Regioselective and solvent-dependent photoisomerization induced internal conversion in red fluorescent protein chromophore analogues

Abstract

Photophysical properties of three red fluorescent protein (RFP) chromophore analogues are reported here. The three RFP chromophore analogues differ in the additional conjugation present in the RFP chromophore. The three chromophores do not exhibit any solvent effect in both absorption and fluorescence spectra. The photoirradiation experiments and recording of ¹H NMR before and after irradiation on one of the three RFP model chromophores show isomerization of the (Z,E) diastereomer to the (E,E) diastereomer. Calculation of S₀ and S₁ potential energy curves shows the preference for isomerization through the exocyclic CC bond with Z-stereochemistry, thus corroborating the experimental results. The computational studies also suggest that torsional motion along the exocyclic CC bond pushes the molecules to a conical intersection, thus paving the pathway for radiationless deactivation.