Single Molecule Spectroscopy of Monomeric LHCII: Experiment and Theory

TL;DR

This paper develops a theoretical and experimental framework for understanding single molecule fluorescence spectra of LHCII, demonstrating Lut 1's role as an efficient quencher and validating the model with experimental data.

Contribution

It introduces a broad-timescale model for LHCII excited state dynamics, combining theory with experiments, and highlights Lut 1's quenching role in fluorescence.

Findings

Model accurately reproduces experimental spectra

Lut 1 effectively quenches fluorescence in LHCII

Conformational changes influence spectral switching

Abstract

We derive approximate equations of motion for excited state dynamics of a multilevel open quantum system weakly interacting with light to describe fluorescence detected single molecule spectra. Based on the Frenkel exciton theory, we construct a model for the chlorophyll part of the LHCII complex of higher plants and its interaction with previously proposed excitation quencher in the form of the lutein molecule Lut 1. The resulting description is valid over a broad range of timescales relevant for single molecule spectroscopy, i.e. from ps to minutes. Validity of these equations is demonstrated by comparing simulations of ensemble and single-molecule spectra of monomeric LHCII with experiments. Using a conformational change of the LHCII protein as a switching mechanism, the intensity and spectral time traces of individual LHCII complexes are simulated, and the experimental statistical distributions are reproduced. Based on our model, it is shown that with reasonable assumptions about its interaction with chlorophylls, Lut 1 can act as an efficient fluorescence quencher in LHCII.