Fluorescence properties of aggregated chlorophyll in vivo and in vitro

Abstract

Intensity of fluorescence was determined as a function of chlorophyll concentration (in ethanol at 77°K) at the maxima of aggregate and monomer emission bands, 720 and 685 mμ, respectively. An expression, derived by relating chlorophyll concentration to the ratio of fluorescence at these two wavelengths, was found to be most consistent with experimental data when the aggregate was assumed to be a dimer. The fluorescence yield of the dimer in ethanol at 77°K was calculated and found to be 0.8. The lifetime of emission from this aggregated species corresponds to ∼10-4 sec. Fluorescence from the chlorophyll monomer and aggregate, in vivo and in vitro, was measured as a function of temperature (from 293° to 77°K). In solution and in algae the activation energy for dimer emission is approximately 1 kcal, the Arrhenius constant is of the order of 104. The latter value reflects a low transition probability for dimer emission, suggesting origin in a triplet or non-binding singlet state.