Identification and characterization of multiple emissive species in aggregated minor antenna complexes

Abstract

Aggregation induced conformational change of light harvesting antenna complexes is believed to constitute one of the pathways through which photosynthetic organisms can safely dissipate the surplus of energy while exposed to saturating light. In this study, Stark fluorescence (SF) spectroscopy is applied to minor antenna complexes (CP24, CP26 and CP29) both in their light-harvesting and energy-dissipating states to trace and characterize different species generated upon energy dissipation through aggregation (in-vitro) induced conformational change. SF spectroscopy could identify three spectral species in the dissipative state of CP24, two in CP26 and only one in CP29. The comprehensive analysis of the SF spectra yielded different sets of molecular parameters for the multiple spectral species identified in CP24 or CP26, indicating the involvement of different pigments in their formation. Interestingly, a species giving emission around the 730nm spectral region is found to form in both CP24 and CP26 following transition to the energy dissipative state, but not in CP29. The SF analyses revealed that the far red species has exceptionally large charge transfer (CT) character in the excited state. Moreover, the far red species was found to be formed invariably in both Zeaxanthin (Z)- and Violaxathin (V)-enriched CP24 and CP26 antennas with identical CT character but with larger emission yield in Z-enriched ones. This suggests that the carotenoid Z is not directly involved but only confers an allosteric effect on the formation of the far red species. Similar far red species with remarkably large CT character were also observed in the dissipative state of the major light harvesting antenna (LHCII) of plants [Wahadoszamen et al. PCCP, 2012], the fucoxanthin-chlorophyll protein (FCP) of brown algae [Wahadoszamen et al. BBA, 2014] and cyanobacterial IsiA [Wahadoszamen et al. BBA, 2015], thus pointing to identical sites and pigments active in the formation of the far red quenching species in different organisms.

Light-harvesting complexes access analogue emissive states in different environments

“… 6 , 26 , 33 One of the long-standing controversies about quenching in the aggregates concerns the origin of the increased far-red emission observed upon clustering. 26 , 27 , 34 This observation has been sometimes used to suggest that redshifted states are involved in the aggregate quenching. 26 , 30 , 35 This interpretation, however, is at variance with experimental evidence from the LHCa's, whose red-emitting states are mostly long-lived.…”

Charge transfer states in phycobilisomes

“…(For interpretation of the references to color in this figure legend, the reader is referred to the online version of this chapter.) revealing the CT character of photoactive species [25][26][27][28], quantify their CT character and excited-state dynamics, and for examining the electronic structures of pigments in protein matrices. It is also an effective experimental tool towards resolving multiple overlapping spectral contributions that give rise to a single broad spectral profile in conventional spectroscopy [26,29] (see also SI on SF).…”

“…revealing the CT character of photoactive species [25][26][27][28], quantify their CT character and excited-state dynamics, and for examining the electronic structures of pigments in protein matrices. It is also an effective experimental tool towards resolving multiple overlapping spectral contributions that give rise to a single broad spectral profile in conventional spectroscopy [26,29] (see also SI on SF). SMS reveals the spectral heterogeneity of photoactive complexes resulting from protein conformational disorder that is averaged out in most ensemble approaches, thus shedding light on the likelihood of accessing, e.g., states with characteristics resembling CT states [30].…”

Correlated fluorescence quenching and topographic mapping of Light-Harvesting Complex II within surface-assembled aggregates and lipid bilayers

“…NPQ is triggered under excess light conditions by the interplay of three factors: the transmembrane ΔpH [ 10 ], the PsbS protein [ 11 ] and the enzymatic de-epoxidation of the LHCII bound xanthophyll violaxanthin to zeaxanthin [ 12 ]. Together these factors bring about a change in conformation of LHCII, and possibly the minor LHC antenna complexes CP24, 26 and 29 [ [13] , [14] , [15] ], that leads to formation of dissipative energy transfer pathways correlated to a dramatic shortening of the Chl excited state lifetime [ 16 , 17 ]. The dynamics and efficiency of NPQ formation and relaxation have been shown to be crucial to plant fitness in fluctuating light environments as experienced in nature [ [18] , [19] , [20] ].…”