The interactions between ribonuclease A and solvent components in aqueous 2-methyl-2,4-pentanediol (MPD) have been investigated by differential refractometry and light scattering at pH 5.8, i.e., conditions similar to those used to crystallize the protein from this solvent system. Application of multicomponent thermodynamic theory shows that, at all solvent compositions up to 50% (v/v) MPD, the protein is preferentially hydrated; i.e., addition of ribonuclease to the mixed solvent leads to an increase in the chemical potential of MPD. This unfavorable thermodynamic interaction leads to phase separation, probably caused by local salting out of the MPD by the charges on the surface of the protein molecule. A parallel examination by circular dichroism (CD) has shown that the CD spectrum of ribonuclease in 50% MPD is indistinguishable from that in dilute buffer.

Interaction of ribonuclease A with aqueous 2-methyl-2,4-pentanediol at pH 5.8

If two good solvents become poor for a polymer when mixed, the solvent pair is called a co-nonsolvent pair for the polymer. The sharp depression of the LCST by the co-nonsolvency in solutions of poly(N-isopropylacrylamide) in the mixed solvent of water and methanol is shown to be caused by the competitive hydrogen bonding of water and methanol molecules onto the polymer chains. On the basis of a new statistical-mechanical model for competitive hydrogen bonds, the degree of hydration θ(w) and of methanol binding θ(m), excess degree ΔθE of solvent binding, preferential adsorption coefficients Γ, LCST spinodal lines, and cloud-point depression ΔTcl are theoretically calculated and compared with the experimental results. The optimal composition xm(M) of methanol at which LCST takes the minimum value is studied as a function of the polymer molecular weight M. In the high molecular weight limit, it takes xm ≃ 0.35. The solution recovers a uniform state in the region of higher methanol composition. Such a peculi...

Preferential Adsorption and Co-nonsolvency of Thermoresponsive Polymers in Mixed Solvents of Water/Methanol

Combining nuclear magnetic resonance (NMR), dynamic light scattering (DLS), and μs long all-atom simulations with two million particles, we establish a delicate correlation between increased side chain organization of PNIPAm and its collapse in aqueous methanol mixtures. We find that the preferential binding of methanol with PNIPAm side chains, bridging distal monomers along the polymer backbone, results in increased organization. Furthermore, methanol–PNIPAm preferential binding is dominated by hydrogen bonding. Our findings reveal that the collapse of PNIPAm is dominated by enthalpic interactions and that the standard poor solvent (entropic) effects play no major role.

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Relating side chain organization of PNIPAm with its conformation in aqueous methanol.

Poly(n-hexyl isocyanate) adopts a helical structure in solution and the preference of one helical sense induced by a nonracemic chiral solvent can be detected by the circular dichroism (CD) of the backbone chromophore of the polymer. Addition of an achiral or racemic cosolvent leads to a reduction of the CD intensity, which can be interpreted quantitatively as reflecting the composition of the solvent mixture in contact with the helical backbone of the polymer. Equilibrium constants characterizing the relation between the solvent composition in contact with the polymer and the bulk of the system are given for mixtures of (S)-1-chloro-2-methylbutane and a number of achiral or racemic cosolvents. The relationship between the structure of the cosolvent and the extent of the preferential solvation was surprising in several instances. The polymer was preferentially solvated by a highly branched hydrocarbon although it was a precipitant and the preferential solvation by alcohols extended up to the point of poly...

Chiral solvation as a means to quantitatively characterize preferential solvation of a helical polymer in mixed solvents

Smart polymers are a modern class of polymeric materials that often exhibit unpredictable behavior in mixtures of solvents. One such phenomenon is co-non-solvency. Co-non-solvency occurs when two (perfectly) miscible and competing good solvents, for a given polymer, are mixed together. As a result, the same polymer collapses into a compact globule within intermediate mixing ratios. More interestingly, polymer collapses when the solvent quality remains good and even gets increasingly better by the addition of the better cosolvent. This is a puzzling phenomenon that is driven by strong local concentration fluctuations. Because of the discrete particle based nature of the interactions, Flory-Huggins type mean field arguments become unsuitable. In this work, we extend the analysis of the co-non-solvency effect presented earlier [D. Mukherji et al., Nat. Commun. 5, 4882 (2014)]. We explain why co-non-solvency is a generic phenomenon, which can only be understood by the thermodynamic treatment of the competitive displacement of (co)solvent components. This competition can result in a polymer collapse upon improvement of the solvent quality. Specific chemical details are not required to understand these complex conformational transitions. Therefore, a broad range of polymers are expected to exhibit similar reentrant coil-globule-coil transitions in competing good solvents.

B. E. Read

Papers

A light-scattering study of preferential adsorption in the system benzene + cyclohexane + polystyrene