Erratum to “Geometries and Vertical Excitation Energies in Retinal Analogues Resolved at the CASPT2 Level of Theory: Critical Assessment of the Performance of CASSCF, CC2, and DFT Methods”

Erratum to “Geometries and Vertical Excitation Energies in Retinal Analogues Resolved at the CASPT2 Level of Theory: Critical Assessment of the Performance of CASSCF, CC2, and DFT Methods

Assessing the Accuracy of Various Ab Initio Methods for Geometries and Excitation Energies of Retinal Chromophore Minimal Model by Comparison with CASPT3 Results

The effect of the quality of the ground-state geometry on excitation energies in the retinal chromophore minimal model (PSB3) was systematically investigated using various single- (within Møller–Plesset and coupled-cluster frameworks) and multiconfigurational [within complete active space self-consistent field (CASSCF) and CASSCF-based perturbative approaches: second-order CASPT2 and third-order CASPT3] methods. Among investigated methods, only CASPT3 provides geometry in nearly perfect agreement with the CCSD­(T)-based equilibrium structure. The second goal of the present study was to assess the performance of the CASPT2 methodology, which is popular in computational spectroscopy of retinals, in describing the excitation energies of low-lying excited states of PSB3 relative to CASPT3 results. The resulting CASPT2 excitation energy error is up to 0.16 eV for the <i>S</i>₀ → <i>S</i>₁ transition but only up to 0.06 eV for the <i>S</i>₀ → <i>S</i>₂ transition. Furthermore, CASPT3 excitation energies practically do not depend on modification of the zeroth-order Hamiltonian (so-called IPEA shift parameter), which does dramatically and nonsystematically affect CASPT2 excitation energies

Mechanism of Co–C Bond Photolysis in the Base-On Form of Methylcobalamin

A mechanism of Co–C bond photodissociation in the base-on form of the methylcobalamin cofactor (MeCbl) has been investigated employing time-dependent density functional theory (TD-DFT), in which the key step involves singlet radical pair generation from the first electronically excited state (S₁). The corresponding potential energy surface of the S₁ state was constructed as a function of Co–C and Co–N_axial bond distances, and two possible photodissociation pathways were identified on the basis of energetic grounds. These pathways are distinguished by whether the Co–C bond (path A) or Co–N_axial bond (path B) elongates first. Although the final intermediate of both pathways is the same (namely a ligand field (LF) state responsible for Co–C dissociation), the reaction coordinates associated with paths A and B are different. The photolysis of MeCbl is wavelength-dependent, and present TD-DFT analysis indicates that excitation in the visible α/β band (520 nm) can be associated with path A, whereas excitation in the near-UV region (400 nm) is associated with path B. The possibility of intersystem crossing, and internal conversion to the ground state along path B are also discussed. The mechanism proposed in this study reconciles existing experimental data with previous theoretical calculations addressing the possible involvement of a repulsive triplet state

Effects of the Protein Environment on the Spectral Properties of Tryptophan Radicals in <i>Pseudomonas aeruginosa</i> Azurin

Many biological electron-transfer reactions involve short-lived tryptophan radicals as key reactive intermediates. While these species are difficult to investigate, the recent photogeneration of a long-lived neutral tryptophan radical in two <i>Pseudomonas aeruginosa</i> azurin mutants (Az48W and ReAz108W) made it possible to characterize the electronic, vibrational, and magnetic properties of such species and their sensitivity to the molecular environment. Indeed, in Az48W the radical is embedded in the hydrophobic core while, in ReAz108W it is solvent-exposed. Here we use density functional theory and multiconfigurational perturbation theory to construct quantum-mechanics/molecular-mechanics models of Az48W^• and ReAz108W^• capable of reproducing specific features of their observed UV–vis, resonance Raman, and electron paramagnetic resonance spectra. The results show that the models can correctly replicate the spectral changes imposed by the two contrasting hydrophobic and hydrophilic environments. Most importantly, the same models can be employed to disentangle the molecular-level interactions responsible for such changes. It is found that the control of the hydrogen bonding between the tryptophan radical and a single specific surface water molecule in ReAz108W^• represents an effective means of spectral modulation. Similarly, a specific electrostatic interaction between the radical moiety and a Val residue is found to control the Az48W^• excitation energy. These modulations appear to be mediated by the increase in nitrogen negative charge (and consequent increase in hydrogen bonding) of the spectroscopic D₂ state with respect to the D₀ state of the chromophore. Finally, the same protein models are used to predict the relaxed Az48W^• and ReAz108W^• D₂ structures, showing that the effect of the environment on the corresponding fluorescence maxima must parallel that of D₀ absorption spectra

Experimental Assessment of the Electronic and Geometrical Structure of a Near-Infrared Absorbing and Highly Fluorescent Microbial Rhodopsin

The recently discovered Neorhodopsin (NeoR) exhibits absorption and emission maxima in the near-infrared spectral region, which together with the high fluorescence quantum yield makes it an attractive retinal protein for optogenetic applications. The unique optical properties can be rationalized by a theoretical model that predicts a high charge transfer character in the electronic ground state (S0) which is otherwise typical of the excited state S1 in canonical retinal proteins. The present study sets out to assess the electronic structure of the NeoR chromophore by resonance Raman (RR) spectroscopy since frequencies and relative intensities of RR bands are controlled by the ground and excited state’s properties. The RR spectra of NeoR differ dramatically from those of canonical rhodopsins but can be reliably reproduced by the calculations carried out within two different structural models. The remarkable agreement between the experimental and calculated spectra confirms the consistency and robustness of the theoretical approach

Experimental Assessment of the Electronic and Geometrical Structure of a Near-Infrared Absorbing and Highly Fluorescent Microbial Rhodopsin

The recently discovered Neorhodopsin (NeoR) exhibits absorption and emission maxima in the near-infrared spectral region, which together with the high fluorescence quantum yield makes it an attractive retinal protein for optogenetic applications. The unique optical properties can be rationalized by a theoretical model that predicts a high charge transfer character in the electronic ground state (S0) which is otherwise typical of the excited state S1 in canonical retinal proteins. The present study sets out to assess the electronic structure of the NeoR chromophore by resonance Raman (RR) spectroscopy since frequencies and relative intensities of RR bands are controlled by the ground and excited state’s properties. The RR spectra of NeoR differ dramatically from those of canonical rhodopsins but can be reliably reproduced by the calculations carried out within two different structural models. The remarkable agreement between the experimental and calculated spectra confirms the consistency and robustness of the theoretical approach