Interactions specific

The classic example in Nature involving multivalent interactions for specific binding is the double helix formation in DNA. Interesting superstructures have been achieved by hybridizing block copolymer based DNA molecules (Jeong and 

In addition to utilizing natural ODNs in block copolymers, ODN mimics have also been studied using polymers with the capability of forming hydrogen bonds. Bazzi and colleagues (Bazzi and Sleiman 2002 Bazzi et al. 2003) have reported diblock and triblock amphiphilic polymers with diamidopyridine (DAP) and dicarboximide moieties as molecular recognition units and studied their micelle formation properties (Chart 2.4). 

Chart 2.4 Structures of diblock and triblock ampbipbiUc pol3fmers with diatnidopyridine (DAP) and dicarboximide moieties. 

Carbohydrate-protein interactions have gained particular interest because of implications in areas such as ceU-specihc drug delivery (Dwek 1996). 

Chart 2.5 Structure of GlcNAc-based homopol3fmer (8) and amphiphilic block copolymer (9). 

Creative concepts in crystal engineering make use of supramolecular synthons to define specific interactions in molecular solids, which may be employed to design crystal packings [1, 4]. The synthon approach has been used to produce close packed structures, and, on the contrary, to build open network structures [21]. In the packing motifs of high-performance pigments several types of specific interactions (synthons) can be identified. 

The packing of aromatic hydrocarbons has been subjected to intense investigations [25]. Many organic pigments are derived from polycycHc aromatic hydrocarbons, and their basic packing behavior often reflects this relationship. 

The solubility parameter can be used as a guide (as noted [92]) but lacks the ability to predict the specific interactions so important in achieving miscibility in many polymer blends. Reviews of the solubility parameter concept applied to polymers and polymer blends include [97,98]. 

Because of its simplicity, the concept of Donnan equilibrium is an attractive approach for the modelling of fibre-ion interactions. However, despite its general success in many cases, the Donnan theory alone is not sufficient to describe the experimentally observed distributions. In those cases, it is necessary to use ion-specific complexation equilibria with the acidic groups as a part of the model to describe the observed phenomena satisfactorily. Such behaviour has been shown to exist, for example, with chemically modified highly charged fibres. An apparent systematic deviation in the ionic distribution from the predictions of the Donnan theory has also been noted with common oxygen-delignified kraft pulps, partly, but 

Plants not only evolved allelochemicals with broad activities (see Section 1.3.1) but also some that can interfere vdth a particular target [3,6,17-19,25]. Targets that are present in animals but not in plants are nerve cells, neuronal signal transduction, and the endocrinal hormone system. Compounds that interfere with these targets are usually not toxic for the plants producing them. Plants have had to develop special precautions (compartmentation resin ducts, trichomes, laticifers) in order to store the allelochemicals with broad activities that could also harm the producer. 

Many alkaloids fall into the class of spedflc modulators and have been modified during evolution in such a way that they mimic endogenous ligands, hormones, or substrates [1,3,18,19]. We have termed this selection process evolutionary molecular modeling [12,13,19,23]. Many alkaloids are strong neurotoxins that were selected for defense against animals [2,3,19]. Table 1.1 summarizes the potential neuronal targets that can be affected by alkaloids. Extensive reviews on this topic have been published [2,3,19]. 

Alkaloids that structurally mimic neurotransmitters can bind to neuroreceptors and either activate (agonists) or inactivate (antagonists) them (Table 1.1). Additional important targets are ion channels, such as the Na , K, and Ca + channels several alkaloids are known that inhibit or activate these ion channels (Table 1.1). 

Many alkaloids are infamous for their strong toxicity towards animals and humans. Most of the deadly alkaloids fall into the class of neurotoxins (see above). The others have cytotoxic properties (Table 1.2). A cytotoxic effect can be generated when cell membranes are made leaky (as by saponins or steroidal alkaloids), or when elements of the cytoskeleton are inhibited. The spindle poisons vinblastine, vincristine, colchicine, and taxol are particularly famous. Actin filament formation is blocked by fungal poisons such as phalloidin from Amanita phalloides. 

