Analysis of fluorescence kinetics and energy transfer in isolated alpha subunits of phycoerythrin from Nostoc sp.

Abstract

Abstract— The fluorescence decay profiles, relative quantum yield, and transmission of the phycoerythrin a subunit, isolated from the photosynthetic antenna system of Nostoc sp., were measured using single picosecond laser excitation. The fluorescence decay profiles were found to be intensity independent for the intensity range investigated (4 × 1013 and 4 × 1015 photons‐cm‐2 per pulse). The decay profiles were fitted to a model assuming both chromophores absorb and fluoresce. The inferred total deactivation rates for the two chromophores, in the absence of energy transfer and when the effects of the response time of the streak camera and the finite pulse width are properly included, are 1.0 × 1010s' and 1.0 × 109 s 1 for the s and f chromophores. respectively, whereas the transfer rate between the two fluorophorcs is estimated to be 1.0 × 1010 s−1 giving a s→ f transfer rate on the order of (100 ps)−1. Steady‐atate polarization measurements were found to be equal to those calculated using the rate parameters inferred from the kinetic model fit to the fluorescence decays. The apparent decrease in the relative fluorescence quantum yield and increase of the relative transmission with increasing excitation intensity is suggestive of ground state depletion and upper excited state absorption. Evidence suggests that exciton annihilation is absent within isolated α subunits for the intensity range investigated (4 × 1013 to 4 × 1015 photons‐cm 2 per pulse).