While much is known about different
allosteric regulation mechanisms,
the nature of the allosteric signal and the time scale on which it
propagates remains elusive. The PDZ3 domain from postsynaptic density-95
protein is a small protein domain with a terminal third α-helix,
i.e., the α3-helix, which is known to be allosterically active.
By cross-linking the allosteric helix with an azobenzene moiety, we
obtained a photocontrollable PDZ3 variant. Photoswitching triggers
its allosteric transition, resulting in a change in binding affinity
of a peptide to the remote binding pocket. Using time-resolved infrared
and UV/vis spectroscopy, we follow the allosteric signal transduction
and reconstruct the timeline in which the allosteric signal propagates
through the protein within 200 ns.
We present systematic kinetic studies of the interaction of a rhenium-based photosensitizer with a cobalt(II) tetrapyridyl water reduction catalyst coadsorbed on ZrO2 by transient IR and visible spectroscopies. The study focuses on the competition between the reduction of the excited photosensitizer by an electron donor in solution and nonproductive quenching between the photosensitizer and the catalyst, either by Dexter energy transfer or by electron transfer followed by ultrafast geminate recombination. The implications of both interactions for the charge transfer reactions on the surface are investigated. We find that the kinetics of the system as a whole and the achievable yield of reduced photosensitizer are determined by the inhomogeneous distribution of next neighbor distances between photosensitizers and the water reduction catalysts at the nanoscale level. This provides insight for rational design of heterogeneous water splitting systems with coimmobilized photosensitizers and catalysts.
A stop-flow sample delivery system for transient spectroscopy is presented, which is, in particular, suited for laser-based instruments (quantum-cascade lasers or amplified femtosecond lasers) with excitation pulse repetition rates in the range 10-100 Hz. Two pulsing micro-valves are mounted onto a flow cuvette designed for transient IR spectroscopy, which is integrated into a flow cycle driven by a peristaltic pump. The performance of the system is demonstrated with transient IR experiments of the trans-to-cis photoisomerization of a water-soluble azobenzene derivative. The sample stands still when the micro-valves are closed and is pushed out from the probe beam focus on a 1 ms timescale when opening the micro-valves. The setup is extremely sample efficient. It needs only small sample volumes, and at the same time, it enables excitation of a large fraction of molecules in solution.
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