Structure, Spectroscopy, and Spectral Tuning of the Gas-Phase Retinal Chromophore: The β-Ionone “Handle” and Alkyl Group Effect

The low-lying singlet states (i.e. S0, S1, and S2) of the chromophore of rhodopsin, the protonated Schiff base of 11-cis-retinal (PSB11), and of its all-trans photoproduct have been studied in isolated conditions by using ab initio multiconfigurational second-order perturbation theory. The computed spectroscopic features include the vertical excitation, the band origin, and the fluorescence maximum of both isomers. On the basis of the S0-->S1 vertical excitation, the gas-phase absorption maximum of PSB11 is predicted to be 545 nm (2.28 eV). Thus, the predicted absorption maximum appears to be closer to that of the rhodopsin pigment (2.48 eV) and considerably red-shifted with respect to that measured in solution (2.82 eV in methanol). In addition, the absorption maxima associated with the blue, green, and red cone visual pigments are tentatively rationalized in terms of the spectral changes computed for PSB11 structures featuring differently twisted beta-ionone rings. More specifically, a blue-shifted absorption maximum is explained in terms of a large twisting of the beta-ionone ring (with respect to the main conjugated chain) in the visual S-cone (blue) pigment chromophore. In contrast, the chromophore of the visual L-cone (red) pigment is expected to have a nearly coplanar beta-ionone ring yielding a six double bond fully conjugated framework. Finally, the M-cone (green) chromophore is expected to feature a twisting angle between 10 and 60 degrees. The spectroscopic effects of the alkyl substituents on the PSB11 spectroscopic properties have also been investigated. It is found that they have a not negligible stabilizing effect on the S1-S0 energy gap (and, thus, cause a red shift of the absorption maximum) only when the double bond of the beta-ionone ring conjugates significantly with the rest of the conjugated chain.

The retinal chromophore/chloride ion pair: structure of the photoisomerization path and interplay of charge transfer and covalent states

Ab initio multi-reference second-order perturbation theory computations are used to explore the photochemical behavior of two ion pairs constituted by a chloride counterion interacting with either a rhodopsin or bacteriorhodopsin chromophore model (i.e., the 4-cis-gamma-methylnona-2,4,6,8-tetraeniminium and all-trans-nona-2,4,6,8-tetraeniminium cations, respectively). Significant counterion effects on the structure of the photoisomerization paths are unveiled by comparison with the paths of the same chromophores in vacuo. Indeed, we demonstrate that the counterion (i) modulates the relative stability of the S0, S1, and S2 energy surfaces leading to an S1 isomerization energy profile where the S1 and S2 states are substantially degenerate; (ii) leads to the emergence of significant S1 energy barriers along all of the isomerization paths except the one mimicking the 11-cis --> all-trans isomerization of the rhodopsin chromophore model; and (iii) changes the nature of the S1 --> S0 decay funnel that becomes a stable excited state minimum when the isomerizing double bond is located at the center of the chromophore moiety. We show that these (apparently very different) counterion effects can be rationalized on the basis of a simple qualitative electrostatic model, which also provides a crude basis for understanding the behavior of retinal protonated Schiff bases in solution.

Photoisomerization Mechanism of 11-cis-Locked Artificial Retinal Chromophores: Acceleration and Primary Photoproduct Assignment

CASPT2//CASSCF/6-31G photochemical reaction path computations for two 4-cis-nona-2,4,6,8-tetraeniminium cation derivatives, with the 4-cis double bond embedded in a seven- and eight-member ring, are carried out to model the reactivity of the corresponding ring-locked retinal chromophores. The comparison of the excited state branches of the two reaction paths with that of the native chromophore, is used to unveil the factors responsible for the remarkably short (60 fs) excited state (S(1)) lifetime observed when an artificial rhodopsin containing an eight member ring-locked retinal is photoexcited. Indeed, it is shown that the strain imposed by the eight-member ring on the chromophore backbone leads to a dramatic change in the shape of the S(1) energy surface. Our models are also used to investigate the nature of the primary photoproducts observed in different artificial rhodopsins. It is seen that only the eight member ring-locked retinal model can access a shallow energy minimum on the ground state. This result implies that the primary, photorhodopsin-like, transient observed in artificial rhodopsins could correspond to a shallow excited state minimum. Similarly, the second, bathorhodopsin-like, transient species could be assigned to a ground state structure displaying a nearly all-trans conformation.

