Insights into Excited State Intramolecular Proton Transfer: An Alternative Model for Excited State Proton Transfer of Green Fluorescence Protein

Yi Hui Chen, Robert Sung, Kuangsen Sung

Research output: Contribution to journalArticlepeer-review

8 Citations (Scopus)

Abstract

The excited state intramolecular proton transfer (ESIPT) that occurs in the o-sulfonamide analogue (o-TsABDI) of the green fluorescent protein (GFP) chromophore provides an alternative model to get insights into the excited state proton transfer (ESPT) related photophysics of GFP. In this article, we explored the ESIPT-related photophysics of o-TsABDI by electronic absorption and fluorescence emission spectra in a wide polarity range of solvents, cis-trans photoisomerization experiment, and Coulomb-attenuating method (CAM)/time-dependent (TD) density functional theory (DFT) calculations. We found that the whole ESIPT process involves four steps. The first step is photoexcitation of o-TsABDI, which does not involve charge transfer (CT). The second step is ESIPT and accompanying electron transfer from the n orbital of the sulfonamide nitrogen to the half-filled π orbital of the 4-benzylideneimidazolone moiety. The third step is fluorescence emission of the zwitterionic o-TsABDI and accompanying CT from the π∗ orbital of the 4-benzylideneimidazolone moiety to the half-filled n orbital of the sulfonamide nitrogen. The last step involves irreversible and barrierless proton recombination. In contrast to the isolated GFP chromophore and its p- and m-amino analogues, the S1 excited state of o-TsABDI does not relax by way of cis-trans photoisomerization through the S1/S0 conical intersection CI(I) by rotating around the I-bond, but follows the ESIPT pathway. The low fluorescence quantum yield of the zwitterionic o-TsABDI might be due to (1) the fluorescence that involves the low-probability π∗ → n charge transfer and (2) nonradiative relaxation through the S1/S0 conical intersection CI(P″) by rotating around the P-bond.

Original languageEnglish
Pages (from-to)5931-5944
Number of pages14
JournalJournal of Physical Chemistry A
Volume122
Issue number28
DOIs
Publication statusPublished - 2018 Jul 2

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry

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