Green Fluorescent Protein (GFP) and Photoactive Yellow Protein (PYP) contain deprotonated, anionic chromophores that absorb light. Both proteins are known to undergo photoinduced oxidation reactions, which potentially limit their biological function and applications. However, the most fundamental property underpinning photooxidation, the electron detachment energy, has only been determined for their deprotonated chromophores in the gas phase. Here, we develop a methodology for calculating the electron detachment energy (VDE) of anionic biological chromophores in aqueous solution and determine the first and higher-lying VDEs of the GFP and PYP chromophores, which are consistent with the experimental data. We also show that in most cases electron detachment occurs resonantly through electronically excited states of the chromophore anions, revealing their importance in electron transfer processes in the condensed phase.<br/>We perform a series of calculations with model systems of increasing size. The structures of these model systems are obtained using the hybrid DFT/EFP/MD approach, and the VDEs are calculated using the invariant version of the multiconfiguration quasi-degenerate perturbation theory XMCQDPT2 coupled to the Effective Fragment Potential (EFP) method with the basis set augmented by a very diffuse p-type function. We show that the first VDE strongly depends on the system size as it relies on the accuracy of the absolute energy estimation of long-range interactions between the negatively charged chromophore and polar water molecules. It converges for a system with ~12500 water molecules (R = 40 Å). The calculated VDE of the GFP chromophore is 6.9 eV. Notably, the first VDE is more than double that of the deprotonated chromophore in vacuo (2.7 eV) as a result of solvent stabilization of the anion. At the same time, the VDE of the GFP protein is similar to that in aqueous solution and equals 7.1 eV. The calculated VDE of the PYP chromophore is 6.6 eV. We conclude that the photooxidation properties of the deprotonated chromophore in aqueous solution are similar to those of the protein. We also show evidence for formation of solvated electrons via a higher-lying excited state of the solvated chromophores, which acts as a gateway for electron transfer processes in the condensed phase.

Theoretical account of multiphoton ionization of anionic biochromophores in aqueous solution

ACS Spring Meeting 2021

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