Structure, initial excited-state relaxation, and energy storage of rhodopsin resolved at the multiconfigurational perturbation theory level

We demonstrate that a "brute force" quantum chemical calculation based on an ab initio multiconfigurational second order perturbation theory approach implemented in a quantum mechanics/molecular mechanics strategy can be applied to the investigation of the excited state of the visual pigment rhodopsin (Rh) with a computational error <5 kcal.mol(-1). As a consequence, the simulation of the absorption and fluorescence of Rh and its retinal chromophore in solution allows for a nearly quantitative analysis of the factors determining the properties of the protein environment. More specifically, we demonstrate that the Rh environment is more similar to the "gas phase" than to the solution environment and that the so-called "opsin shift" originates from the inability of the solvent to effectively "shield" the chromophore from its counterion. The same strategy is used to investigate three transient structures involved in the photoisomerization of Rh under the assumption that the protein cavity does not change shape during the reaction. Accordingly, the analysis of the initially relaxed excited-state structure, the conical intersection driving the excited-state decay, and the primary isolable bathorhodopsin intermediate supports a mechanism where the photoisomerization coordinate involves a "motion" reminiscent of the so-called bicycle-pedal reaction coordinate. Most importantly, it is shown that the mechanism of the approximately 30 kcal.mol(-1) photon energy storage observed for Rh is not consistent with a model based exclusively on the change of the electrostatic interaction of the chromophore with the protein/counterion environment.

Mechanism of the N-Cyclopropylimine-1-pyrroline Photorearrangement

We present here a combined experimental and computational investigation into the photorearrangement of N-cyclopropylimines to yield pyrrolines. We show that the photochemistry, regiochemistry, and stereochemistry of the reaction can be understood in terms of a mechanism involving barrierless evolution in three different (S(2), S(1), S(0)) singlet states and sequential decay through two different (S(2)/S(1), and S(1)/S(0)) conical intersection funnels. We provide evidence that the reaction mechanism involves the generation of a nonequilibrated (i.e., transient) excited state diradical, whose decay can lead not only to pyrrolines but also to cyclopropylimine isomers. It is concluded that the reaction outcome depends on the details of the structure of such transient diradical and on the nature of the dynamics of its decay through the S(1)/S(0) conical intersection.

Counterion Controlled Photoisomerization of Retinal Chromophore Models: a Computational Investigation

CASPT2//CASSCF photoisomerization path computations have been used to unveil the effects of an acetate counterion on the photochemistry of two retinal protonated Schiff base (PSB) models: the 2-cis-penta-2,4-dieniminium and the all-trans-epta-2,4,6-trieniminium cations. Different positions/orientations of the counterion have been investigated and related to (i) the spectral tuning and relative stability of the S0, S1, and S2 singlet states; (ii) the selection of the photochemically relevant excited state; (iii) the control of the radiationless decay and photoisomerization rates; and, finally, (iv) the control of the photoisomerization stereospecificity. A rationale for the results is given on the basis of a simple (electrostatic) qualitative model. We show that the model readily explains the computational results providing a qualitative explanation for different aspects of the experimentally observed "environment" dependent PSB photochemistry. Electrostatic effects likely involved in controlling retinal photoisomerization stereoselectivity in the protein are also discussed under the light of these results, and clues for a stereocontrolled electrostatically driven photochemical process are presented. These computations provide a rational basis for the formulation of a mechanistic model for photoisomerization electrostatic catalysis.

