Volker Buss

Electronic structure calculations on helicenes. Concerning the chirality of helically twisted aromatic systems

Abstract CNDO/S calculations including configuration interaction of up to 300 singly excited states have been performed on the MMP2 and PM3 optimized geometries of [ n ]-helicenes, with n from 3 to 0. Oscillator and rotatory strengths have been calculated for all states down to 200 nm, with generally good agreement in the long wavelength regions. The computed data are analyzed on the basis of a four-orbital model yielding two forbidden L and two allowed B states. The latter ones including signs correlate with two high intensity absorptions of opposite signs observed in the spectra of all helicenes except the smallest one. The description in terms of tangential and radial transition dipoles provides a physical picture of the excited states. It is shown that the −, + pattern of these bands is conclusive evidence for the M -helical absolute configuration of these compounds.

Inherent chirality of the retinal chromophore in rhodopsin - a nonempirical theoretical analysis of chiroptical data.

CASSCF and GAUSSIAN CIS calculations were performed on ground and excited states of different conformations of 11-cis-retinal protonated Schiff bases, the chromophore of rhodopsin, in order to study their chiroptical properties and attempt a correlation between absolute conformation and CD-spectral data. Geometries were taken from molecular models, from published rhodopsin models, and from retinal conformations obtained from molecular dynamics with geometry restraints. In all the cases studied we find that a positive sense of twist about the C12-C13 bond correlates with a calculated positive CD of the long wavelength absorption band; the twist of the C6-C7 bond modulates this primary contribution of the C12-C13 bond. The correlation of the beta-band with structural features is not straightforward. Calculations on bathorhodopsin lend support to the idea that this intermediate is a highly twisted all-trans-conformation.

All-trans- and 11-cis-retinal, their N-methyl Schiff base and N-methyl protonated Schiff base derivatives: a comparative ab initio study

Abstract Ab initio calculations have been performed on all- trans -retinal, 1 , 11- cis -12-s- cis -retinal, 2 , and 11- cis -12-s- trans -retinal, 3 , as well as their corresponding N -methyl Schiff bases and protonated N -methyl Schiff base derivatives. Geometries preoptimized at the RHF/3-21G level were fully optimized with the RHF/6-31G ∗∗ basis set. The stabilities in order of increasing energy are 1, 2 and 3 for the aldehydes and Schiff bases, but 1, 3 and 2 for the protonated species. Geometries are in excellent agreement with available X-ray results. Minute details are reproduced with surprising accuracy, except for the large dihedral angles at the C6C7 bond (and the C12C13 bond for the 11- cis isomers) where packing effects may play an important role. Geometry changes from the aldehydes to the corresponding Schiff bases are negligible, while protonation is accompanied by the loss of double bond fixation and an increased tendency towards planarization. The geometry about the C6C7 bond is distorted 6-s- cis in the aldehydes and the Schiff bases, while in the protonated Schiff bases an additional almost planar 6-s- rans conformation is found. The out-of-plane deformation about the C12C13 bond is 49 ° in 2 , 5 ° in 3 and 0 ° in the protonated Schiff base of 3 .

Excited state molecular dynamics of retinal model chromophores

A molecular dynamics study of the photoreactions of the penta-3,5-dieniminium cation, a model system of the biologically relevant chromophore retinal, was performed using a high level CASSCF methodology with a statistical ensemble of molecular trajectories. The simulations give insight into the dynamic effects on the possible photo-isomerization reactions and confirm a model proposed for the surface topology near the intersection region of the excited and ground state.

Glutamic Acid 181 Is Uncharged in Dark-Adapted Visual Rhodopsin

We have performed a high-level quantum chemical analysis to study the chromophore-protein interaction involving the charged (B) and/or uncharged (C) form of E181 and also a mutant E181Q model in the presence of the primary counterion E113 (A). As the magnitude of the calculated spectral shifts on either side remains within +/-10 nm, we show that the orientation of the dipole moment vector is the key to unlocking the puzzle on this contentious issue. We find that E181 is present in the uncharged (or) protonated form in the dark-adapted visual Rhodopsin, and therefore an electrostatically neutral environment is envisaged.

