Stability of collagen in ionic liquids: Ion specific Hofmeister series effect

Highlights

• This work outlines the interaction of imidazolium-based ionic liquids (IILs) with varying anions and collagen.

• It focuses on comparing the effect of collagen induced by varying the anions obeying Hofmeister series.

• The role of IILs in stabilizing/destabilizing collagen has been evaluated using varied experimental techniques.

Abstract

In protein-ionic liquids (ILs) interactions, anions play an important role. In this work, imidazolium-based ILs (IILs) with varying anions namely dicyanamide (DCA), hydrogen sulfate (HS), dimethyl phosphate (DP), acetate (A), sulfate (S) and dihydrogen phosphate (DHP) have been chosen with the aim of understanding the role of anions in bringing about the destabilization effect on collagen based on the kosmotropicity and chaotropicity of ions. Imidazolium-based ILs destabilized the triple helical structure of collagen, thereby proving as strong denaturants for collagen and this was confirmed by various spectroscopic techniques viz., CD, FT-IR, viscosity and impedance measurements. The solution studies were in accordance to the changes in the dimensional stability of RTT collagen fibres at the fibrillar level. Imidazolium cations with varied anions have exhibited destabilizing effect on collagen in order of ions in Hofmeister series; IDP < IDHP < IA < IDCA < IS < IHS. Presumably, these notable effect and changes were facilitated by electrostatic interactions between the anions and amine functional groups of collagen.

Introduction

Ionic liquids, lime lighted as “green” and “designer” materials have instigated a paradigm shift in every field due to its unique and attractive physicochemical properties [[1], [2]]. Depending on the cations and anions, they can be specifically and effectively tuned for a wide range of applications in green chemistry to pharmaceutical and biotechnological applications [[3], [4]]. In general, a pair of strong kosmotropic anions and chaotropic cations stabilize the protein, whereas chaotropic anions and kosmotropic anions destabilize the protein and selection of ions are based on Hofmeister series [5].

Research on the interaction and behavior between ionic liquid and protein has nested stance in scientific community due to its remarkable properties. The spectral and thermodynamic behavior of IL-BSA system suggested the loss of secondary and tertiary structure and the anionic moiety seemed to have more obvious effect than the cationic ones [6] and the denaturation was caused by electrostatic and hydrophobic interaction, hydrogen bonding and van der Waals forces as proved by Zhu and Yan et al. [[7], [8]]. The stability of insulin and heme protein in imidazolium based ILs with varying anions was also evaluated and it was disappointing to note that ILs failed to protect the native state, which led to denaturation [[9], [10]]. Therefore, protein stability is receiving rapid interest in the active research in biological and physical sciences providing an adequate and fundamental understanding of the behavior and stability of proteins in ILs.

Collagen, the most abundant protein in mammals has been used widely as a biomaterial [[11], [12]] and they have been explored with ionic liquids [[13], [14]]. The dissolution and regeneration of collagen using BMIM Cl studied by Meng et al. reported partial destruction in triple helical structure [15]. Zhang et al., has prepared cellulose/collagen film using [EMIM] [Ac] as common solvent [16] and they have been used in regenerating triple helical structure, which is indicative of potential applications in tissue engineering and it has been proved using experimental and computational studies [[17], [18]].

We have studied the influence of various ionic liquids on collagen on the different hierarchical orderings of collagen. The influence of BMIM Cl has brought significant changes at higher hierarchical level of collagen [19]. At molecular and fibrillar level, choline amino acid based ILs has also demonstrated destabilizing effect due to competitive hydrogen bonding between its molecules [20]. Also, ammonium and phosphonium based ILs had demonstrated destabilizing effect on collagen due to their chaotropicity of ions [[21], [22]], whilst choline dihydrogen phosphate stabilized collagen by exerting an electrostatic force, thus making it suitable for preparation of biomaterials for tissue engineering applications [[23], [24]]. Also, the changes in the structure and hydration dynamics of collagen has been studied upon interaction with imidazolium and choline based ILs [25]. In addition to, imidazolium based ILs has been employed as unhairing cum fibre agent for skin matrix and it has paved way for cleaner and greener processing systems [[26], [27]].

Still the interactions between proteins and ionic liquids in aqueous media are not well understood and the underlying interpretations are mostly speculative. This emphasizes the need to understand the interaction of collagen with various additives, thus providing a new probe for elucidating protein- IL interaction. Herein, a series of imidazolium-based ionic liquid (IILs) with wide range of varying anions such as dicyanamide, hydrogen sulfate, dimethyl phosphate, acetate, sulfate and dihydrogen phosphate has been comprehensively investigated at both molecular and fibrillar level. The choice of cations and anions was based on chaotropic and kosmotropic behavior of ions as described in Hofmeister series. The interaction of IILs with collagen has been studied using various characterization techniques viz., circular dichroic studies, Fourier transform infrared spectroscopy, viscosity and UV–Vis studies, dimensional stability and impedance measurements to influence the varying anions on collagen.