Design and photochemical characterization of a biomimetic light-driven Z/E switcher

Protonated Schiff bases (PSBs) of polyenals constitute a class of light-driven switchers selected by biological evolution that provide model compounds for the development of artificial light-driven molecular devices or motors. In the present paper, our primary target is to show, through combined computational and experimental studies, that it is possible to approach the design of artificial PSBs suitable for such applications. Below, we use the methods of computational photochemistry to design and characterize the prototype biomimetic molecular switchers 4-cyclopenten-2'-enylidene-3,4-dihydro-2H-pyrrolinium and its 5,5'-dimethyl derivative both containing the penta-2,4-dieniminium chromophore. To find support for the predicted behavior, we also report the photochemical reaction path of the synthetically accessible compound 4-benzylidene-3,4-dihydro-2H-pyrrolinium. We show that the preparation and photochemical characterization of this compound (together with three different N-methyl derivatives) provide both support for the predicted photoisomerization mechanism and information on its sensitivity to the molecular environment.

Origin, nature, and fate of the fluorescent state of the green fluorescent protein chromophore at the CASPT2//CASSCF resolution

Ab initio CASPT2//CASSCF relaxation path computations are employed to determine the intrinsic (e.g., in vacuo) mechanism underlying the rise and decay of the luminescence of the anionic form of the green fluorescent protein (GFP) fluorophore. Production and decay of the fluorescent state occur via a two-mode reaction coordinate. Relaxation along the first (totally symmetric) mode leads to production of the fluorescent state that corresponds to a planar species. The second (out-of-plane) mode controls the fluorescent state decay and mainly corresponds to a barrierless twisting of the fluorophore phenyl moiety. While a "space-saving" hula-twist conical intersection decay channel is found to lie only 5 kcal mol(-1) above the fluorescent state, the direct involvement of a hula-twist deformation in the decay is not supported by our data. The above results indicate that the ultrafast fluorescence decay observed for the GFP chromophore in solution is likely to have an intrinsic origin. The possible effects of the GFP protein cavity on the fluorescence lifetime of the investigated chromophore model are discussed.

Photoactivation of the photoactive yellow protein: why photon absorption triggers a trans-to-cis Isomerization of the chromophore in the protein

Atomistic QM/MM simulations have been carried out on the complete photocycle of Photoactive Yellow Protein, a bacterial photoreceptor, in which blue light triggers isomerization of a covalently bound chromophore. The "chemical role" of the protein cavity in the control of the photoisomerization step has been elucidated. Isomerization is facilitated due to preferential electrostatic stabilization of the chromophore's excited state by the guanidium group of Arg52, located just above the negatively charged chromophore ring. In vacuo isomerization does not occur. Isomerization of the double bond is enhanced relative to isomerization of a single bond due to the steric interactions between the phenyl ring of the chromophore and the side chains of Arg52 and Phe62. In the isomerized configuration (ground-state cis), a proton transfer from Glu46 to the chromophore is far more probable than in the initial configuration (ground-state trans). It is this proton transfer that initiates the conformational changes within the protein, which are believed to lead to signaling.

A Fast Photoswitch for Minimally Perturbed Peptides: Investigation of the trans → cis Photoisomerization of N-Methylthioacetamide

Thio amino acids can be integrated into the backbone of peptides without significantly perturbing their structure. In this contribution we use ultrafast infrared and visible spectroscopy as well as state-of-the-art ab initio computations to investigate the photoisomerization of the trans form of N-methylthioacetamide (NMTAA) as a model conformational photoswitch. Following the S2 excitation of trans-NMTAA in water, the return of the molecule into the trans ground state and the formation of the cis isomer is observed on a dual time scale, with a fast component of 8-9 ps and a slow time constant of approximately 250 ps. On both time scales the probability of isomerization to the cis form is found to be 30-40%, independently of excitation wavelength. Ab initio CASPT2//CASSCF photochemical reaction path calculations indicate that, in vacuo, the trans-->cis isomerization event takes place on the S1 and/or T1 triplet potential energy surfaces and is controlled by very small energy barriers, in agreement with the experimentally observed picosecond time scale. Furthermore, the calculations identify one S2/S1 and four nearly isoenergetic S1/S0 conical intersection decay channels. In line with the observed isomerization probability, only one of the S1/S0 conical intersections yields the cis conformation upon S1-->S0 decay. A substantially equivalent excited-state relaxation results from four T1/S0 intersystem crossing points.

