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In the present contribution, we demonstrated that surface-enhanced resonance Raman scattering spectra from single green fluorescent proteins (GFPs) were obtained. The most important findings are the direct detection of the conversion between a deprotonated and a protonated form of the chromophore at the single-molecule level via the corresponding vibrational fingerprints, and the fact that the enhanced green fluorescent protein (EGFP) also shows a high surface enhanced resonance Raman scattering (SERRS) signal. Our findings show the potential of the technique to study structural dynamics of protein molecules at a single-molecule level.

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Abstract:Recently, it has been shown that the red fluorescent protein DsRed undergoes photoconversion on intense irradiation, but the mechanism of the conversion has not yet been elucidated. Upon irradiation with a nanosecond-pulsed laser at 532 nm, the chromophore of DsRed absorbing at 559 nm and emitting at 583 nm (R form) converts into a super red (SR) form absorbing at 574 nm and emitting at 595 nm. This conversion leads to a significant change in the fluorescence quantum yield from 0.7 to 0.01. Here we demonstrate that the photoconversion is the result of structural changes of the chromophore and one amino acid. Absorption, fluorescence, and vibrational spectroscopy as well as mass spectrometry suggest that a cis-to-trans isomerization of the chromophore and decarboxylation of a glutamate (E215) take place upon irradiation to form SR. At the same time, another photoproduct (B) with an absorption maximum at 386 nm appears upon irradiation. This species is assigned as a protonated form of the DsRed chromophore. It might be a mixture of several protonated DsRed forms as there is at least two ways of formation. Furthermore, the photoconversion of DsRed is proven to occur through a consecutive two-photon absorption process. Our results demonstrate the importance of the chromophore conformation in the ground state on the brightness of the protein as well as the importance of the photon flux to control/avoid the photoconversion process.

The complex photophysics of the wild-type green fluorescent protein (GFP), one of the most popular fluorescent probes in biology, has been extensively documented in literature. The excited-state dynamics of GFP was explained by means of a model implying excited-state proton transfer (ESPT) and three forms of the chromophore, a protonated A form absorbing at 400 nm and two deprotonated I and B forms absorbing at around 475 nm. We report here a systematic picosecond time-resolved fluorescence study of the enhanced green fluorescent protein (EGFP) variant, carrying the Ser65-Thr and Phe64-Leu mutations. By means of multiple excitation wavelength time-resolved experiments, we were able to distinguish between the fluorescence decay times of the deprotonated I* and B* states (3.4 and 2.7 ns). Spectrally, we found the I form being red shifted in comparison with the B form, both in absorption and in emission. Evidence for an excited-state reaction, namely, proton transfer, is also reported. An additional protonated species is proposed in the photophysical scheme in order to explain the excited-state dynamics of EGFP on the basis of our results as well as previous reported data. Two alternative models are presented, both of them applicable also to the data reported in relation with wild-type GFP.

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