Pheophytins and the photosynthetic reaction center

Concentration quenching in chlorophyll-alpha and relation to functional charge transfer in vivo

Chlorophyll-alpha in ordinary solvents exhibits concentration quenching. Dimeric chlorophyll is reasonably well confirmed as the quenching species, by a critical reanalysis of available data on concentration dependence and on spectral features, in ordinary solvents, and in several analogous quenching environments. This quenching in the dimer in vitro is somewhat less firmly analyzed as due to a new fast internal conversion. Much peripheral evidence supports transient charge transfer as the cause of internal conversion. The same evidence points to a strong similarity to functional charge transfer in vivo. I suggest that inability to extract P680 may be due to its conversion to a form resembling P700 by addition of water. A number of straightforward experiments are suggested to test these proposals. In particular, it is desirable to test for the existence of a vibronic perturbation (from a higher npi* state) in the dimer, as an alternative to charge transfer for explaining the "observed" internal conversion. Such a vibronic cause would raise interesting problems for phototrap function in vivo.

Nitrogen fixation and hydrogen metabolism in photosynthetic bacteria

The photosynthetic bacteria are found in a wide range of specialized aquatic environments. These bacteria represent important members of the microbial community since they are capable of carrying out two of the most important processes on earth, namely, photosynthesis and nitrogen fixation, at the expense of solar energy. Since the discovery that these bacteria could fix atmospheric nitrogen, there has been an intensification of studies relating to both the biochemistry and physiology of this process. The practical importance of this field is emphasized by a consideration of the tremendous energy input required for the production of artificial nitrogenous fertilizer. The present communication aims to briefly review the current state of knowledge relating to certain aspects of nitrogen fixation by the photosynthetic bacteria. The topics that will be discussed include a general survey of the nitrogenase system in the various photosynthetic bacteria, the regulation of both nitrogenase biosynthesis and activity, recent advances in the genetics of the nitrogen fixing system, and the hydrogen cycle in these bacteria. In addition, a brief discussion of some of some of the possible practical applications provided by the photosynthetic bacteria will be presented.