Room Temperature Excitation-Emission Spectra of Single LH2 Complexes Show Remarkably Little Variation

Abstract

Excitation spectroscopy gives direct insight in excited state manifold, energy transfer, transient intermediates, vibrations, etc. Unfortunately, excitation spectroscopy of single molecules at ambient conditions has remained challenging. Here, we present excitation spectra alongside emission spectra of the same individual light harvesting complex LH2 of the purple bacteria Rps. acidophila. Acquisition of both the excited and ground state spectra allows to quantify disorder and interband correlations, which are key variables for the interpretation of observed long-lasting coherences. We have overcome the low photostability and small fluorescence quantum yield, inherent to many biologically relevant systems, by combining single molecule Fourier transform spectroscopy, low excitation intensities, and effective data analysis. We find that LH2 complexes show little spectral variation (130-170 cm-1), that their two absorption bands (B800-B850) act uncorrelated, and that the Stokes shift is not constant. The low amount of spectral disorder underlines the protective role of the protein scaffold, benefitting the efficient energy transport throughout the light-harvesting membrane