Complexation of green and red Kaede fluorescent protein chromophores by a zwitterion to probe electrostatic and induction field effects

Eleanor K. Ashworth, Mark H. Stockett, Christina Kjaer, Philip C. Bulman Page, Stephen R. Meech, Steen Brøndsted Nielsen, James N. Bull

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The photophysics of green fluorescent protein (GFP) and red Kaede fluorescent protein (rKFP) are defined by the intrinsic properties of the light-absorbing chromophore and its interaction with the protein binding pocket. This work deploys photodissociation action spectroscopy to probe the absorption profiles for a series of synthetic GFP and rKFP chromophores as the bare anions and as complexes with the betaine zwitterion, which is assumed as a model for dipole microsolvation. Electronic structure calculations and energy decomposition analysis using Symmetry-Adapted Perturbation Theory are used to characterize gas-phase structures and complex cohesion forces. The calculations reveal a preponderance for coordination of betaine to the phenoxide deprotonation site predominantly through electrostatic forces. Calculations using the STEOM-DLPNO-CCSD method are able to reproduce absolute and relative vertical excitation energies for the bare anions and anion–betaine complexes. On the other hand, treatment of the betaine molecule with a point-charge model, in which the charges are computed from some common electron density population analysis schemes, show that just electrostatic and point-charge induction interactions are unable to account for the betaine-induced spectral shift. The present methodology could be applied to investigate cluster forces and optical properties in other gas-phase ion–zwitterion complexes.
Original languageEnglish
Pages (from-to)1158–1167
Number of pages10
JournalThe Journal of Physical Chemistry A
Issue number7
Early online date9 Feb 2022
Publication statusPublished - 24 Feb 2022

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