Structure of the intersection space associated with Z/E photoisomerization of retinal in rhodopsin proteins

In this paper we employ a CASSCF/AMBER quantum-mechanics/molecular-mechanics tool to map the intersection space (IS) of a protein. In particular, we provide evidence that the S1 excited-state potential-energy surface of the visual photoreceptor rhodopsin is spanned by an IS segment located right at the bottom of the surface. Analysis of the molecular structures of the protein chromophore (a protonated Schiff base of retinal) along IS reveals a type of geometrical deformation not observed in vacuo. Such a structure suggests that conical intersections mediating different photochemical reactions reside along the same intersection space. This conjecture is investigated by mapping the intersection space of the rhodopsin chromophore model 2-Z-hepta-2,4,6-trieniminium cation and of the conjugated hydrocarbon 3-Z-deca-1,3,5,6,7-pentaene.

Excited-state singlet manifold and oscillatory features of a nonatetraeniminium retinal chromophore model

In this paper we use ab initio multireference Møller-Plesset second-order perturbation theory computations to map the first five singlet states (S(0), S(1), S(2), S(3), and S(4)) along the initial part of the photoisomerization coordinate for the isolated rhodopsin chromophore model 4-cis-gamma-methylnona-2,4,6,8-tetraeniminium cation. We show that this information not only provides an explanation for the spectral features associated to the chromophore in solution but also, subject to a tentative hypothesis on the effect of the protein cavity, may be employed to explain/assign the ultrafast near-IR excited-state absorption, stimulated emission, and transient excited-state absorption bands observed in rhodopsin proteins (e.g. rhodopsin and bacteriorhodopsin). We also show that the results of vibrational frequency computations reveal a general structure for the first (S(1)) excited-state energy surface of PSBs that is consistent with the existence of the coherent oscillatory motions observed both in solution and in bacteriorhodopsin.

Computational study on the origin of the stereoselectivity for the photochemical denitrogenation of diazabicycloheptene

The origin of the inversion stereoselectivity of housane formation via photochemical nitrogen extrusion of diazabicycloheptene (DBH) has been investigated using reaction path computations and multireference second-order perturbation theory within a CASPT2//CASSCF scheme. We show that the primary photoproduct of the reaction is an exo-axial conformer of the diazenyl diradical ((1) DZ) which displays a cyclopenta-1,3-diyl moiety with a Cs-like structure. (1) DZ is selectively generated via decay at a linear-axial conical intersection, and it is located in a shallow region of the ground state potential energy surface that provides access to five different reaction pathways. Reaction path analysis (including probing with classical trajectories) indicates that production of inverted housane can only occur via impulsive population of an axial-to-equatorial pathway, and it is thus inconsistent with thermal equilibration of the primary (1) DZ conformer. Similarly, according to the same analysis, the decrease of inversion stereoselectivity and even the retention (stereochemical memory effect) observed for suitably substituted DBHs are explained by dynamics effects where the axial-to-equatorial impulsive motion is restrained by the inertia and/or steric hindrance of the substituents. These results shade light on the poorly understood mechanisms that allow a photochemical reaction, in which a large amount of energy is deposited in the reactant by photon absorption, to show a high degree of stereoselectivity.

