Nanodiscoidal Lipid-Protein Assembly Modulated Ultrafast Excitation Relaxation in an Algal LHCII Complex

Bryopsis (Bry.) corticulans is a marine green alga thriving in the intertidal zone. To utilize the underwater blue-green light, its major light-harvesting antenna LHCII adopts a high composition of chlorophyll (Chl) b and the keto-carotenoid (Car), siphonin (Spn), and siphonaxanhin (Spx). This work is intended to examine the intracomplex, ultrafast excitation transfer dynamics under blue-light excitation, emphasizing on the influence from the biomimicking lipid environment. From BryLHCII solubilized in aqueous phase to that embedded in a lipid-membrane nanodisc (mnd-BryLHCII), the overall singlet excitation transfer (SET) efficiency drops for (5∼13)% over 400-550 nm. Femtosecond time-resolved absorption spectroscopy and comprehensive spectral analysis reveal highly heterogeneous excitation relaxation pathways in terms of pigments and their binding sites in both LHCII preparations. With reference to BryLHCII, mnd-BryLHCII switches off the Spx521 S₁ and the neoxanthin S₁ pathways of Car-to-Chl a SET, whereas it keeps the S₁ pathways of Spn520 and Spx522 active and even accelerated. Thus, the functionality of Spn520 at L1 and Spx521 at L2 is oriented toward light harvesting and triplet photoprotection, respectively. In addition, a minor portion of Chl b in mnd-BryLHCII fails to transfer the Q_y excitation to the lowest-energy cluster of Chls, i.e., the terminal emitter. The inactivated or deficient SET pathways account for the observed decline in SET efficiency. Our findings demonstrate the role of the lipid environment in optimizing the light-harvesting and photoprotective functions of the pigment cofactors of the algal LHCII complex, shedding light on deeper understanding of the underlying molecular mechanisms.