The Effect of an Electric Field on the Spectroscopic Properties of the Isolated Green Fluorescent Protein Chromophore Anion

• Published in: Chemphyschem Vol. 19; no. 14; pp. 1686 - 1690
• Main Authors: Langeland, Jeppe, Kjær, Christina, Andersen, Lars H., Brøndsted Nielsen, Steen
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 17.07.2018
• Subjects: Absorption; Anions; Anions - chemistry; Charge transfer; Chromophores; Electric fields; Electricity; Fluorescence; GFP; Green Fluorescent Proteins - chemistry; Ion spectroscopy; Ionization; Ions; Molecular chains; Molecular Structure; Open channels; Photon absorption; Photophysics; Proteins; Quantum Theory; Spectrometry, Mass, Electrospray Ionization; GFP; Ion spectroscopy; Absorption; Charge transfer; Photophysics
• ISSN: 1439-4235; 1439-7641; 1439-7641
• DOI: 10.1002/cphc.201800225

Here we uncover the direct effect of a high electric field on the absorption by the Green Fluorescent Protein chromophore anion isolated in vacuo based on gas‐phase action spectroscopy. Betaine is a strong molecular dipole that creates an electric field of ∼70 MV/cm when attached to the ion at the phenolate oxygen, more than half the actual field from the protein matrix and pointing in the same direction. Nevertheless, the shift in absorption is limited (0.08 eV), supporting earlier conclusions, but subject to much debate, that the protein is rather innocent in perturbing the transition energy. The betaine complexes are readily made by electrospray ionization and in contrast to the bare ions, they dissociate after one‐photon absorption. Also, electron detachment is not an open channel complicating the bare ion case. As steric constraints are absent in vacuo, the possibility of turning on fluorescence by an electric field can be tested from experiments on complexes with betaine. One field at a time: It has been up for much debate how much the matrix of the Green Fluorescent Protein shifts the electronic absorption by its buried chromophore anion. Based on gas‐phase ion spectroscopy of complexes between the anion and the betaine zwitterion that has a significant dipole moment, the authors determine that the effect of a strong asymmetric electric field on the absorption band maximum is limited to 0.08 eV.