Ribosomal protein biosynthesis is often inhibited by alkaloids that interact with nucleic acids [23]. There are also more specific inhibitors, such as emetine. 

Coleman and Painter [4-6] consider hydrogen bonding as the central strong interaction in polymers that cause the observed phase behavior and miscibility. They have used Fourier transform infrared (FTIR) spectroscopy to study the hydrogen bonding in polymer blends in systems such as polyamides and polyurethanes. A large class 

Early attempts to model polymer miscibility started with the Flory-Huggins theory. Modifications to the theory to account for the free volume effects have been proposed in the literature. The Flory interaction parameter, x, was allowed to assume negative values to explain observed polymer-polymer miscibility. This approach was not found to be satisfactory for four reasons  

The number of hydrogoi bonding contacts are not randffln. The interaction tom has to take a more complex composititxi dependent fimn than the Pi P2Xi2- 

Formation of hydrogen bonds results in loss of rotational degree of freedom in the molecules. 

A specific and nonspecific interactions cannot be lumped into a single interaction parameter, % 

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The interest in vesicles as models for cell biomembranes has led to much work on the interactions within and between lipid layers. The primary contributions to vesicle stability and curvature include those familiar to us already, the electrostatic interactions between charged head groups (Chapter V) and the van der Waals interaction between layers (Chapter VI). An additional force due to thermal fluctuations in membranes produces a steric repulsion between membranes known as the Helfrich or undulation interaction. This force has been quantified by Sackmann and co-workers using reflection interference contrast microscopy to monitor vesicles weakly adhering to a solid substrate [78]. Membrane fluctuation forces may influence the interactions between proteins embedded in them [79]. Finally, in balance with these forces, bending elasticity helps determine shape transitions [80], interactions between inclusions [81], aggregation of membrane junctions [82], and unbinding of pinched membranes [83]. Specific interactions between membrane embedded receptors add an additional complication to biomembrane behavior. These have been stud-... 

Figure C2.3.14. Isolated surfactant modes of adsorjDtion at liquid-solid interfaces for a surfactant having a distinct headgroup and hydrophobic portion (dodecyltrimetlrylammonium cation) (a), (b) headgroup specific interaction (c), (d) hydrophobic tail interaction, (e),(f) headgroup and tail interactions.

Mark, A. E., van Gunsteren, W. F. Decomposition of the free energy of a system in terms of specific interactions. Implications for theoretical and experimental studies. J. Mol. Biol. 240 (1994) 167-176... 

Boresch S and M Karplus 1995. The Meaning of Component Analysis Decomposition of the Free Energy in Terms of Specific Interactions. Journal of Molecular Biology 254 801-807. 

Furthermore, the number of diene - dienoplrile combinations that can be expected to undergo a Lewis-acid catalysed Diels-Alder reaction is limited. Studies by Wijnen leave little doubt that the rate of typical Diels-Alder reactions, where the dienophile is activated by one or more carbonyl functionalities, does not respond to the presence of Lewis acids in aqueous solution , at least not beyond the extent that is expected for non-specific interactions (salt effects). No coordination of the Lewis acid to the dienophile was observed in these cases, which is perhaps not surprising. Water is... 

For ammonium surfactants there is evidence for the existence of an additional specific interaction between the headgroups of the surfactant and the aromatic solubilisate . This is in line with the observation that partition coefficients for benzene in CTAB solutions are much higher than those for... 

Ph.CH2.OMe, Ph.(CH2)2.0Me, Ph.(CH2)3.0Me (2-3, 3-4, 1-3), does not decrease steadily, but goes through a maximum. These two circumstances point to a specific -interaction in nitrations of the ethers with acetyl nitrate which is important with benzyl methyl ether, more important with methyl phenethyl ether, and not important with methyl phenpropyl ether. This interaction is the reaction with dinitrogen pentoxide already mentioned, and the variation in its importance is thought to be due to the different sizes of the rings formed in the transition states from the different ethers. 

