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.