Polyelectrolytes-hypercoiling

A further application of time-resolved fluorescence measurements is in the study of conformational dynamics of polymer chains in solution. Fluorescence anisotropy measurements of macromolecules incorporating suitable fluorescent probes can give details of chain mobility and polymer conformation (2,14). A particular example studied in this laboratory is the conformational changes which occur in aqueous solutions of polyelectrolytes as the solution pH is varied (15,16). Poly(methacrylic acid) (PMA) is known to exist in a compact hypercoiled conformation at low pH but undergoes a transition to a more extended conformation at a degree of neutralization (a) of 0.2 to 0.3 (1 6). Similar conformational transitions are known to occur in biopolymer systems and consequently there is considerable interest in understanding the nature of the structures present in model synthetic polyelectrolyte solutions. 

In later work it was shown that water-soluble antennas could be made by copolymerizing aa oinatic monomers such as vinyl naphthalene and naphthylmethyl methacrylate with polyelectrolytes such as acrylic acid [3,4]. The high efficiency of these antennas in dilute aqueous base was attributed to the hypercoiling of the poly-(acrylic acid) chain to give a pseudo micellar stincture such as that illustrated schematically in Figure 2. We believe that such structures are formed spontaneously in solution due to the hydro-phobic interactions of the large aromatic ccmiponents stabilized by the interaction of water with the hydrophillic carboxyl anions. 

It was later shown that other types of polyelectrolytes involving partial sulfonation of poly(vinylnai ithalene) [5,6] and copolymers of aromatic monomers with styrene sulfonate would also lead to polymers which in aqueous solution achieved this hypercoiled configuration. 

Water-soluble probes have also been reported to undergo marked changes in their fluorescence characteristics when dispersed in polyelectrolyte solutions [18,52,61,72-78]. For example, the cationic dye, auramine O (AuO) is virtually nonfluorescent in aqueous solution but an increase in intensity is observed in the presence of PMAA at low pH as a result of enhanced binding in the hypercoiled state [72,73,75,78]. On neutralization, the fluorescence from AuO decreases as the compact structure breaks down forming the expanded state and the probe is released to the aqueous phase [52]. In a recent extension of this theme, the sensitivity of the emission spectrum of AuO to the environment in which it resides has been further exploited by covalently bonding the dye to PMAA [61]. Figure 2.1 shows the... 

The polarization of a DMA-labeled PMAA sample was monitored [18] as afunction of pH, and rc was later derived [46] at various degrees of ionization, via Equation 2.28. tc varies from ca. 32 ns at a = 0 to ca. 6 ns at a = 0.8. Not surprisingly, the authors [46] offered a similar explanation for the pH dependence of xc to that of Anufrieva and Gotlib [16] essentially, abreakdown in the hypercoil structure occurs as a increases and the polyelectrolyte expands allowing increased mobility of the chain segments. 

When TRAMs are made on dispersed probes, tc will reflect the speed of rotation of the fluorophore [20,46,60,76], which can be related to the microviscosity of the medium. In the context of probing the structure of PMAA, the fluorescent dyes have been occluded in the hypercoiled conformation [46,60,76] allowing an estimate of the size of the rotating, solubilizing cluster to be derived from the resultant tc. Alternatively, if the fluorophore is covalently attached to the polyelectrolyte in the form of a label then, depending on its mode of attachment, information concerning motion of the chain ends [46,60,76], the backbone [26,88,112,113], and chain substituents [26,88] can be derived from tc. 

Early time-resolved anisotropy experiments on PMAA used a combination of anthryl-based labels and probes [18,46,60,76] in an effort to fully characterize the conformational switch of the polyelectrolyte in aqueous solution. In their study of probes dispersed in and labels incorporated into PMAA, Treloar and coworkers [60,76] derived information from anisotropy experiments pertinent not only to the cluster size of the rotating units, but also to the structure of the hypercoil itself. 

Polyelectrolytes modified via covalent bonding with long hydrocarbon chains are expected to behave as hypercoils over the entire pH range. The hydrophobic interaction between the paraffinic side chains in these systems leads to stabilization of compact structures even at high pH. Such polymers have been called polysoaps or intramolecular micelle-forming polymers. 

Auramine O and ethidium bromide interactions with hypercoiling olefin maleic acid copolymers have been used as probes to study the polyelectrolyte conformations.The two cationic dyes responded in fluorescence or in visible spectra to changes in conformation of aqueous solutions of maleic acid copolymers with ethylene, isobutene, 2-methyl-l-pentene, or 2,4,4-trimethyl-1-pentene. The fraction of bound auramine is very sensitive to the presence of polyelectrolyte compact coils and to changes in ionic strength and type of counterions. The fluorescence spectra of the ethidium bromide systems also depended on the structure of the compact coils. Dye properties are correlated with the viscosity and equilibrium thermodynamic data for the various copolymers. 

The overall yield and the kinetics of photoinduced electron transfer (ET) for a polyelectrolyte-bound chromophore are modified by steric effects arising from hydrophobic interactions between the polymer and chromophore [43-45] these are termed hydrophobic protection. Partially sulfonated poly(vinylnaphthalene)s form a hypercoiled structure in water, and photoexcitation energy migrates through naphthalene units in the hypercoil [46]. Such antenna polyelectrolytes, with photochemically reactive molecules incorporated inside the hypercoil, exhibit efficient photosensitized reactions owing to the antenna effect, and are termed photozymes [46]. Hydrophobic protection and photozymes are based on the same principles as compartmentalization. 

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