Among the ultimate goals of protein physics, the complete, experimental description of the energy paths leading
to protein conformational changes remains a challenge. Single protein fluorescence spectroscopy constitutes an approach of
choice for addressing protein dynamics, and, among naturally fluorescing proteins, light-harvesting (LH) proteins from purple
bacteria constitute an ideal object for such a study. LHs bind bacteriochlorophyll a molecules, which confer on them a high
intrinsic fluorescence yield. Moreover, the electronic properties of these pigment-proteins result from the strong excitonic
coupling between their bound bacteriochlorophyll a molecules in combination with the large energetic disorder due to slow
fluctuations in their structure. As a result, the position and probability of their fluorescence transition delicately depends on
the precise realization of the disorder of the set of bound pigments, which is governed by the LH protein dynamics. Analysis
of these parameters using time-resolved single-molecule fluorescence spectroscopy thus yields direct access to the protein dynamics.
Applying this technique to the LH2 protein from Rhodovulum (Rdv.) sulfidophilum, the structure—and consequently the
fluorescence properties—of which depends on pH, allowed us to follow a single protein, pH-induced, reversible, conformational
transition. Hence, for the first time, to our knowledge, a protein transition can be visualized through changes in the electronic
structure of the intrinsic cofactors, at a level of a single LH protein, which opens a new, to our knowledge, route for understanding
the changes in energy landscape that underlie protein function and adaptation to the needs of living organisms.