Introduction Fluorescent spectrum is an important photophysical property of fluorescent proteins, and it includes some useful information on electronically excited states of the protein. However, the origin of the spectral shape of the fluorescent spectra has not been discussed sufficiently. In analyzing the spectral shape theoretically, two important factors have to be considered: electronic structure of the excited state and structural fluctuations of the protein. Accurate electronic description of the excited state necessitates ab initio (non-empirical) quantum mechanical (QM) calculation, and sufficient consideration of protein fluctuations requires long molecular dynamics simulation with molecular mechanical (MM) force fields. To consider accurately and efficiently both aspects, we extended a hybrid QM/MM method (QM/MM-RWFE-SCF method[1]) to the excited state calculations. This new method enables one to simulate realistic fluorescence spectra of fluorescent proteins. Results & Discussion The method of spectrum calculation was applied to enhanced green fluorescent protein (EGFP), which possesses the remarkably broader absorption spectrum than the fluorescence one. The present simulation successfully reproduced the characteristic experimental spectra (Fig. 1). It is shown that in the electron transitions charge transfers occur within the chromophore and the magnitude of the charge transfer in the absorption is larger than that in the fluorescence. Our simulation suggests that the difference in the spectral width between the absorption and the fluorescence originate from the difference in the magnitude of the charge transfer, because the variance in transition energy is proportional to the change in charge distribution. References (1) Kosugi, T.; Hayashi, S. J. Chem. Theory Comput. 2012, 8, 322.

Theoretical study on the fluorescent spectrum of enhanced green fluorescent protein

249th National Meeting (2015)

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