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

Abstract

The development of stimuli-responsive biomimetic systems to understand interactions between proteins and surfaces is of an increasingly important scientific interest due to its potential applications in diagnostics and fundamental biological research. In this study, we report a simple and label-free method utilizing interfacial properties of liquid crystals (LCs), mediated by self-assembly of a naturally occurring cyclic lipopeptide, surfactin (SFN). We demonstrated that SFN molecules promote the homeotropic alignment of LC at the LC–aqueous interface giving rise to dark optical appearances under cross polars. The ordering transition of LC is mainly caused by the lateral hydrophobic interactions between hydrocarbon chains of SFN and LC molecules at the interface. Interestingly, the optical state of LC changed to bright when protein (five proteins studied herein) molecules were in the vicinity of the interface thereby, allowing label-free imaging of protein adsorption at those interfaces. It was further realized that the shapes of bright spatial patterns at the SFN-laden interface, which are formed in the presence of proteins, are directly associated with the native secondary conformations of proteins, i.e., elongated/fibrillar (β-sheet-rich) and globular domains (α-helix-rich). Thus, the designed LC system can also be applied to detect amyloidogenic proteins, mainly involved in neurological disorders. In addition, the LC-based method presents additional advantages over existing spectroscopic/biological techniques such as simple optical readout, high sensitivity (nanomolar concentration regime), and easy sample preparation. We believe that the SFN-decorated LC-based interfaces will open a new avenue to detect other subtle biomolecular interactions at LC–aqueous interfaces.

Interfacial Polyelectrolyte–Surfactant Complexes Regulate Escape of Microdroplets Elastically Trapped in Thermotropic Liquid Crystals

“…The tilted domains coalesced and grew in size over time but were still observed as distinct domains after 48 h of equilibration. Because PSS induces planar alignment and DTAB induces homeotropic LC alignment (see Section S12), these observations suggest that mixtures of DTAB and PSS phase separate into DTAB-rich and DTAB-lean domains at the LC interface. − This observation may be connected to past studies using ellipsometry that have reported formation of micrometer-scale domains (aggregates) at air–water interfaces at stoichiometric ratios of DTAB to PSS similar to those used in our study at LC interfaces. , At stoichiometric ratios above 1:1 (Figure d,e), we observed the LCs to exhibit a discontinuous transition from planar to homeotropic alignment (patterned homeotropic and planar domains), providing further support for the proposed formation of micrometer-sized domains rich in DTAB. This contrasts with the LC response to PDDA-SDS, which, at all stoichiometric ratios of SDS and PDDA investigated in our experiments, involved a transition from planar to a homeotropic orientation via a continuous change in tilt angle (see Section S13).…”

Elucidating liquid crystal-aqueous interface for the study of cholesterol-mediated action of a β-barrel pore forming toxin

“…First, the dendritic texture of the LCs observed as a result of the lipid-binding of VCC is suggestive of a b-sheet-rich secondary structure of the protein at the LC-aqueous interface consistent with the prior reports. 28,29 Second, cholesterol is a requisite in the lipid composition to induce a reorientation of LCs for the interfacial interactions between VCC and the lipid mixture. Third, for a particular concentration of VCC, the degree of LC ordering transition shows a linear correlation with the amount of cholesterol present, indicative of a continuous change in the orientation of the LCs at the aqueous interface.…”

“…28 Proteins with a predominant β-sheet-rich secondary structure result in dendritic patterns of LCs, while α-helical proteins result in rounded or ellipsoidal bright domains. 29 With this idea kept in mind, we monitored the optical appearance of LCs at the LC–aqueous interface decorated with the lipid mixtures (0–50% cholesterol in PC) upon addition of VCC. We note that the optical appearance of the mixed lipid-laden LC–aqueous interfaces was stable (Fig.…”

Liquid Crystals: Versatile Self-Organized Smart Soft Materials

“…The identification of secondary structures of proteins has been achieved using LC biosensors. 323 Nanoscale interaction of lipid with proteins has been probed at the interface of LC droplets. 324…”

“…Such disruption could be easily visualed under a microscope as the dark appearance of the cell changes to bright. The identification of secondary structures of proteins has been achieved using LC biosensors . Nanoscale interaction of lipid with proteins has been probed at the interface of LC droplets …”