Influence of isopropanol-water solvent mixtures on the conformation of poly-L-lysine.

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AbstractThe helix–coil transition of poly‐L‐lysine (PLL) in water–isopropanol solvent mixtures has been investigated at room temperature by circular dichroism measurements. Within the range of 70%–80% isopropanol concentration (by volume), the polymer undergoes a sharp transition, characterized by the formation of a fully charged α‐helical structure. On the basis of some experimental evidence the role of the organic component in solution appears more complicated than that of strengthening the intramolecular hydrogen bonds in the polymer. By analogy with the distribution of the components of alcohol–water mixtures in simple ionic systems, it is thought that only an high co‐solvent concentration brings about an extensive and possible cooperative depletion of the clusters of firmly‐bound water molecules in the domain of the polylelectrolyte, favoring the transition to the α‐helical structure. On the other hand, CD spectral patterns show that the addition of NaCl in the alcohol‐rich–water mixtures of charged poly‐L‐lysine gives rise to a transition from the α‐helical to a β‐structures conversion obeys a first‐order rate law at all times, with a rate constant dependent on solvent composition and ionic strength. In these conditions, the rate of the process is close to that found for the thermally induced α–β transition. Higher polymer concentration and/or ionic strength cause a phase separation of β‐PLL, suggesting that in this case interchain reactions (where hydrogen bonding should play the major role) predominate. Titration experiments on charged α‐helical poly‐L‐lysine in 85% or 90% isopropanol mixtures confirm the occurrence of a conformational transition, which takes place within a degree of dissociation α of 0.2–0.75. The transition is accompanied by a visible turbidity, which increases as the titration proceeds. Implications of the solvent distribution around the macroion on the observed conformational phenomena are also discussed.