Hula-twisting in green fluorescent protein

Abstract

In the hula-twist (HT) process two adjacent bonds twist concertedly to give a product from simultaneous conformational and configurational isomerization. Based on quantum mechanical (QM) calculations and an isoviscosity analysis of internal conversion in green fluorescent protein (GFP) it has been proposed, in the literature, that the GFP chromophore undergoes a HT upon excitation. A volume analysis of HTs and, τ and ϕ one-bond-flips (OBF) in the chromophore model, 4-hydroxybenzylidene-2,3-dimethylimidazolidinone ( I ), revealed that the τ OBF displaces a larger volume than the HT or the ϕ OBF. However both the HT and ϕ OBF processes displace the same volume. Therefore the volume conserving property of the HT is not a sufficient reason for the excited chromophore to undergo a HT. Since the chromophore is unsymmetric there are 4 different possible HTs. Molecular mechanics (MM) calculations have been used to show that the protein matrix of GFP forms a cavity around the chromophore that is complementary to an excited state conformation in which the phenol and imidazolidinone rings are perpendicular to each other – a conformation that was obtained by a concerted positive 45° HT of both chromophore dihedral angles. The twisted chromophore obtained by concertedly rotating both dihedral angles in a negative direction is sterically disfavored relative to the planar and positively rotated forms. Rotating the τ and ϕ bonds in opposite directions does not produce a large change in strain energy, as the two ring systems stay in the same plane.