Properties of biomolecules at the interfaces: studies and characterization of chromonic mesogens, from the basis to applications

Abstract

The study of the interaction between molecules, in particular biological molecules and liquid crystals (LC), has experienced a huge growth in the recent years because of the development in devices engineering applied not only in photonics but also in the biomedical eld. In order to design more e cient LC devices, it is rst necessary to understand the behavior and properties of newly-synthesized liquid crystals and to garner a more indepth understanding of currently-existing LCs in order to answer pending questions about them. The aim of this thesis work, is to better understand the interactions involved at the interface between liquid crystals and other materials, whatever is their nature, i.e. polymeric or biological. We started studying the e ect of di erent con ning surfaces on the alignment of a special class of lyotropic liquid crystals, called \chromonics", which, in addition of LC properties, are biocompatible. Di erently from the most common liquid crystals, i.e. thermotropic LC, the mesogens that constitute the chromonic LC phases are not amphiphilic, but they are \plan-like" aromatic compound. This class of molecules embraces not only dyes and drugs, but also DNA and its bases. Using the knowledge acquired with chromonic mesogen, we tried to understand a more complicate system, such as the phenomena involved at the biomolecules decorated-liquid crystals lms interfaces. More speci cally, it is possible to divide the work in two macro-parts. The rst part concerns the alignment of a chromonic molecule, \disodium cromoglycate" (DSCG). The study of chromonic LC behaviour in con ned geometries and its physical properties, could be a model for more complex biological assemblies. Hence, we demonstrated the role of alignment layer's surface energy in the alignment of nematic phase of DSCG, achieving both alignments and for the rst time, a stable-in-time homeotropic anchoring of this LC solution. With the knowledge acquired from DSCG, we were able to align also DNA bases liquid crystal solutions. In particular, guanosine monophosphate in pre-cholesteric and cholesteric liquid crystals phases were perfectly aligned homeotropically without means of external elds, as was done until now, and partially planar aligned. Moreover, we observed that if ionic and/or silver doped solutions are added to the LC guanosine phases, it is possible to control the pitch of the cholesteric phase, modifying the helix structure. Instead, varying the nature of the con ning surfaces, in such conditions, it is possible to obtain guanosine vesicles. Other studies have been carried out on new chromonic complexes, synthesized at Chemistry Department of UNICAL, with possible application as anticancer drugs. A complete characterization of these compounds were done (XRD, phase diagrams, etc) and also for these compounds, we developed a\route"to drive the alignment, particularly important for future application in biophotonic devices. The second part of the work is focused on LC based biosensors. From the biotechnological and biomedical applications point of view, the studies on interactions of proteins with lipids are an area of fundamental interest, due to enormous biological importance. In fact, studies on biosensor devices are tremendously increased in recent years, focusing the attention also on nding low cost raw materials with high e ciency: liquid crystals, thanks to their high sensitivity to the external conditions, represent the best candidate. It has been demonstrated that aqueous interface of LC has an instantaneous response when exposed to phospholipids. This is a good base to study the interaction between biomolecules using LC as probe. Starting from the results found in literature, we studied the e ect of phospholipids on protein decoratede-liquid crystal interfaces by means of optical microscopy and FT-IR measurements. The rst technique allowed us to observe the response of decorated LC lm when exposed to phospholipids vesicles, while the second, gave us insight on conformational changes involved in secondary structure of the protein in function of the time of interaction between protein and LC, and the pH of the surrounding environment. The results obtained show a new methods to report speci c binding of vesicles on protein decorated interfaces.