Helically twisted chiral cyanine dyes: Influence of chromophore length on observed and calculated rotatory strengths

Synthesis, chiroptical properties, and quantum-mechanical calculations of the monomethine dye 3 and of the trimethine dyes 7 and 11 are reported. In 3 and 11, the chromophore is forced into a twisted all-Z-conformation by steric interaction of the end groups in the former and the presence of a t-butyl group in the mesoposition of the latter, which is manifest in the UV/Vis spectra not only in the reduced intensity of the longest wavelength absorption, but also in the occurrence, at shorter wavelength, of a “cis-peak.” Chiral substitution of the end groups serves as chiral anchor to discriminate between otherwise enantiomeric forms and makes them amenable to chiroptical investigation. The results are in agreement with theoretically calculated chiroptical data based on helically twisted cyanine chromophores. They support the contention that not only the sense of the helix, but also its length determines the sign of the associated Cotton effect. Chirality 9:243–249, 1997.

A First-Principles Study of 11-Cis-Retinal: Modelling the Chromophore-Protein Interaction In Rhodopsin

The 11- cis -retinal protonated Schiff base is the chromophore of rhodopsin, the photoreceptor in the vertebrate eye. The photochemical isomerization from 11- cis to the all- trans form triggers a series of enzymatic reactions known as the visual cascade which eventually leads to a neural signal. Experiments such as resonance Raman, NMR etc., have shown that 11- cis -retinal is probably highly twisted in the protein pocket. Because detailed knowledge about the kind of interaction with the protein is missing, a theoretical description of the chromophore conformation is difficult. In the simulations the results of which will be presented here, we assume that the retinal chromophore, as a consequence of the steric fit into the protein binding pocket, undergoes a specific kind of conformational change.The structure we obtain is in good agreement with the experimentally observed highly twisted conformation of the chromophore backbone.

Circular dichroism of axially chiral methaqualone, 3‐Aryl‐2‐mercapto‐ and 3‐aryl‐2‐alkylthio‐4(3H)‐quinazolinones: conformational dependence of CD and assignment of absolute configuration

Rotational strengths calculated on the basis of quantum-mechanically obtained minimum energy geometries were used to determine the absolute configurations of axially chiral 3-aryl-4(3H)-quinazolinones from the sign of the observed Cotton effects (CEs). For the spectral data, CNDO/S calculations were used; for the geometries, ab initio (RHF/6-31G) and semiempirical (AM1) theories were used. Oscillator and rotational strengths of all excited states down to 200 nm were compared to experimental absorption and circular dichroism (CD) data. It was found that the sign of the 1Lb Cotton effects obtained for the enantiomers of methaqualone and derivatives of 3-aryl-2-alkylthio-4(3H)-quinazolinones can be correlated unambiguously with the absolute configuration. Furthermore, the sign of the Cotton effect of the π-π* transition of the thiocarbonyl chromophore of 3-aryl-2-mercapto-4(3H)-quinazolinones is suitable for a successful stereochemical correlation. Chirality 10:253–261, 1998.

Unexpected behavior of the retinylidene chromophore in the exciton interaction of chiral 1,2-cyclohexanediamine schiff bases

Abstract The bis-chromophoric Schiff Bases derived from (1R,2R)-(−)-trans-1,2-cyclohexanediamine and two conjugated polyenes, 2,4-hexadienal and all-trans-retinal, show opposite exciton coupling patterns in the CD - without any apparent reason.

9-Demethylrhodopsin: theoretical evidence for a relaxed batho intermediate.

The 9-methyl group of retinal is crucial for the photoreaction of rhodopsin. On the basis of the results of QM/MM simulations, we propose that the primary function of the methyl group is not to properly align the chromophore in the ground state, but that it is a prerequisite for the peculiarly twisted and strained chromophore observed in the batho state. With the methyl group firmly anchored in the protein binding pocket the protein, at the cost of the incipient photon energy, manages to increase the strain energy stored in the chromophore by 25%, which may be crucial for driving the subsequent transformations.