Single Molecule Spectroscopy of Monomeric LHCII: Experiment and Theory.

Pavel Malý,Rienk Van Grondelle,J Michael Gruber,Tomáš Mančal

doi:10.1038/srep26230

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.

Highlights

Photosynthesis, arguably the most important photo-induced process on Earth, converts the energy of light into its chemically/biologically useful form
The localized excited states can be denoted as i = ei ∏n≠i gn, where |gi〉and |ei〉are the electronic ground and excited states of the chromophore denoted by index i, respectively
We have shown that the weak illumination regime, in which the single-molecule spectroscopy (SMS) experiment is performed, allows for an effective source term description of the light-matter interaction in which short time transient effects arising in the photoinduced evolution of molecular systems can be consistently accommodated side by side with the slow evolution of the protein bath observed in SMS experiments

Summary

Introduction

Photosynthesis, arguably the most important photo-induced process on Earth, converts the energy of light into its chemically/biologically useful form. Despite the fact that the experiment we analyze in this work does not provide more insight into the sub-nanosecond dynamics of individual complexes than previous works, we formulate the theory consistently in such a way that it enables the description of such processes. This is done in order to highlight the existence of a less studied type of processes on an intermediate time scale and to stress the need to search for experimental techniques which can cover the range of timescales from femtoseconds to nanoseconds in single molecular spectroscopy. We have recently reported a progress towards measuring processes on the order of hundreds of femtoseconds in single light-harvesting complexes[29]

Methods

Results

Conclusion