Tris , anion binding

When the drug and the CD are both charged, electrostatic effects may be observed. Adverse electronic effects have been observed for the complexation between the anionic form of indomethacin and the dianion of carboxymethyl-p-CD, CM2-p-CD.At pH 6.6, indomethacin exists as an anion and under these conditions, the anionic carboxymethyl CD did not complex the drug at all, probably due to electrostatic repulsions. However, the tri-anion, CM3-p-CD l has been reported to complex the anionic forms of warfarin and indomethacin (Table 5) although only at 71 and 60% of the binding observed for the neutral p-CD. 

Steed and co-workers have also utilized a tripodal backbone to construct a number of tri-pyridinium venus flytrap receptors (71a-c) and investigated their anion binding and sensing properties [84,85]. 

Strong anion binding capacity (towards and Cl ) is also observed for non-cyclic methylene bridged, tri- and tetratin hosts, X(Ph2SnCH2) SnPh2X ( = 2 or 3, X = F and Cl) which form [X(Ph2SnCH2) SnPh2X](X ) i anionic complexes [59]. 

The active center of A. has two parts, the anionic binding site for the quatemaiy nitrogen (which is responsible for the alcohol specificity) and the esterase center (where a catalytic serine and histidine lyse the ester bond). The enzyme is inactivated by blockage of either the serine hydroxyl (by organic phosphate esters, such as diisopropylfluorophosphate or diethyl p-nitrophenylphosphate), or the anionic center by tri-methylammonium derivatives. If the enzyme has been blocked by organophosphates, it can be reactivated by pralidoxime salti which are therefore used as antidotes to organophosphate poisoning. 

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