Bifurcate link

ACS Symposium Series American Chemical Society Washington, DC, 1980. 

For these reasons, absorption of an iron chelating drug will be favored if (a) the drug is lipophilic, and (b) the pKg of any acidic group(s) in the molecule is 3 and the pK of any basic group(s) in the molecule if 8, or (c) a specific carrier mechanism is available. 

Various examples of this technique, know as drug latentiation , have been documented by Harper (46) and more recently by Sinkula and Yalkowsky (47). Interestingly, Catsch (48) has suggested that the improved efficacy of certain EDTA-like chelating agents in promoting heavy metal excretion may be related to their ability to form internal esters with enhanced bioavailability e.g.. 

Following its absorption from the gastrointestinal tract, an iron chelating drug will be transported to the liver via the portal vein. Here the drug must survive detoxification by 

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The chemical significance of this rather abstract idea is that the IRC bifurcates at the VRI, which is shown by the blue fine in Figure 21.3. Consequently, the reactant and product that are linked by TS2 are either both products for TS1 or both reactants for TSl, depending on which way the reaction proceeds. In principle, there could be a VRI on each side of TSl, meaning that two reactants and two products would be interconverted via a single transition state. There could even be additional VRIs on either side of TS2, which would increase the number of reactants and products that could share TSl. 

Figure 12.27. Molecules of 58 linked into chains by bifurcated C—FI - 0 interactions, similar to the a-network formed by urea.

Three-center (bifurcated) hydrogen bonding between adjacent base pairs explains unusual properties of poly(dA) poly(dT). The stability of the B-form of this duplex (and of its ribo analog) formed by two homopolymers might be a consequence of the extreme propeller twist (Box 20.1) reported for the central portions (the A-tracts) of the double helical dodecamer d(CGCAAAAAAGCG) [702] and d(CGCAAATTTGCG) [703]. In both crystal structures the DNA molecules show the same unusual three-center bonds linking adjacent base pairs. 

Bifurcated hydrogen bonds between dipyridinium cations and coordinated dithiooxalates have been used to assemble tapes [130] with similar geometry to those in which coordinated halides act as acceptors [15]. Replacement of dipyridinium with 4-carboxypiperidinium gave a structure in which the anions are linked by carboxylic acid dimer assembled dications. Similar bifurcated hydrogen bonds were observed in the structures of bipyridinium salts of anilic acids [131]. 

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