The initial energy transfer steps in photosynthesis occur on ultrafast timescales. We analyze the carotenoid to bacteriochlorophyll energy transfer in LH2 Marichromatium purpuratum as well as in an artificial light-harvesting dyad system by using transient grating and two-dimensional electronic spectroscopy with 10 fs time resolution. We find that Förster-type models reproduce the experimentally observed 60 fs transfer times, but overestimate coupling constants, which lead to a disagreement with both linear absorption and electronic 2D-spectra. We show that a vibronic model, which treats carotenoid vibrations on both electronic ground and excited states as part of the system's Hamiltonian, reproduces all measured quantities. Importantly, the vibronic model presented here can explain the fast energy transfer rates with only moderate coupling constants, which are in agreement with structure based calculations. Counterintuitively, the vibrational levels on the carotenoid electronic ground state play the central role in the excited state population transfer to bacteriochlorophyll; resonance between the donor-acceptor energy gap and the vibrational ground state energies is the physical basis of the ultrafast energy transfer rates in these systems.

LH2 from the purple photosynthetic bacterium Marichromatium (formerly known as Chromatium) purpuratum is an integral membrane pigment-protein complex that is involved in harvesting light energy and transferring it to the LH1-RC 'core' complex. The purified LH2 complex was crystallized using the sitting-drop vapour-diffusion method at 294 K. The crystals diffracted to a resolution of 6 Å using synchrotron radiation and belonged to the tetragonal space group I4, with unit-cell parameters a = b = 109.36, c = 80.45 Å . The data appeared to be twinned, producing apparent diffraction symmetry I422. The tetragonal symmetry of the unit cell and diffraction for the crystals of the LH2 complex from this species reveal that this complex is an octamer.

Photophysical properties of two typical aryl carotenoids, okenone and chlorobactene, were studied with application of femtosecond and microsecond time-resolved absorption spectroscopies. These carotenoids are structurally similar and differ only by keto-group and character of the aryl ring. The studies have concentrated on aspects of the photochemistry of these carotenoids as possibility of solvent polarity induced formation of intramolecular charge transfer state in okenone, which contains a keto-group directly attached to the carbon-carbon double bond conjugation, estimating the energy of the forbidden first excited singlet electronic state, S1 (2(1)Ag(-)) and testing the photoprotective capabilities of okenone and chlorobactene in real biological systems. The energies of the S1 (2(1)Ag(-)) state obtained for these carotenoids are 12 750 cm(-1) for okenone and 13 450 cm(-1) for chlorobactene and are not affected either by temperature or solvent polarity. The effect of cryogenic temperature on the excited states lifetimes and energies was also studied at 77 K in 2-methyltetrahydrofuran, which forms a transparent glass upon freezing. The ability to quench bacteriochlorophylls triplets was studied on model bacteriochlorophyll a-carotenoid mixtures with application of flash photolysis. The triplet state lifetime obtained from the anticipated kinetic modelling of the rise and decay of the pool of carotenoid triplets are 2.1 μs for okenone and 2.8 μs for chlorobactene.