No abstract

Ultrafast excited state energy transfer to the primary electron donor or special pair in photosynthetic reaction centers has been measured following excitation of the lowest electronic state of the other chromophores. This was achieved by observing the rise time and induced anisotropy in the spontaneous fluorescence from the special pair using fluorescence up-conversion at 85 K. Very fast energy transfer is observed when exciting either of the bacteriochlorophyll monomers. Energy transfer from the bacteriopheophytins, which are considerably further from the special pair than the bacteriochlorophyll monomers, is about 50% slower. The rate, distance, and temperature dependence of energy transfer in both cases are very different from what is predicted by conventional Fo ¨rster dipole-dipole theory. By working at low temperature and with the reaction center mutant (M)L214H (the beta mutant), which assembles with a bacteriochlorophyll monomer in place of a bacteriopheophytin in the H L binding site (the location of the primary electron acceptor), it is possible to selectively initiate the energy transfer process on the functional and nonfunctional sides of the reaction center. The observed rates of energy transfer to the special pair are found to be similar. Thus energy transfer rates are comparable on the functional and nonfunctional sides, while electron transfer rates differ by at least 2 orders of magnitude. This suggests that the dominant source of functional asymmetry for electron transfer involves differences in the association of the functional-side chromophores with their environment (e.g. free and reorganization energy differences), rather than differences in electronic coupling.

Papillary renal cell carcinoma (RCC) is known by its tendency to avascularity by angiography; however, data concerning its clinicopathologic spectrum and prognosis are not available. In a review of 224 renal cell carcinomas accesioned in our files, 34 were found to be papillary and 190 of other histologic types. A comparative analysis of these two gropus revealed marked differences. The majority of papillary tumors (85.3%) were in pathologic stage I, whereas more than half of the nonpapillary tumors had extended beyond the limits of the kidney. Follow-up data revealed that the survival for papillary RCC was significantly higher than that for nonpapillary tumors. This difference held true even when tumors in the same pathologic stage were compared. Many papillary tumors, particularly those with a favorable course, were massively necrotic, densely infiltrated by macrophages, or both. In view of these findings, the possibility that host mechanisms are involved in destruction and confinement of the tumor is discussed. Examination of kidney tissue distant from the tumor disclosed, in some cases, atypical hyperplastic changes of collecting tubules; this raises the possibility that some papillary tumors arise from distal tubular epithelium. Hypo- or avascularity was present in all papillary RCC's studied by angiography.

We have measured the excited-state dynamics and spectral evolution of oxidized flavin−adenine dinucleotide (FADox) and flavin mononucleotide (FMNox) in simple solvents with subpicosecond time resolution by one-color and white-light continuum transient absorption spectroscopy. In water, the FADox transient shows significant quenching of the excited state with a lifetime of ∼4 ps while for FMNox this component is of much lower amplitude. However, when caffeine is added to the FMN solution the amplitude of the fast component is recovered. Additionally, the excited-state dynamics of FAD in neat formamide, a solvent that breaks up stacking interactions in FAD, is similar to that obtained for FMN in water. These measurements provide conclusive evidence that the excited-state quenching observed in FAD or flavin−purine complexes occurs in less than 5 ps. The observed spectral evolution shows that internal conversion from S2 to S1 occurs in less than 100 fs.

The spontaneous fluorescence from the special pair primary electron donor in bacterial photosynthetic reaction centers has been measured at low temperature using fluorescence up-conversion following excitation of the special pair with 80 fs pulses from a mode-locked Ti:sapphire laser. Oscillations are observed during the first few picoseconds of the decay. The frequency of the oscillations and the detection wavelength dependence of their intensity are similar to results obtained by Vos, Martin, and co-workers using stimulated emission. This demonstrates that the oscillations are associated with the excited state of the special pair, not with stimulated Raman scattering. Vibrational coherence persists on the time scale of electron transfer. Possible relationships between these results and other observables are discussed.The nature of the electronic excited state(s) which initiates photosynthesis is a key element for understanding the mechanism of ultrafast electron transfer. The excited species in photosynthetic reaction centers (RCs) from bacteria is a dimer or special pair of bacteriochlorophylls (BChls) denoted P. Although the electronic properties of BChl monomers are typical of any large aromatic molecule, the interactions between the BChls within the special pair and between the special pair and its protein environment lead to unique properties. Many static spectroscopies such as absorption, circular dichroism,* photochemical hole b~r n i n g ,~.~ Stark,5 and resonance Raman63' spectroscopies have been used to characterize these unusual properties of P.Recently, two new dynamical approaches have been added. Vos, Martin, and co-workers, in a series of tour de force experiments, demonstrated oscillations in the stimulated emission from 'P when the special pair is impulsively excited with very short pulses.' Oscillations were initially observed in RCs which are incapable of electron transfer, where the 'P lifetime is about 200 ps, and were subsequently demonstrated in functional RCs, at both cryogenic and room temperature. At the same time, two groups independently reported the direct measurement of spontaneous fluorescence from lP at room temperature using fluorescence up-conver~ion.~~~ Both groups observed that the excited decay is biphasic, a result which had not previously been reported using transient absorption spectroscopy. The observation of a biphasic decay was also perplexing as photochemical hole-burning experiments at cryogenic temperatures exhibit zero-phonon line ~i d t h s~,~ which agree quite closely with those predicted from the single exponential kinetics measured by transient absorption spectroscopy. In order to investigate this further and to establish the experimental capability for electric-field-modulated measurements on the spontaneous fluorescence, we have developed the methodology for measuring spontaneous fluorescence with high signal-to-noise at low temperatures. In the course of these experiments, oscillations analogous to those reported by Vos et al.' were discovered, and this is t...