Signaling state formation

Initial Events Signaling State Formation and Ground State Recovery The Photocycle Model Structural Relaxation... 

Signaling State Formation and Ground State Recovery... 

The relaxation processes of Br2 from the photoexcited state in carbon tetrachloride (CC14) was investigated by the diffusive component of the thermal grating [95]. A long-lived component (18 ns) and a fast-rising component were observed in the TG signal. The slower dynamics was assigned to the decay from the lowest excited state A ( n2u). From the amplitude of these components, the quantum yield of the A state formation was estimated to be 0.50 + 0.08. 

Recently, photocycle models changed significantly with respect to events that occur after the formation of the pR intermediate (pre-pR events are discussed in detail in Section 123.4.1). Much of the early work assumed a simple three-state photocycle model, i.e., with the pG, pR, and pB states only. In these models, formation of the signaling state, pB (from pR), was described as a kinetically biexponential and as a monoexponential event. Actually, the first detailed analysis of the photocycle of PYP postulated a photocycle containing an additional intermediate with similar spectral properties as pB, because of the observed biexponential kinetic character of pB formation. However, this model was abandoned in a... 

After formation of the signaling state, the photocycle enters the recovery phase. For recovery to take place, the chromophore has to be re-isomerized, and the protein needs to return to its ground state fold. Before re-isomerization can take place, the chromophore needs to be deprotonated, and the protein needs to adopt a specific fold that allows for the re-isomerization to take place. This situation is represented by the intermediate, which is in equilibrium with pB. In addition to this recovery in the dark, it... 

The degree of surface cleanliness or even ordering can be determined by REELS, especially from the intense VEELS signals. The relative intensity of the surface and bulk plasmon peaks is often more sensitive to surface contamination than AES, especially for elements like Al, which have intense plasmon peaks. Semiconductor surfaces often have surface states due to dangling bonds that are unique to each crystal orientation, which have been used in the case of Si and GaAs to follow in situ the formation of metal contacts and to resolve such issues as Fermi-level pinning and its role in Schottky barrier heights. 

It is important to emphasize that direet studies sueh as those earned out on the eyelopropylmethyl radieal ean be done with low steady-state eoneentrations of the radical. In the case of the study of the eyelopropylmethyl radical, removal of the source of irradiation leads to rapid disappearance of the EPR spectrum, because the radicals react rapidly and are not replaced by continuing radical formation. Under many conditions, the steady-state concentration of a radical intermediate may be too low to permit direct detection. Failure to observe an EPR signal, therefore, cannot be taken as conclusive evidence against a radical intermediate. 

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