Organic cation binding

The examples just presented give initial impressions of how DNA can be utilized as a template in the synthesis of nanometric and mesoscopic aggregates. However, the studies emphasize the importance of fundamental research on the interaction between DNA and the various binders, such as metal and organic cations. Of particular importance are the consequences of binding events on the structure and topology of the nucleic acid components involved. 

Bednarczyk D, Ekins S, Wikel JH and Wright SH. Influence of molecular structure on substrate binding to the human organic cation transporter, hOCTl. Mol Pharmacol 2003 63 489-98. 

Suhre WM, Ekins S, Chang C, Swaan PW and Wright SH. Molecular determinants of substrate/inhibitor binding to the human and rabbit renal organic cation transporters hOCT2 and rbOCT2. Mol Pharmacol 2005 67 1067-77. 

The fact that f.a.b.-m.s. can be used to observe cluster ions has been exploited in a study of metal binding to cyclodextrins, and in an investigation of the complexes formed between a 3-O-methylmannose dodecasac-charide and alkyltrimethylammonium ions having decyl and hexadecyl as alkyl chains. In the latter study, the larger organic cation was shown to form the stronger complex. 

The foregoing discussion of micellar charge effects has implicitly assumed that differences in water activity or substrate location in cationic and anionic micelles are not of major importance. If such differences were all important it would be difficult to explain the differences in k+/k for carbonyl addition and SN reactions, because increase of water content in an aqueous-organic solvent speeds all these reactions (Johnson, 1967 Ingold, 1969). As to substrate location, there is very extensive evidence that polar organic molecules bind close to the micelle-water interface in both anionic and cationic micelles, although the more hydrophobic the solute the more time it will spend in the less polar part of the micelle. Substrate hydrophobicity has a marked effect on the overall rate effects in both cationic and anionic micelles, but less so on values of k+/k. It seems impossible to explain all these charge effects in terms of differences in the location of substrates in cationic and anionic micelles. 

In view of the compensatory enthalpy-entropy relationship observed for a wide variety of ionophore types, we may conclude that the cation-binding behavior, where the weak ion-dipole and dipole-dipole interaction is the major driving force for complexation, can be quantitatively analyzed and characterized by the slope and intercept of the AH-TAS plot without any exception. In this context, it is stimulating to extend the scope of this theory to the inclusion complexation of organic guests with molecular hosts. 

Sodium ion exchange. Sodium ion exchange on zeolites (Section 7.3) or on synthetic organic cation-exchange resins such as Dowex-50 (a sulfonated polystyrene Fig. 14.1), in most circumstances, is superior to the above softening methods.13 The exchange process favors binding of Ca2+ or Mg2+ over Na+ in the solid resin phase ... 

The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

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