Excited-state dynamics of bacteriorhodopsin probed by broadband femtosecond fluorescence spectroscopy

AbstractThe impact of varying excitation densities (∼0.3 to ∼40 photons per molecule) on the ultrafast fluorescence dynamics of bacteriorhodopsin has been studied in a wide spectral range (630–900 nm). For low excitation densities, the fluorescence dynamics can be approximated biexponentially with time constants of <0.15 and ∼0.45 ps. The spectrum associated with the fastest time constant peaks at 650 nm, while the 0.45 ps component is most prominent at 750 nm. Superimposed on these kinetics is a shift of the fluorescence maximum with time (dynamic Stokes shift). Higher excitation densities alter the time constants and their amplitudes. These changes are assigned to multi-photon absorptions

Photoacid for Extremely Long-Lived and Reversible pH-Jumps

The ability to change the acidity of an aqueous solution within the time of a short laser pulse and use the resulting low pH to drive acid-catalyzed reactions in the irradiated volume opens news perspectives for the spatial and temporal control of a variety of processes. Persistent and reversible acidification of an aqueous solution is achieved with a new molecule, 1-(2-nitroethyl)-2-naphthol, that combines the fast photoacid properties of an aromatic alcohol with the slow proton transfer rates of a nitroalkane. The protons released in a few nanoseconds by pulsed laser excitation of the photoacid last for nearly one second in aqueous solutions. We show that acid−base equilibria with other species is established at the lower pHs of the irradiated volume, that the process is reversible and that it can be maintained under continuous irradiation

pH-Jump overshooting

Acid – base systems are commonly expected to equilibrate on a timescale much faster than any other chemical reaction, so their composition can be deduced from the corresponding pKa or pKb values. In a pH-jump experiment done on a multi acid/base pair system, it was found that it takes tens of microseconds before an equilibrium is established. Within that time, the system is kinetically driven reaching surprising states far different from its final equilibrium, for example carboxylate groups were protonated in the presence of hydroxyl ions

UV-Induced Isomerization Dynamics of N

The photoisomerization dynamics of N-methyl-2-pyridone (NMP) dissolved in CH3CN have been interrogated by time-resolved electronic and vibrational absorption spectroscopy. Irradiation at two different wavelengths (330 or 267 nm) prepares NMP(S1) molecules with very different levels of vibrational excitation, which rapidly relax to low vibrational levels of the S1 state. Internal conversion with an associated time constant of 110(4) ps, leading to reformation of NMP(S0) molecules, is identified as the dominant (&gt;90%) decay pathway. Much of the remaining fraction undergoes a photoinitiated rearrangement to yield two ketenes (revealed by their characteristic antisymmetric C═C═O stretching modes at 2110 and 2120 cm(-1)), which are in equilibrium. The rate of ketene formation is found to be pump-wavelength dependent, consistent with ab initio electronic structure calculations which predict a barrier on the S1 potential energy surface en route to a prefulvenic conical intersection, by which isomerization is deduced to occur. Two kinetic models-differentiated by whether product branching occurs in the S1 or S0 electronic states-are presented and used with equal success in the analysis of the experimental data, highlighting the difficulties associated with deducing unambiguous mechanistic information from kinetic data alone.</p