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:
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:
The next two figures show the structure of some complexes used for modeling molecular quinone-protein interactions:
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.
The EGO/CPMD method is also used for the simulation of enzymatic reactions.
Last update: 24-MAI-99 / MN