IMO - LMU Nonella group

Calculation of electronically excited states

The primary step of the photocycle of Bacteriorhodopsin occurs after light excitation. The potential energy surface of the first electronically excited state can therefore be decisive for the mechanism of the primary step. A still open question concerns the conformation of the first spectroscopically detected state J. According to one model the conformation of the chromophore in this state corresponds to a 13-cis conformation whereas in a second model, a double isomerization is proposed which would result in a 13,14-dicis conformation.

In collaboration with the group of Prof. Paul Tavan at the LMU in Munich, we plan to combine our ab initio calculation of the excited state with classical dynamics simulations in order to get more insight into the mechanism of this very fast primary step. Calculated potential energy surfaces of a model molecule for a retinal protonated Schiff base can be seen in the picture below.

Determination of accurate vibrational force fields

Experimentally, quinones are used for the investigation of structural changes occuring during electron transfer reactions in photosynthetic reaction centers. In order to successfully interpret experimental data the spectra of free quinones and quinone radicals have to be well understood. Density functional methods have been shown to predict excellent frequencies of the experimentally important C=C and C=O modes. In future application we will investigate environmental effects of the protein onto the vibrational spectrum. This work is in close collaboration with the experimental group of Dr. Jacques Breton at the CNRS at Saclay.

The picture shows how accurate vibrational frequencies and intensities of 2-methoxy-1,4-benzoquinone are predicted by a BP86/6-31G** calculation:

Exp. and calc. spectra

Modeling of Protein-Chromophore interactions

By applying standard quantum chemical methods simple chromophore-protein systems are modeled. The influence of molecular interactions on the structure and vibrational spectrum of a chromphore is studied. For such investigation we also apply a QM/MM hybrid method (EGO/CPMD) which had been recently developed in the groups of Michele Parrinello (MPI Stuttgart) and Paul Tavan (LMU München).

The first figure shows how the torsional barrier of the methoxy group depends on the method of calculation and on intermolecular interactions:

pes of methoxy group

The next two figures show the structure of some complexes used for modeling molecular quinone-protein interactions:

quinone-water complexquinone nh4+ complex

By means of QM/MM hybrid methods we simulate the behavior of molecules in a realistic environment, i.e. a ubiquinone model molecules in solution to learn more about the dynamics of the flexible substituents.

Simulation of Enzymatic Reactions

The EGO/CPMD method is also used for the simulation of enzymatic reactions.

Last update: 24-MAI-99 / MN