Light-Harvesting Complex II
(LHCII) is a membrane protein found in plant chloroplasts that has the crucial
role of absorbing solar energy and subsequently performing excitation energy
transfer to the reaction centre subunits of Photosystem II. LHCII provides
strong absorption of blue and red light, however, it has minimal absorption in
the green spectral region where solar irradiance is maximal. In a recent
proof-of-principle study, we enhanced the absorption in this spectral range by
developing a biohybrid system where LHCII proteins together with lipid-linked
Texas Red (TR) chromophores were assembled into lipid membrane vesicles. The
utility of these systems was limited by significant LHCII quenching due to
protein-protein interactions and heterogeneous lipid structures. Here, we
organise TR and LHCII into a lipid nanodisc, which provides a homogeneous, well-controlled
platform to study the interactions between TR molecules and single LHCII complexes.
Fluorescence spectroscopy determined that TR-to-LHCII energy transfer has an
efficiency of at least 60%, resulting in a 255% enhancement of LHCII fluorescence,
two-fold greater than in the previous system. Ultrafast transient absorption
spectroscopy revealed two time constants of 3.7 and 128 ps for TR-to-LHCII
energy transfer. Structural modelling and theoretical calculations indicate
that these timescales correspond to TR-lipids that are loosely- or
tightly-associated with the protein, respectively, with estimated TR-to-LHCII
separations of ~3.5 nm and ~1
nm. Overall, we demonstrate that a nanodisc-based biohybrid system provides an
idealised platform to explore the photophysical interactions between extrinsic
chromophores and membrane proteins with potential applications in understanding
more complex natural or artificial photosynthetic systems.
Ultrafast Energy Transfer Between Lipid-Linked Chromophores and Plant Light-Harvesting Complex II