A simple approach for improving the hybrid MMVB force field: application to the photoisomerization of s-cis butadiene

MMVB is a QM/MM hybrid method, consisting of a molecular mechanics force field coupled to a valence bond Heisenberg Hamiltonian parametrized from ab initio CASSCF calculations on several prototype molecules. The Heisenberg Hamiltonian matrix elements Q(ij) and K(ij), whose expressions are partitioned here into a primary contribution and second-order correction terms, are calculated analytically in MMVB. When the original MMVB force field fails to produce potential energy surfaces accurate enough for dynamics calculations, we show that significant improvements can be made by refitting the second-order correction terms for the particular molecule(s) being studied. This "local" reparametrization is based on values of K(ij) extracted (using effective Hamiltonian techniques) from CASSCF calculations on the same molecule(s). The method is demonstrated for the photoisomerization of s-cis butadiene, and we explain how the correction terms that enabled a successful MMVB dynamics study [Garavelli, M.; Bernardi, F.; Olivucci, M.; Bearpark, M. J.; Klein, S.; Robb, M. A. J Phys Chem A 2001, 105, 11496] were refitted.

Probing the rhodopsin cavity with reduced retinal models at the CASPT2//CASSCF/AMBER level of theory

We show that the ab initio CASPT2//CASSCF strategy previously used to investigate the ground and excited states of the chromophore of the vision receptor rhodopsin (Rh) in vacuo can be successfully implemented in a QM/MM scheme allowing for CASPT2//CASSCF/AMBER geometry optimization and excited state property evaluation in proteins. Two receptor models (Rh-1 and Rh-2) incorporating different reduced chromophores are investigated. It is shown that Rh-2 features a chromophore equilibrium structure with the correct helicity and a lambdamax that is only 52 nm blue-shifted from the observed value. This result should open the way to a qualitatively correct ab initio QM/MM modeling of the early excited state transient species involved in the vision process.

Relationship between photoisomerization path and intersection space in a retinal chromophore model

A low-lying segment of the intersection space (IS) between the excited-state and the ground-state energy surfaces of a retinal chromophore model has been mapped using ab initio CASSCF computations. Analysis of the structural relationship between the computed IS cross-section and the excited state Z --> E isomerization path shows that these are remarkably close both in energy and in structure. Indeed, the IS segment and the Z --> E path remain roughly parallel and merge only when the double bond reaches a 70 degree twisting. This finding supports the idea that, in certain chromophores, a more extended segment of IS, and not a single conical intersection, contributes to the decay to the ground state.

Ground and excited state CASPT2 geometry optimizations of small organic molecules

A method for computing second-order multiconfigurational perturbation theory (CASPT2) energy gradients numerically has been implemented and applied to a range of elementary organic chromophores, including 1,3 butadiene, acrolein, and two protonated Schiff bases. Geometries of ground and excited states-as well as conical intersections-are compared with the corresponding CASSCF structures, illustrating the effect of including the correction for dynamical electron correlation. It is shown that the differences between the two methods are not readily categorized, but that, while individual changes in bond lengths can be quite large ( approximately 0.01-0.02 A), the natures and CASPT2 energetics of the structures remain similar. Exceptions to this tend to be systems that have a strong ionic character and that are not well described at the CASSCF level. Basis set effects (double- vs. triple-zeta) were examined for a limited number of examples, and found to be quite dramatic at both levels of theory.

Photoisomerization acceleration in retinal protonated Schiff-base models

The results of new and recently reported CASSCF/6-31G* photoisomerization path computations of a series of models of the 11-cis retinal chromophore of the visual pigment rhodopsin are discussed. The results indicate that, with respect to the chromophore in vacuo, certain structural, intramolecular and environmental factors are capable of speeding up the excited-state decay associated with the cis --> trans isomerization motion. Using suitable protonated Schiff-base models, it is shown that three structural factors can potentially speed up the isomerization: (i) reducing the length of the conjugated chain, (ii) twisting of the hydrocarbon end of the conjugated chain with respect to the protonated Schiff-base end and (iii) ring locking of the conjugated chain with an eight-membered ring. All these factors operate through increasing the slope of the excited-state energy surface and enhancing the coupling between stretching and torsional modes. We argue that the protein catalysis seen in rhodopsin may, at least partly, exploit the same principles.