Signal transfer mechanism

Mechanistic studies on the formation of PPS from polymerization of copper(I) 4-bromobenzenethiolate in quinoline under inert atmosphere at 200°C have been pubUshed (91). PPS synthesized by this synthetic procedure is characterized by high molar mass at low conversions and esr signals consistent with a single-electron-transfer mechanism, the Sj l-type mechanism described earlier (22). 

The advantage of employing periodic perturbation of light intensity, e.g., using a chopper, and phase-sensitive detection are beyond a simple enhancement of the signal-to-noise ratio. For photoinduced electron-transfer mechanisms, as schematized in Fig. 11, the... 

Receptors are also located on the presynaptic membrane. These autc-reivptors regulate the release and synthesis of the neurotransmitter and are part of a feedback mechanism that aims to keep the activation or inhibition within the synapse for a discrete time interval and to terminate the signal transfer once the information has reached the adjacent cell via removal (mactivation) of the neurotransmitter from the synaptic cleft. 

We may now assemble the foregoing information into a molecular description of a few biological processes in which the interaction between water and metal ions plays an important role. First some problems related to signal transfer in nerve cells are discussed. This is followed by some comments on the mechanism operating at nerve synapses in which, in addition to the sodium and potassium ions, a specific transmitter substance and calcium ions take part. 

From Fig. 6.14, it can be deduced that for A Ef = —142.4 mV, the two mechanisms show distinctly different voltammograms for small values of %cw(= (k 1 + ki) ja), whereas the responses become more similar as the chemical kinetics is faster. Thus, for// v >100 the voltammetric signal of the ECE mechanism is equivalent to that of an EE mechanism where the half-wave potentials correspond to those of the EC (first electron transfer) and CE (second electron transfer) mechanisms under fully labile conditions (Eqs. (3.201c) and (3.221b) for <5, > 0, respectively). 

Many chemosensors exploit electron transfer as the signal transduction mechanism. Owing to different design constraints, we distinguish among systems whose function is derived from PET as described by Fig. 6 versus those systems whose function relies on electron (or energy) transfer between discrete donor and acceptor sites. 

Interestingly, if the same titration experiment is performed in presence of any other 3d divalent cation (Mn Fe, Co, Zn ), no fluorescence quenching is observed over the entire 2-12 pH interval. In this connection, it must be noted that deprotonation of the amide groups of the dioxocyclam subunit is an especially endothermic process, which can take place only if the unfavorable heat effect is compensated by the exothermic formation of strong metal-ligand bonds. This situation occurs only with metal ions late in the 3d series, e.g., Ni and Cu , but not with cations earlier in the same series. Thus, the anthracene functionalized dioxocyclam system represents a selective fluorescent sensor for Cu" and Ni , whose signal transduction mechanism is an electron transfer process [13]. 

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