Aqueous ionic liquids influence the disulfide bond isoform equilibrium in conotoxin AuIB: a consequence of the Hofmeister effect?

Abstract

The appearance of several disulfide bond isoforms in multiple cysteine containing venom peptides poses a significant challenge in their synthesis and purification under laboratory conditions. Recent experiments suggest that careful tuning of solvent and temperature conditions can propel the disulfide bond isoform equilibrium in favor of the most potent, native form. Certain aqueous ionic liquids (ILs) have proven significantly useful as solvents for this purpose, while exceptions have also been noted. To elucidate the molecular level origin behind such a preference, we report a detailed explicit solvent replica exchange molecular dynamics study of a conotoxin, AuIB, in pure water and four different aqueous IL solutions (~45-60% v/v). The ILs studied here are comprised of cations like 1-ethyl-3-methyl-imidazolium (Im₂₁⁺) or 1-butyl-3-methyl-imidazolium (Im₄₁⁺) coupled with either acetate (OAc^-) or chloride (Cl^-) as the counter anion. Our simulations unfold interesting features of the conformational spaces sampled by the peptide and its solvation in pure water and aqueous IL solutions. Detailed investigation into populations of the globular disulfide bond isoform of AuIB in aqueous IL solutions reveal distinct trends which might be related to the Hofmeister effect of the cation and anion of the IL and of specific interactions of the aqueous IL solutions with the peptide. In accordance with experimental observations, the aqueous [Im₂₁][OAc] solution is found to promote the highest globular isoform population in AuIB.

Keywords: Conopeptide; Disulfide scrambling; Hofmeister effect; Ionic liquid; REMD.

Fig. 1

Disulfide bond isoforms of AuIB. a Globular (Cys2–Cys8; Cys3–Cys15) and b ribbon isoform (Cys2–Cys15; Cys3–Cys8). Ionic liquid cations c 1-butyl-3-methylimidazolium, [Im₄₁⁺] and d 1-ethyl-3-methylimidazolium, [Im₂₁⁺] are depicted without hydrogens. The counter anions used are e acetate, [OAc⁻] and f chloride, [Cl⁻]

Fig. 2

Internal energy distributions for AuIB/water-[Im₂₁][OAc] system for 15 different replicas: a total system, b only peptide. The temperatures of the corresponding replicas are given

Fig. 3

S–S distance distributions for the three disulfide bond isoforms of AuIB (B beads, G globular, R ribbon) in neat water and four aqueous IL solutions. The plots show the distribution of snapshots sampled in the lowest temperature replica alone and contain 5000 scattered points for the aqueous IL solutions and 4000 points for pure water. The heat maps show the density of snapshots for a given combination of the distance identifiers, d_ij and d_kl

Fig. 4

Correlation between the percentage of disulfide bond isoforms (black square globular; red triangle beads) and the difference in the Jones–Dole viscosity B-coefficients of anion and cation (in dm³ mol⁻¹). The viscosity B-coefficient values for the cations and anions are taken from Zhao (2006), following equations 4 and 5

Fig. 5

Radial distribution functions involving AuIB and the solvating species in neat water and four aqueous IL solutions: a peptide–cation; b peptide–anion; c water–cation; d water–anion; (e) between oxygen atoms of water within 2.5 Å from the peptide and f peptide–water. All the distributions except (e) are calculated within 7 Å from the peptide surface

Fig. 6

Intra-peptide and peptide-solvent H-bond distributions: a intra-peptide; b peptide–all components of the solvent/solvent mixture; c peptide–anion and d peptide–water. Except for (d), for all the other plots, the calculations are carried out by considering that a hydrogen bond is formed between any atom with a hydrogen bonded to it (the donor, D) and another atom (the acceptor, A) provided that the distance D–A is less than the cutoff distance of 4 Å and the angle D–H–A is less than the cutoff angle of 40°. For (d) only the polar atoms (N, O, S, F) are specifically considered in the H-bond calculation

Fig. 7

Distributions of the peptide backbone length of AuIB in neat water and four aqueous IL solutions for: a all snapshots belonging to the lowest temperature replica; b snapshots belonging to the lowest temperature replica and falling within the 5 × 5 Å² cutoff for the distance identifiers of either globular, beads or ribbon isoforms of AuIB; c the same as in (b), but with the area normalized

Fig. 8

Non-bonding potential energies (van der Waals and electrostatic) of interaction between AuIB and the components of the solvent mixture: water (W), anion (A), cation (C) and total solvent (T)