Single-molecule spectroscopy of intrinsically fluorescing systems

State-of-the-art light detectors allow tracking of spectroscopic processes of individual fluorescing systems on millisecond timescales. Single-molecule spectroscopy has revealed the existence of switching phenomena that have never been observed by using conventional ensemble-averaging techniques. A classic example of such a single-molecule signature is rapid fluorescence intensity fluctuations, commonly known as fluorescence intermittency. Fig. 1 shows an example of fluorescence intermittency from a single system of the main plant light-harvesting complex. Exploitation of these intensity switches in different environmental conditions can provide valuable insight on the regulation of energy transfer in light-harvesting complexes in vivo. In addition, spectral fluctuations can be followed in real time (Fig. 2), thereby shedding light on dynamic processes of the complexes. Although a relatively new technique, single-molecule spectroscopy has already aided significantly in the understanding of the structure and function of bacterial light-harvesting complexes involved in the earliest reactions in photosynthesis. At the Biophysics group we focus mainly on intrinsically fluorescent systems at conditions close to room temperature.

Contact: Rienk van Grondelle, e-mail: or Tjaart Krüger, e-mail:
Rapid fluorescence intensity fluctuations (left) and spectral fluctuations (right) from a single LHCII trimeric complex at 5°C. The distinct spectroscopic states are denoted by I, II and III, respectively. Spectra on top are the time averages of the respective spectral states, where fluorescence (FL) is expressed in counts per second (cps).