Effects of Hydrogen Bonding on Internal Conversion of GFP-like Chromophores. II. The meta-Amino Systems

Abstract

To rationalize the efficient quenching of the fluorescence and the Z → E photoisomerization of m-ABDI, the meta-amino analogue of the green fluorescent protein (GFP) chromophore, in protic solvents, the femtosecond time-resolved fluorescence and transient infrared (TRIR) spectra of m-ABDI in CD3CN, CH3OH, and CD3OD are determined. For solutions in CD3CN, the fluorescence decay lifetime is ∼7.9 ns and IR absorption lines near 1513, 1531, 1557, and 1613 cm(-1) of m-ABDI in its electronically excited state were observed with a decay time >5 ns. For solutions in CH3OH, the fluorescence decay is double exponential with time constants of ∼16 and 62 ps. In addition to IR absorption lines of m-ABDI in its electronically excited state with a decay time of ∼16 ps, new features near 1513, 1532, 1554, and 1592 cm(-1) were observed to have a rise time of ∼19 ps and a decay constant of ∼58 ps, indicating formation of an intermediate. The assignments for the IR spectra of the ground and excited states were assisted with DFT and TDDFT calculations, respectively. We conclude that the torsion of the exocyclic C═C bond (the τ torsion) is responsible for the nonradiative decay of electronically excited m-ABDI in CD3CN. However, in CH3OH and CD3OD, the solute-solvent hydrogen bonding (SSHB) interactions diminish significantly the barrier of the τ torsion and induce a new pathway that competes successfully with the τ torsion, consistent with the efficient fluorescence quenching and the diminished yield for Z → E photoisomerization. The new pathway is likely associated with excited-state proton transfer (ESPT) from the solvent to m-ABDI, particularly the carbonyl group, and generates an intermediate (ESPT*) that is weakly fluorescent.

Fluorescence of a Histidine-Modified Enhanced Green Fluorescent Protein (EGFP) Effectively Quenched by Copper(II) Ions. Part II. Molecular Determinants

“…The results regarding to planar geometries are in good agreement with those reported in previous studies [20,35]. It can be seen, that the HOMO orbital has π-bonding and antibonding character on the γ and β bond, respectively.…”

“…Moreover, the probability of finding electrons at imidazolinone ring is negligible in the HOMO, but is significant in the LUMO. The enhanced charge-transfer character entails a reduction in S 1 →S 0 emission [20] which is supported by the short fluorescence lifetimes for all complexes [34]. From the HOMO delocalization we can infer that the enhanced charge transfer occurs during the deexcitation process and the extra electron density on phenol ring will attracts a proton since it is positively charged.…”

“…In summary, by combining the computational and experimental techniques we attempted to fill the unknown parts of the quenching mechanism which are still unclear [18][19][20][21]. Hence, this analysis appears to be a new approach in simulating the chromophore behavior during the quenching process and offers a further opportunity to get a powerful insight into the deexcitation mechanism.…”

Strong electron donation induced differential nonradiative decay pathways for para and meta GFP chromophore analogues

“…[9][10][11] Their excited state fluorescence lifetimes (t f ) are also at least one order of magnitude higher than that of the para analogues. [9][10][11] Thus, enhancement of F f and t f could be achieved by structural modification. It has been shown that the extent of charge transfer is much higher for the meta analogues in comparison to the para analogues.…”

“…[1][2][3][4][5][6][7][8] This proposition has been extended to other unconstrained GFP chromophore analogues. [9][10][11][12] For example both p-HBDI and p-ABDI (Chart 1) have a Z-E isomerisation quantum yield (F ZE ) B 0.5 and F f B 0.0001. 9 However, meta analogues of these two molecules i.e.…”