Low-temperature fluorescence excitation spectra for long-wavelength emission as a function of greening in Euglena gracilis and chlorophyll A concentration in vitro: a mathematical model to describe both systems.

A systematic study was made of the spectrum for exciting long-wave-length fluorescence (at 77 degrees K) during the first 100 hr of greening in Euglena gracilis. A band at 705-710 nm is observable after cells have been greening in light for 30 hr. The ratio of the 705-nm to the 675-nm peak increases during greening, reaching a maximum value at 85 hr, then declining. With concentrated solutions of chlorophyll a, fluorescence excitation spectra are similar to those observed in vivo. The ratio of aggregate to monomer bands increases with concentration of chlorophyll, reaching a maximum value in ethanol and in pyridine at about 3 x 10(-2) M and 6 x 10(-2) M respectively, then declining. Several model systems were analyzed. It is shown that the band observed in solution with maximum at 705-710 nm is not an artifact of the fluorescence apparatus; it does not arise from undissolved chlorophyll; it does not arise from a fluorescent or nonfluorescent impurity; it does not arise solely from light absorption by a dimer or larger aggregate of chlorophyll. Agreement is obtained between the experimental observations and the results of a mathematical model by including terms for the efficiency of energy transfer from monomeric to dimeric chlorophyll, as well as for the formation of dimers by an equilibrium reaction.