Resent results concerning photosynthetic light harvesting

1.1 A central question in the long-standing debate concerning resolution and origin of spectral substructure of absorption bands of photosynthetic antenna complexes and their assignment to molecular structure was answered:

The final state of the intracomplex excitation energy transfer (EET) chain in the plant main light-harvesting complex (LHC II) was resolved as an excitonically coupled, chlorophyll (Chl) a/b dimer. Moreover it was identified as Chl a2/ b2 in the LHC II structural model of Kühlbrandt et al. (1994), Fig. 1:



Fig. 1 Assignment of the red-most excitonic transition (peaking at 678 nm) to the pigment binding site a2 in the structural model of LHC II.

The energy level (located at 678 nm) is the donor state in intercomplex EET. At variance, the corresponding state in the minor antenna complex, CP29, belongs to a monomeric Chl a. These results were obtained by two independent tailor-made techniques, thus overcoming serious problems posed by spectral congestion typical for plant photosynthetic antenna complexes: i) Two-photon excitation spectroscopy of a novel weak fluorescence emission from higher excited Chl states. ii) Nonlinear polarization spectroscopy in the frequency domain with spectrally separated pump- and probe ranges



Fig. 2 NLPF spectra of trimeric LHC II pumped in the Chl a/b-Qy range and probed at different wavelengths in the Soret band.

1.2 Work concerning determination of the energetic position of optically dark excited states of carotenoids and their role in EET, started recently in the present project, was successfully continued: the location of the 2Ag- state was identified for two carotenoids (rhodopin, neurosporene) in a native environment. It was shown, that this state does not participate in EET. The method of two-photon excitation spectroscopy of BChl fluorescence (after careful re-examination of its content of information) was used.- In close cooperation with project 2.26, pulsed soft X-ray absorption spectroscopy at the C1s edge produced for the first time a XANES spectrum of a native antenna complex (viz the peridinin-Chl a-protein, PCP, dominated by carotenoid absorption). Following recent successful determination of optically forbidden excited states of amorphous carotenoid samples by this method, this is a new important step towards the ultimate goal of investigating the dynamics of excitation energy flow through the dark states of carotenoids.

1.3 The refractive index of the environment of the respective chromophores is an essential parameter for comparison of experimental and theoretical EET rates. Reliable experimental values are scarce. A suitable sensor molecule was characterized. Thus, the refractive index of the microenvironment of the 800 -> 850 nm EET step in purple bacteria was obtained (n = 1.59) - solving a problem of pivotal interest in photosynthetic research.

Recent results concerning ultraweak melanin fluorescence
2.1 Several types of melanin have been characterized by their two-photon excited fluorescence spectra as well as decay times



Fig. 3 Two-photon femtosecond pulse-excited fluorescence spectra of normal skin tissue (dashed line) and malignant melanoma (solid).

2.2 Based on this, the malignant transformation of skin tissue could be definitely characterized by a spectral red shift and lifetime-shortening of the ultraweak melanin fluorescence. It reflects an accumulation of pheomelanin at the expense of eumelanin.

2.3 A mobile laboratory femtosecond fluorometer for early detection of skin cancer has been completed (Cooperation with LTB Lasertechnik Berlin and Becker & Hickl); cp. medicinews newsletter 6/2002, 1-2