title: Photo-induced protonation/deprotonation in the GFP-like fluorescent protein Dronpa: mechanism responsible for the reversible photoswitching.
abstract: Recently, reversible photoswitching in bulk samples or in individual molecules of Dronpa, a mutant of a green fluorescent protein (GFP)-like fluorescent protein, has been demonstrated. Intense irradiation at 488 nm changed Dronpa in a dim protonated form, and weak irradiation at 405 nm restored it to the bright deprotonated form. Here, we report on the mechanism of photoswitching of Dronpa by means of ensemble and single-molecule spectroscopy. Ensemble spectroscopy shows that the photoswitching can be described, in first approximation, by a three-state model including a deprotonated (B), a protonated (A1), and a photoswitched protonated (A2) forms of the chromophore. While the B and the A1 forms are in a ground state acid-base equilibrium, the B and the A2 forms are reversibly photoswitched upon irradiation with 488 and 405 nm light. At the single-molecule level, the on-times in fluorescence intensity trajectories excited at 488 nm decrease with increasing the excitation power, consistent with the photoswitching from the B to A2 form. The on-times agree well with expected values, which are calculated based on the ensemble spectroscopic properties of Dronpa. The fluorescence trajectory obtained with simultaneous dual-color excitation at 488 and 405 nm demonstrates reversible photoswitching between the B and the A2 forms at the single-molecule level. The efficiency of the photoswitching from the A2 to B form increased with increasing the excitation power of the 405 nm light. Our results demonstrate that Dronpa holds its outstanding photoswitching properties, based on a photo-induced protonation/deprotonation process, even at the single-molecule level.
authors.affiliation: Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium. satoshi_habuchi@hms.harvard.edu
authors.forename: Satoshi
authors.initials: S
authors.lastname: Habuchi
authors.forename: Peter
authors.initials: P
authors.lastname: Dedecker
authors.forename: Jun-ichi
authors.initials: J
authors.lastname: Hotta
authors.forename: Cristina
authors.initials: C
authors.lastname: Flors
authors.forename: Ryoko
authors.initials: R
authors.lastname: Ando
authors.forename: Hideaki
authors.initials: H
authors.lastname: Mizuno
authors.forename: Atsushi
authors.initials: A
authors.lastname: Miyawaki
authors.forename: Johan
authors.initials: J
authors.lastname: Hofkens
mesh_terms: D049452:Green Fluorescent Proteins / Q000737:chemistry / Q000528:radiation effects*; D010777:Photochemistry; D011522:Protons; D013050:Spectrometry, Fluorescence
chemical_list: D011522:Protons; D049452:Green Fluorescent Proteins