Ordering Transitions in Liquid Crystals Triggered by Bioactive Cyclic Amphiphiles: Potential Application in Label-Free Detection of Amyloidogenic Peptides

Abstract

We report the orientational behavior of nematic liquid crystals (LCs) influenced by a cyclic lipopeptide, polymyxin B (PmB). It was found that PmB can spontaneously self-assemble at aqueous-LC interfaces and induce a homeotropic ordering of the LCs at those interfaces, thus resulting in dark optical appearance of the LC under cross polarizers. Density functional theory studies substantiate the experimental findings that the stability of homeotropic anchoring of the LC is strongly influenced by the hydrophobic interactions between aliphatic tails of PmB and LC molecules along with the additional supramolecular interactions between their head groups. Interestingly, exposure of the PmB-laden aqueous-LC interface to anionic serum proteins such as bovine serum albumin (BSA) and human hemoglobin triggered a planar reorientation of the LC, leading to a bright optical state of the LC. This allows label-free characterization of the biomolecular interactions between proteins and antibiotics (i.e., PmB) in vitro at those interfaces. Such peptidic (PmB)-based LC interfaces can also distinctly amplify the adsorption of β-sheet-rich proteins (fibronectin and concanavalin A) through appearances of fibril-like spatial patterns which are, however, not observed in the presence of α-helix-rich proteins (BSA). Such changes in the optical patterns of the LC in contact with β-sheet-rich proteins occur at nanomolar concentrations at those interfaces, and thus the method could be useful to detect toxic amyloids at a low concentration regime. We envision that our simple label-free optical system may open a wide avenue to detect an extensive assortment of interfacial biochemical events occurring at the aqueous-LC interfaces.

Surfactin-Laden Aqueous–Liquid Crystal Interface Enabled Identification of Secondary Structure of Proteins

“…First, we selected four proteins for the study: BSA, Hb, Cyto, and ConA. Structurally, each one of them consists of at least a hydrophobic pocket and adopts mainly α-helical secondary structure in the bulk phase − (except ConA, which is an all β-sheet protein). However, they differ in their physicochemical properties such as overall ionic charge as recently confirmed by us through zeta potential measurements at physiological pH: BSA (most anionic) > ConA (anionic) > Hb (slightly anionic) > Cyto (highly cationic).…”

“…Structurally, each one of them consists of at least a hydrophobic pocket and adopts mainly α-helical secondary structure in the bulk phase − (except ConA, which is an all β-sheet protein). However, they differ in their physicochemical properties such as overall ionic charge as recently confirmed by us through zeta potential measurements at physiological pH: BSA (most anionic) > ConA (anionic) > Hb (slightly anionic) > Cyto (highly cationic). Prior to exposure to proteins, the SFN-laden (7.5 μM) aqueous/E7 interface was exchanged with tris-buffered saline (TBS) (1 mM, pH 7.2) to remove free SFN from the bulk solution.…”

“…Thus, previous protein-related studies at LC–aqueous interfaces vastly comprise linear amphiphiles such phospholipids and conventional water-soluble surfactants and polymers. − Recently, Yang et al reported the synthesis and examination of the self-assembly behavior of a linear lipopeptide at LC–aqueous interfaces . Regarding protein adsorption studies, we recently made an advancement using a cyclic lipopeptide, polymyxin B (PmB), which could trigger a homeotropic ordering of LC supported by topological flexibility of its cyclic peptidic headgroup and the linear acyl tail . Furthermore, we demonstrated that ordering of LC beneath PmB-decorated LC system could readily be triggered by presence of proteins at nanomolar concentration regime.…”

“…Concerning protein-based studies, careful investigation using LC has been carried out by several groups to understand the interfacial phenomenon regarding protein binding events and consequent lateral organization of the biomolecules at LC–aqueous interfaces. − It is well-known that proteins trigger a negligible change in orientations of LC; therefore, to achieve an optical signal for protein, it is important, prior to protein adsorption, to modify the LC–aqueous interface with amphiphiles that promote homeotropic orientations of LC. Thus, previous protein-related studies at LC–aqueous interfaces vastly comprise linear amphiphiles such phospholipids and conventional water-soluble surfactants and polymers. − Recently, Yang et al reported the synthesis and examination of the self-assembly behavior of a linear lipopeptide at LC–aqueous interfaces .…”

“…We speculated that hydrophobic lipid tails of SFN might facilitate the homeotropic alignment of LC at aqueous interface, while the two anionic peptide functionalities (in headgroup) may direct the adsorption of proteins at interface in controlled manner leading to a change in ordering of LCs at the interface as proposed in Figure b. The study is highly encouraged as the interfacial behavior of cyclic lipopeptides remains relatively unexplored at LC–aqueous interface and such systems could be useful as highly sensitive and multifunctional detection assays . Herein, we used five biologically relevant proteins as model proteins to account the detailed study of LC’s orientations driven by intermolecular interactions between SFN and protein molecules at those interfaces.…”