Reversible dissociation, high-concentration

The interactions between transmitters and their receptors are readily reversible, and the number of transmitter-receptor complexes formed is a direct function of the amount of transmitter in the biophase. The length of time that intact molecules of acetylcholine remain in the biophase is short because acetylcholinesterase, an enzyme that rapidly hydrolyzes acetylcholine, is highly concentrated on the outer surfaces of both the prejunctional (neuronal) and postjunctional (effector cell) membranes. A rapid hydrolysis of acetylcholine by the enzyme results in a lowering of the concentration of free transmitter and a rapid dissociation of the transmitter from its receptors little or no acetylcholine escapes into the circulation. Any acetylcholine that does reach the circulation is immediately inactivated by plasma esterases. 

Both of these equations can be universally expressed through the Stern-Volmer constant k. The latter one has been used already in Section XII.B as a standard for comparison between the different theories of irreversible quenching. In the case of the reversible reaction (3.703), the problem is more difficult, especially at high concentrations of B molecules. After the dissociation, other B molecules can be involved in the reaction with A. This many-particle competition for the partner couples the motion of the molecules, making the problem unsolvable analytically. Thus only approximate solutions were obtained by means of different methods and assumptions whose validity very often remains unclear. 

These data show that the polypeptide chain of /3-lactoglobulin has a molecular weight of 18.(XX). and that in high concentration, the protein exists as a dimer of Mr = 36,000. However, on dilution, the dimer, maintained by reversible, noncovalent interactions, undergoes a partial dissociation A2 2A. 

Thus, immobilized HgCl2 could form the basis of a readily reversible sensor for aqueous chloride at moderate to high concentrations, though a tiny concentration of Cl would have to be present at all times in the contacting solution to prevent dissociation of the HgC complex. 

In the lungs the process is reversed. A high oxygen concentration drives the conversion from the deoxyHb configuration to that of oxyHb. The change in the protein s three-dimensional structure initiated by the binding of the first oxygen molecule releases bound COz, H+, and BPG. The H+ recombine with HCOj to form carbonic acid, which then dissociates to form C02 and H20. C02 subsequently diffuses from the blood into the alveoli. 

Coordinatively Unsaturated Cobalt Carbonyls Relevant to Hydro-formylation. The negative effect of carbon monoxide partial pressure on the rate of hydroformylation was the first indication of the participation of coordinatively unsaturated cobalt carbonyls in the catalysis of aldehyde formation and of the accompanying olefin isomerization. The retarding effect of carbon monoxide has also been observed in cobalt-catalyzed olefin and aldehyde hydrogenation and in various other reactions of cobalt carbonyls as well. It was assumed that in these reactions in fast reversible carbon monoxide dissociation highly reactive coordinatively unsaturated complexes are formed in very low concentrations, undetectable by conventional analytical methods. By using sophisticated new methods, in some cases the detection and characterization of these elusive species has become possible. 

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