems

We present an efficient approach for surface hopping-based nonadiabatic dynamics in the condensed phase. For the systems studied, a restricted Kohn–Sham orbital formulation of the delta self-consistent field (ΔSCF) method was used for efficient calculation of excited electronic states. Time-dependent density functional theory (DFT) is applied to aid excited-state SCF convergence and provide guess electronic state densities. Aside from that the Landau–Zener procedure simplifies the surface hopping between electronic states. By utilizing the combined Gaussian and plane waves approach with periodic boundary conditions the method is easily applicable to full atomistic DFT simulations of condensed-phase systems and was used to study the nonradiative deactivation mechanism of photoexcited diimide in water solution

Nonradiative Relaxation Mechanisms of UV Excited Phenylalanine Residues: A Comparative Computational Study

The present work is directed toward understanding the mechanisms of excited state deactivation in three neutral model peptides containing the phenylalanine residue. The excited state dynamics of theγL(g+)folded form of N-acetylphenylalaninylamide (NAPA B) and its amide-N-methylated derivative (NAPMA B) is reviewed and compared to the dynamics of the monohydrated structure of NAPA (NAPAH). The goal is to unravel how the environment, and in particular solvation, impacts the photodynamics of peptides. The systems are investigated using reaction path calculations and surface hopping nonadiabatic dynamics based on the coupled cluster doubles (CC2) method and time-dependent density functional theory. The work emphasizes the role that excitation transfer from the phenylππ*to amidenπ*state plays in the deactivation of the three systems and shows how the ease of out-of-plane distortions of the amide group determines the rate of population transfer between the two electronic states. The subsequent dynamics on thenπ*state is barrierless along several pathways and leads to fast deactivation to the ground electronic state

ΔSCF with Subsystem Density Embedding for Efficient Nonadiabatic Molecular Dynamics in Condensed-Phase Systems

An approach combining subsystem density embedding with the variational delta self-consistent field is presented, which extends current capabilities for excited-electronic-state calculations. It was applied on full-atomic nonadiabatic dynamics of a solvated diimide system, demonstrating that comparable accuracy can be achieved for this system for the investigated configuration space and with a shorter simulation time than the computationally more expensive conventional Kohn–Sham density functional theory-based method. This opens a new pragmatic technique for efficient simulation of nonadiabatic processes in the condensed phase, in particular, for liquids

Spin–Orbit Couplings for Nonadiabatic Molecular Dynamics at the ΔSCF Level

A procedure for the calculation of spin–orbit coupling (SOC) at the delta self-consistent field (ΔSCF) level of theory is presented. Singlet and triplet excited electronic states obtained with the ΔSCF method are expanded into a linear combination of singly excited Slater determinants composed of ground electronic state Kohn–Sham orbitals. This alleviates the nonorthogonality between excited and ground electronic states and introduces a framework, similar to the auxiliary wave function at the time-dependent density functional theory (TD-DFT) level, for the calculation of observables. The ΔSCF observables of the formaldehyde system were compared to reference TD-DFT values. Our procedure gives all components (energies, gradients, nonadiabatic couplings, and SOC terms) at the ΔSCF level of theory for conducting efficient, full-atomistic nonadiabatic molecular dynamics with intersystem crossing, particularly in condensed phase systems