Evaluating the Nature of So-Called S*-State Featurein Transient Absorption of Carotenoids in Light-Harvesting Complex2 (LH2) from Purple Photosynthetic Bacteria

摘要

Carotenoids are a class of natural pigments present in all phototrophic organisms, mainly in their light-harvesting proteins in which they play roles of accessory light absorbers and photoprotectors. Extensive time-resolved spectroscopic studies of these pigments have revealed unexpectedly complex photophysical properties, particularly for carotenoids in light-harvesting LH2 complexes from purple bacteria. An ambiguous, optically forbidden electronic excited state designated as S* has been postulated to be involved in carotenoid excitation relaxation and in an alternative carotenoid-to-bacteriochlorophyll energy transfer pathway, as well as being a precursor of the carotenoid triplet state. However, no definitive and satisfactory origin of the carotenoid S* state in these complexes has been established, despite a wide-ranging series of studies. Here, we resolve the ambiguous origin of the carotenoid S* state in LH2 complex from Rba. sphaeroides by showing that the S* feature can be seen as a combination of ground state absorption bleaching of the carotenoid pool converted to cationsand the Stark spectrum of neighbor neutral carotenoids, induced bytemporal electric field brought by the carotenoid cation–bacteriochlorophyllanion pair. These findings remove the need to assign an S* state,and thereby significantly simplify the photochemistry of carotenoidsin these photosynthetic antenna complexes.