trans−cis Photoisomerization of a Photoactive Yellow Protein Model Chromophore in Crystalline Phase

Photoisomerisation quantum yield and non-linear cross-sections with femtosecond excitation of the photoactive yellow protein

The quantum yield of photoisomerisation of the photoactive yellow protein (PYP) strongly depends on peak power and wavelength with femtosecond optical excitation. Using systematic power titrations and addition of second order dispersion resulting in 140, 300 and 600 fs pulse durations, the one and multi-photon cross-sections at 400, 450 and 490 nm have been assessed from transient absorption spectroscopy and additionally the Z-scan technique. Applying a target model that incorporates photoselection theory, estimates for the cross-sections for stimulated emission and absorption of the first excited state, the amount of ultrafast internal conversion and the underlying species associated dynamics have been determined. The final quantum yields for photoisomerisation were found to be 0.06, 0.14-0.19 and 0.02 for excitation wavelengths 400, 450 and 490 nm and found to increase with increasing pulse durations. Transient absorption measurements and Z-scan measurements at 450 nm, coinciding with the maximum wavelength of the ground state absorption, indicate that the photochemical quantum yield is intrinsically limited by an ultrafast internal conversion reaction as well as by stimulated emission cross-section. With excitation at 400 nm photoisomerisation quantum yield is further significantly limited by competing multi-photon excitation into excited state absorption at 385 nm previously proposed to result in photoionisation. With excitation at 490 nm the photoisomerisation quantum yield is predominantly limited further by the significantly higher stimulated emission cross-section compared to ground state cross-section as well as multi-photon processes. In addition to photoionisation, a second product of multi-photon excitation is identified and characterised by an induced absorption at 500 nm and a time constant of 2 ps for relaxation. With power densities up to 138 GW cm(-2) the measurements have not provided indication for coherent multi-photon absorption of PYP. In the saturation regime with 450 nm excitation, the limit for the photoisomerisation quantum yield was found to be 0.14-0.19 and the excited state absorption cross-section 6.1 × 10(-17) cm(2) or 0.36 times the ground state cross-section of 1.68 × 10(-16) cm(2) per molecule. This places a fundamental restriction on the maximum populations and sample penetration that may be achieved for instance in femtosecond pump-probe experiments with molecular crystals of PYP.

Topochemistry and photomechanical effects in crystals of green fluorescent protein-like chromophores: effects of hydrogen bonding and crystal packing

To obtain insight into the effects of the environment on the photophysics and photochemistry of the green fluorescence protein (GFP), eight crystal structures of six synthetic aryl-substituted analogues (2-fluoro, 2-methyl, 3-hydroxy, 3-methoxy, 2,4-dimethyl and 2,5-dimethyl) of the GFP chromophore (4-hydroxy-benzylidenedimethylimidazolinone) were determined and correlated with their two-dimensional steady-state and time-resolved solid-state excitation-emission spectra. The stacking between the molecules greatly affected the emission energy and the lifetime of the emission of the chromophore, implying that pi-pi interactions could be critical for the photophysics of GFP. The reaction pathways were dependent on the excitation energy, resulting either in [2 + 2] photodimerization at the bridging double bond (UV excitation) or flipping of the imidazolone ring (visible excitation). The meta-hydroxy chromophore (3-HOBDI) was the only GFP-chromophore analogue that was obtained as more than one stable polymorph in the pure state thus far. Due to the asymmetric substitution with hydrogen bond donors and acceptors, 3-HOBDI is tetramorphic, the forms showing distinctly different structure and behavior: (1) while one of the polymorphs (3-HOBDI-A), having multilayer structure with alternating stereochemistry of linear hydrogen-bonded motifs, undergoes photodimerization under UV light, (2) another (3-HOBDI-C), which has dimeric head-to-tail structure, shows Z-to-E isomerization via tau-one-bond flip of the imidazolone ring by excitation in the visible region. X-ray diffraction analysis of a partially reacted single crystal of 3-HOBDI-C provided the first direct evidence of tau-one-bond flip occurring in a GFP-like compound. Moreover, the cooperative action of the photodimerization of 3-HOBDI-A appears as a photomechanical effect of unprecedented magnitude for a single crystalline specimen, where photoexcited single crystals bend to more than 90 degrees without breaking.

Ultrafast light-induced response of photoactive yellow protein chromophore analogues

The fluorescence decays of several analogues of the photoactive yellow protein (PYP) chromophore in aqueous solution have been measured by femtosecond fluorescence up-conversion and the corresponding time-resolved fluorescence spectra have been reconstructed. The native chromophore of PYP is a thioester derivative of p-coumaric acid in its trans deprotonated form. Fluorescence kinetics are reported for a thioester phenyl analogue and for two analogues where the thioester group has been changed to amide and carboxylate groups. The kinetics are compared to those we previously reported for the analogues bearing ketone and ester groups. The fluorescence decays of the full series are found to lie in the 1-10 ps range depending on the electron-acceptor character of the substituent, in good agreement with the excited-state relaxation kinetics extracted from transient absorption measurements. Steady-state photolysis is also examined and found to depend strongly on the nature of the substituent. While it has been shown that the ultrafast light-induced response of the chromophore in PYP is controlled by the properties of the protein nanospace, the present results demonstrate that, in solution, the relaxation dynamics and pathway of the chromophore is controlled by its electron donor-acceptor structure: structures of stronger electron donor-acceptor character lead to faster decays and less photoisomerisation.