Preprocess dependence of optical properties of ensembles and single siphonaxanthin-containing major antenna from the marine green alga Codium fragile

Abstract

The siphonaxanthin-siphonein-Chl-a/b-protein (SCP) is the light-harvesting complex of the marine alga Codium fragile. Its structure resembles that of the major light-harvesting complexes of higher plants, LHC II, yet it features a reversed Chl a:Chl b ratio and it accommodates other variants of carotenoids. We have recorded the fuorescence emission spectra and fuorescence lifetimes from ensembles and single SCP complexes for three diferent scenarios of handling the samples. While the data obtained from ensembles of SCP complexes yield equivalent results, those obtained from single SCP complexes featured signifcant diferences as a function of the sample history. We ascribe this discrepancy to the diferent excitation intensities that have been used for ensemble and single complex spectroscopy, and conclude that the SCP complexes undergo an aging process during storage. This process is manifested as a lowering of energetic barriers within the protein, enabling thermal activation of conformational changes at room temperature. This in turn leads to the preferential population of a red-shifted state that features a signifcant decrease of the fuorescence lifetime.