Effects of Hydrogen Bonding on Internal Conversion of GFP-like Chromophores. I. The para-Amino Systems

Abstract

To understand the effects of solvent-solute hydrogen bonding (SSHB) on the excited-state dynamics of two GFP-like chromophores, p-ABDI and p-CFABDI, we have determined the quantum yields for fluorescence (Φf) and the isomerization Z → E (ΦZE) and the femtosecond fluorescence and transient infrared absorption in selected solvents. The behavior that ΦZE ≅ 0.50 in aprotic solvents, such as CH3CN, indicates that the E-Z photoisomerization adopts a one-bond-flip mechanism through the torsion of the exocyclic C═C bond (the τ torsion) to form a perpendicular species (τ ∼90°) in the singlet excited state followed by internal conversion (IC) to the ground state and partition to form the E and Z isomers with equal probabilities. The observed ΦZE decreased from 0.50 to 0.15-0.28 when CH3CN was replaced with the protic solvents CH3OH and CF3CH2OH. In conjunction with the solvent-independent rapid (<1 ps) kinetics for the fluorescence decay and the solvent-dependent slow (7-20 ps) kinetics for the ground-state recovery, we conclude that the SSHB modifies the potential energy surface for the τ torsion in a way that the IC occurs also for the twisted intermediates with a τ-torsion angle smaller than 90°, which favors the formation of the Z isomers. The possibility of IC induced by torsion of the exocyclic C-C bond (the φ torsion) is also considered but excluded.

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

“…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.…”

“…Significant charge transfer character within the chromophore implies large changes in molecular electron density which triggers the non-radiative decay by transition between anionic and neutral form. This is supported by the increase of λ abs for 2His-EGFP-Cu 2+ at 395 nm which means that the push−pull electronic character of the appropriate moieties in chromophore is strengthened and thereby the tendency to form a twisted intramolecular charge transfer state is increased [34].…”

“…It is also relevant that the internal conversion at a γ-distortion angle smaller than 90°can become more favorable energetically due to the presence of copper. This process was presented in detail and illustrated with a scheme by Huang et al [34].…”

TICT excited states of o‐ and p‐N,N‐dimethylamino analogues of GFP chromophore

“…However, p ‐HBDI significantly loses fluorescence after it is stripped of the protein environment, because it decays mostly through cis ‐ trans photoisomerization in aprotic solvents via the S 1 /S 0 conical intersection at the torsional angle of 90° around the I‐bond . In protic solvents, it was suggested that solvent‐solute hydrogen bonding modifies the potential energy surface (PES) of the S 1 excited state in a way that the S 1 excited state decays through cis ‐ trans photoisomerization via the S 1 /S 0 conical intersection at the torsional angle smaller than 90° around the I‐bond . On the other hand, ab initio calculations at CASPT2//CASSCF level suggested the S 1 excited state of the p ‐HBDI anion decays mainly through barrierless torsional deformation around the P‐bond .…”

“…To explore or tune the photophysical properties of the GFP chromophore, chemists prepared p ‐HBDI and its various analogues. Thus, the cis ‐ o ‐ and cis ‐ p ‐ N , N ‐dimethylamino analogues ( cis ‐ p ‐DMABDI and cis ‐ o ‐DMABDI) of GFP chromophore were prepared for their possible interesting photophysical properties (Scheme ) . One of the interesting properties is, unlike the dual fluorescence of p ‐DMABN, they were reported to have single fluorescence, and their fluorescent emissions shift to higher wavelengths when they stay in a more polar solvent .…”