The argtiments of Norman and his co-workers seem to give affirmative answers to the first and second of these questions, but it is doubtful if the available data further require such an answer for the third question. It can be argued that the crucial comparison made between the behaviour of benzyltrimethylammonium ion and protonated benzyl methyl ether is invalid, and that it is possible to interpret the results in terms of nitration by the nitronium ion, modified by protonation of the oxygen atom of the ether a case for the possible involvement of the nitro-nium ion in specific interaction leading to o-substitution has been made. ... 

It is only the contribution of AH to AG that we are discussing here, but we see the effect of this contribution-in the systems for which the approximation is valid-is that a solvent becomes less suitable to dissolve a polymer the greater the difference is between their 6 values. At best, when 61 = 62, the solvent effect is neutral. Cases for which a favorable specific interaction between solvent and polymer actually promotes solution are characterized by negative values of AH and are therefore beyond the capabilities of this model. 

One noteworthy neurotoxic response was demonstrated in laboratory pyrolysis studies using various types of phosphoms flame retardants in rigid urethane foam, but the response was traced to a highly specific interaction of trimethylolpropane polyols, producing a toxic bicycHc trimethylolpropane phosphate [1005-93-2] (152). Formulations with the same phosphoms flame retardants but other polyols avoided this neurotoxic effect completely. 

Unlike most crystalline polymers, PVDF exhibits thermodynamic compatibiUty with other polymers (133). Blends of PVDF and poly(methyl methacrylate) (PMMA) are compatible over a wide range of blend composition (134,135). SoHd-state nmr studies showed that isotactic PMMA is more miscible with PVDF than atactic and syndiotactic PMMA (136). MiscibiUty of PVDF and poly(alkyl acrylates) depends on a specific interaction between PVDF and oxygen within the acrylate and the effect of this interaction is diminished as the hydrocarbon content of the ester is increased (137). Strong dipolar interactions are important to achieve miscibility with poly(vinyhdene fluoride) (138). PVDF blends are the object of many papers and patents specific blends of PVDF and acryflc copolymers have seen large commercial use. 

Specific Interactions. Ideas oa the subject of specific iateractions between PVC and a plasticizer molecule, as a basis of plasticization, can be considered a more detailed form of some of the ideas already discussed. Clearly some mechanism of attraction and interaction between PVC and plasticizer must exist for the plasticizer to be retained in the polymer after processing. 

The role of specific interactions in the plasticization of PVC has been proposed from work on specific interactions of esters in solvents (eg, hydrogenated chlorocarbons) (13), work on blends of polyesters with PVC (14—19), and work on plasticized PVC itself (20—23). Modes of iateraction between the carbonyl functionaHty of the plasticizer ester or polyester were proposed, mostly on the basis of results from Fourier transform infrared spectroscopy (ftir). Shifts in the absorption frequency of the carbonyl group of the plasticizer ester to lower wave number, indicative of a reduction in polarity (ie, some iateraction between this functionaHty and the polymer) have been reported (20—22). Work performed with dibutyl phthalate (22) suggests an optimum concentration at which such iateractions are maximized. Spectral shifts are in the range 3—8 cm . Similar shifts have also been reported in blends of PVC with polyesters (14—20), again showing a concentration dependence of the shift to lower wave number of the ester carbonyl absorption frequency. 

M. M. Coleman,. E. Graf, and P. C. Painter, Specific Interactions and the Miscibility ofiPoljmer Blends, Technomic, Lancaster, Pa., 1991. 

SAMs provide the needed design flexibUity, both at the individual molecular and at the material levels, and offer a vehicle for investigation of specific interactions at interfaces, and of the effect of increasing molecular complexity on the stmcture and stabUity of two-dimensional assembHes. These studies may eventuaUy produce the design capabUities needed for assembHes of three-dimensional stmctures (109). 

Electroanalytical chemistry is one of the areas where advantage of the unique properties of SAMs is clear, and where excellent advanced analytical strategies can be utilized, especially when coupled with more complex SAM architectures. There are a number of examples where redox reactions are used to detect biomaterials (357,358), and where guest—host chemistry has been used to exploit specific interactions (356,359). Ion-selective electrodes are an apphcation where SAMs may provide new technologies. Selectivity to divalent cations such as Cu " but not to trivalent ions such as Fe " has been demonstrated (360). 

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