Hydrophobic hypercoil

Dubin, P., and U. P. Strauss Hydrophobic hypercoiling in copolymers of maleic add and alkyl vinyl ethers. J. Phys. Chem. 71, 2757 (1967). 

A dramatic reduction in the quenching efficiency is apparent from the labeled PMAA samples (see Table 2.2) on use of / at a low pH. This reflects the fact that the anionic quencher is inhibited from accessing the hydrophobic hypercoil. At pH values in excess of 6, although PMAA is in an extended conformation, repulsion occurs between the I and the carboxylate ions which results in reduced kq values (ca. 0.2-0.05 x 109mol 1dm3 s 1) compared to that observed while using CH3NO2 under similar pH conditions (see Table 2.2). 

Finally the results recently reported by DUBIN and Strauss are certainly worth mentioning, Potentiometric titration of alternating copolymers of maleic anhydride and n-butyl ether indicates that there occurs a conformational transition which is not exhibited by a copolymer of maleic anhydride and ethyl vinyl ether (Fig. 7). The anomalous behavior of the butyl copol3mier is clearly cormected with the establishment of hydrophobic interactions among the butyl groups. Dubin and Strauss conclude that the maleic anhydride and n-butyl vinyl ether copolymer is hypercoiled in dilute aqueous solution. These authors also point out that the behavior of the latter copolymer is identical with that previously found for highly charged polysoaps. 

For example, simple fluorescence intensity measurements on dispersed hydrocarbon probes such as anthracene [17], perylene [18], pyrene [6,17,22], 9,10-dimethylanthracene (DMA) [60], and coumarin dyes [62] have confirmed that PMAA displays pH-dependent solution behavior. A marked decrease in the intensity of the probe occurs between pH 5 and 6, which coincides with the conformational transition of PMAA as determined by classical methods [2-4,47-50]. Two interrelated effects account for this behavior the solubilizing capacity of the polymer promotes an increase in the concentration of the probe in the solution [6,17,18,60,62] and because the intensity of the fluorescence observed is proportional to the excited state population the resulting emission is enhanced. The hydrocarbons may also be considered to be preferentially solubilized within the hydrophobic domains or structures of the hypercoiled state [6,22]. This results in a degree of protection from the deactivating effects of the aqueous phase and a concomitant increase in the fluorescence observed [6,17,18,22,60,62]. 

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. 

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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