Effects of Hofmeister anions on the phase transition temperature of elastin-like polypeptides

Abstract

The modulation of the lower critical solution temperature (LCST) of two elastin-like polypeptides (ELPs) was investigated in the presence of 11 sodium salts that span the Hofmeister series for anions. It was found that the hydrophobic collapse/aggregation of these ELPs generally followed the series. Specifically, kosmotropic anions decreased the LCST by polarizing interfacial water molecules involved in hydrating amide groups on the ELPs. On the other hand, chaotropic anions lowered the LCST through a surface tension effect. Additionally, chaotropic anions showed salting-in properties at low salt concentrations that were related to the saturation binding of anions with the biopolymers. These overall mechanistic effects were similar to those previously found for the hydrophobic collapse and aggregation of poly(N-isopropylacrylamide), PNIPAM. There is, however, a crucial difference between PNIPAM and ELPs. Namely, PNIPAM undergoes a two-step collapse process as a function of temperature in the presence of sufficient concentrations of kosmotropic salts. By contrast, ELPs undergo collapse in a single step in all cases studied herein. This suggests that the removal of water molecules from around the amide moieties triggers the removal of hydrophobic hydration waters in a highly coupled process. There are also some key differences between the LCST behavior of the two ELPs. Specifically, the more hydrophilic ELP V5A2G(3)-120 construct displays collapse/aggregation behavior that is consistent with a higher concentration of anions partitioning to polymer/aqueous interface as compared to the more hydrophobic ELP V(5)-120. It was also found that larger anions could bind with ELP V5A2G(3)-120 more readily in comparison with ELP V(5)-120. These latter results were interpreted in terms of relative binding site accessibility of the anion for the ELP.

Figure 1

Structure of PNIPAM.

Figure 2

Proposed mechanisms for specific anion effects on the LCST of ELP V₅-120. (a) Direct interactions of anions with water involved in hydrogen bonding to the amide. Kosmotropic anions polarize these water molecules and thereby weaken the hydrogen bonding of water to the macromolecule, a salting-out effect. (b) The blue lines represent the hydrophobically hydrated regions of the biomacromolecule. The cost of such hydration increases as salt is added to solution. (c) Direct ion binding of chaotropic anions to the amide moieties along the backbone of the polypeptide should cause a salting-in effect.

Figure 3

LCST vs salt concentration curves for a series of sodium salts with ELP V₅-120. Each data point represents the average of eight measurements, and the standard deviations are within the size of the circular data points in all cases. The dashed lines are fits to the data using eqs 1 and 2.

Figure 4

Plot of the linear slope, c, from eq 2 against (a) ΔS_hydr and (b) σ for ELP V₅-120 with 11 different sodium salts. The dashed red lines are fits to the kosmotropes in (a) and the chaotropes in (b).

Figure 5

Residual LCST vs salt concentration data for the chaotropic anions with ELP V₅-120 after subtracting out the linear portion of the data. The dashed lines represent Langmuir isotherm fits to the data points.

Figure 6

LCST vs salt concentration curves for ELP V₅A₂G₃-120 with a series of sodium salts. Each data point represents the average of eight measurements, and the standard deviations are within the size of the circles used to plot the data. The dashed lines are fits to the data using eqs 1 and 2.

Figure 7

Plot of the linear slope, c, from eq 2 against (a) ΔS_hydr and (b) σ for ELP V₅A₂G₃-120 with 11 different sodium salts. The dashed blue lines are fits to the kosmotropes in (a) and the chaotropes in (b).

Figure 8

Residual LCST vs salt concentration data for the chaotropic anions with ELP V₅A₂G₃-120 after subtracting out the linear portion of the data. The dashed lines represent Langmuir isotherm fits to the data points.

Figure 9

Plot of the ratio of B_max values for ELP V₅A₂G₃-120/ ELP V₅-120 vs ionic volume of chaotropes ions.