Quantum chemical simulations of excited-state absorption spectra of photosynthetic bacterial reaction center and antenna complexes.

The semiempirical ZINDO/S CIS configuration interaction method has been used to study the ground- and excited-state absorption spectra of wild type and heterodimer M202HL reaction centers from purple bacterium Rhodobacter sphaeroides as well as of peripheral LH2 and LH3 light harvesting complexes from purple bacterium Rhodopseudomonas acidophila. The calculations well reproduce the experimentally observed excited-state absorption spectra between 1000 and 17,000 cm(-1), despite the necessarily limited number of chromophores and protein subunits involved in the calculations. The electron density analysis reveals that the charge transfer between adjacent chromophores dominates the excited-state absorption spectra. Clear spectroscopic differences observed between the wild type and heterodimer reaction centers as well as between the LH2 and LH3 antenna complexes arise from differences in the energy level manifolds of the complexes, particularly those of the charge transfer states. The calculations also imply that the lowest excited state of the bacterial reaction centers has charge transfer character that is related to charge transfer within the special pair and between the special pair and the accessory bacteriochlorophyll of the photosynthetically active electron transfer branch.

A new potential toxaphene congener: synthesis, GC/EI-MS study, crystal structure, NMR analysis, and ab initio calculations of 3-endo,5-endo-dichloro-7,7-bis-chloromethyl-4-dichloromethyl-tricyclo[2.2.1.0(2,6)]heptane.

A new potential toxaphene congener 3-endo,5-endo-dichloro-7,7-bis-chloromethyl-4-dichloromethyl-tricyclo[2.2.1.0(2,6)]heptane 2 has been isolated from reaction mixture obtained by the chlorination of 2-exo, 10,10-trichlorobornane 1. The X-ray structural analysis of 2 revealed an unusual tricyclic structure, where the two chlorine atoms occupying endo-positions are in close spatial proximity with each other and near to the neighbouring CHCl2 group. Further, it revealed that the symmetry of the molecule is distorted. The 1H and 13C NMR spectra of 2 have been assigned by means of 1H, 1H double-quantum filtered correlation spectroscopy (DQF COSY), PFG 1H, 13C HMQC (pulsed field gradient heteronuclear multiple-quantum coherence), 1H, 13C heteronuclear multiple bond correlation (HMBC) experiments, and computer aided 1H NMR spectral analysis. The asymmetry of 2 is also discernible on the 1H NMR parameters. In addition, gas chromatographic (GC) properties and electron impact (EI) mass spectrum of 2 has been studied. Ab initio Hartree-Fock (HF) method with the basis set 6-31G(d) has been used for the optimization of the equilibrium geometry and calculation of total energy for 2. The optimized geometry is in good agreement with the crystal structure. According to the rotation energy profile calculated at the HF/6-31G(d) level, rotation of the chloromethyl and dichloromethyl groups are highly unlikely at the room temperature.