Abstract
One of the most intriguing findings in single molecule spectroscopy (SMS) is the observation of Raman spectra of
individual molecules, despite the small cross section of the transitions involved. The observation of the spectra can be
explained by the surface enhanced Raman scattering (SERRS) effect. At the single-molecule level, the SERRS-spectra
recorded as a function of time reveal inhomogeneous behaviour such as on/off blinking, spectral diffusion, intensity
fluctuations of vibrational line, and even splitting of some lines within the spectrum of one molecule. Single-molecule
SERRS (SM-SERRS) spectroscopy opens up exciting opportunities in the field of biophysics and biomedical
spectroscopy. The first example of single protein SERRS was performed on hemoglobin. However, the possibility of
extracting the heme group by silver sols can not be excluded. Here we report on SM-SERRS spectra of enhanced green
fluorescent protein (EGFP) in which the chromophore is kept in the protein. The time series of SM-SERRS spectra
suggest the conversion of the EGFP chromophore between the deprotonated and the protonated form. Autocorrelation
analysis of SM-SERRS trajectory reveals the presence of fast dynamics taking place in the protein. Our findings show
the potential of the technique to study structural dynamics of protein molecules.