Unveiling the hydrogen bonding network in liquid crystalline natural-based glycosides containing polymeric complexes: Experimental and theoretical assessment

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Abstract

In this work we present a facile and versatile strategy to prepare new amphiphilic compounds obtained from natural sources, avoiding costly covalent synthetic stages, and we introduce a powerful methodology to describe hydrogen-bonding networks in carbohydrates liquid crystals A series of new glycosides has been prepared by mixing a natural-based mannoside, αManPKO, with three different polymeric substrates: poly(ethylene oxide), PEG, poly(4-vinyl pyridine), P4VP, and a block-copolymer containing PEG and P4VP segments, PEG45-b-P4VP18. The materials have been characterised by differential scanning calorimetry, polarised optical microscopy and small-angle X-ray diffraction. The resulting complexes are assembled by hydrogen-bonding and form smectic A phases, with the polymeric chains spread along the surface of the glycosides bilayers. By using Fourier-transform infrared spectroscopy, FT-IR, and molecular simulations, we have assessed the selectivity of the hydrogen bonds formed between αManPKO and the polymeric segments. Our results suggest that the assembly of the polymeric complexes must be explained by a combination of interfacial mixing between the polymer/glycoside units at the bilayer boundaries (favoured by PEG) and the formation of strong hydrogen bonds (favoured by P4VP).

Introduction

Hydrogen-bonding is a versatile technique to yield new supramolecular liquid crystals [1,2], thanks to the directional character of the hydrogen bonds that facilitates the arrangement of anisotropic structures. The typical strength of a hydrogen bond (1 60 kJ·mol−1) [3,4] can guarantee the stability of the new materials above their processing melting points, whilst providing some degree of “softness”. Some early examples of complexes with mesogenic character include the pyridine-benzoic assemblies, reported by Kato and co-workers [5,6], or the seminal works by Bruce and co-workers, using alkoxystilbazoles [[7], [8], [9]]. To date, a wide range of liquid crystals continues to be prepared by hydrogen-bonding, including, chiral bent-core with supramolecular induced chirality [10,11], photosensitive liquid crystals [12,13], modular assemblies showing broad blue phases [14], supramolecular dimers exhibiting the twist-bend nematic phase [[15], [16], [17]], or smectic networks for selective mass and ionic transport [[18], [19], [20]], among many others.

Carbohydrate liquid crystals can be considered as early precedents of supramolecular mesomorphic compounds, and were already reported in the first half of the 20th century [21,22]. More specifically, the different hydroxyl groups can form multiple hydrogen bonds between glycosides, resulting in microphase separation between polar and non-polar regions, ultimately favouring smectic behaviour [[23], [24], [25], [26], [27], [28], [29]]. We note, however, that the exhibition of liquid crystalline phases is not based in the formation of new “rod-like” or “disc-like” moieties by hydrogen-bonding, but instead on segregation due to the amphiphilic character of the glycosides, including hydrogen-bonding between the sugar heads. Carbohydrate liquid crystals experienced a fast development in the 1980′s and 1990′s [[26], [27], [28]], and the mesomorphic behaviour of new glycosides continues to be the object of systematic investigation by varying their composition and stereochemistry [[30], [31], [32], [33]].

Due to its important role on the formation of liquid crystal phases, in this work we investigate with detail the hydrogen-bonding network of a natural-based glycoside, a palm kernel oil-based mannoside, αManPKO, 1, and its complexes with different polymeric substrates,

The alkyl chains of αManPKO were obtained from palm kernel oil, and then added to a mannose head by glycosidation, resulting in a mixture containing different chain lengths, and the effect of composition of 1 is currently under investigation [34]. The formation of liquid crystalline structures and its non-toxicity, makes αManPKO a promising candidate for drug delivery applications [35]. αManPKO has been complexated to three different substrates: poly(ethylene oxide), PEG, 2; poly(4-vinyl pyridine), P4VP, 3; and a block copolymer with both PEG and P4VP segments, PEG45-b-P4VP18, 4,

Whilst PEG is considered as a polymer substrate of great interest for biological applications due to its bio-compatibility [36,37], P4VP has been widely applied as a building block to yield supramolecular polymers [13,[38], [39], [40], [41], [42], [43]]. Finally, block copolymers not only facilitate the introduction of new functionalities in different segments of the polymer chain, but they also offer further control over microphase separation by regulating their hydrophobic/hydrophilic ratios [[44], [45], [46]].

The materials are characterised by a combination of thermal, structural, spectroscopic and modelling techniques, in order to provide relevant insights into the role of the hydrogen-bonding network to assemble liquid crystalline glycosides [23]. Complexation of block-copolymers has been used for different applications and materials, including light-responsive materials studied in our lab [45]. More specifically, Ikkala and co-workers have reported several examples using P4VP as a polymeric matrix; and, for selected examples, they describe the self-assembly of P4VP block copolymers complexed with cholesteryl hemisuccinate [47] and with 3-pentadecylphenol [48]. These and other precedent works, however, focus on the structural and compositional analysis, whilst a detailed model of the hydrogen-bonding network is still crucial to describe and predict complexation. The assembly of glycosides, tackled in the present work, as well as other systems containing multiple and resonating hydrogen bonds [49], is particularly challenging, and requires accounting for several hydroxyl groups potentially acting as hydrogen-donors and hydrogen-acceptors. Our approach can then open new forefronts to prepare supramolecular liquid crystal polymers as drug-delivery and cosmetic formulations [[50], [51], [52], [53], [54]]. In the long-term, the use of amphiphilic polymers will be beneficial to provide nanocarriers stealth effects that suppress opsonisation, to reduce interactions with the reticular-endothelial system, and to ultimately prolong circulation lifetime in blood [[55], [56], [57]].

Section snippets

Materials preparation

The mannoside αManPKO, 1, was synthesised according to the process described in detail in [34], and can be reviewed as electronic supplementary information (ESI, section A). D(+)-mannose monohydrate and boron trifluoride, BF3, were purchased from Sigma Aldrich and used without further purification. The palm kernel oil, PKO, was obtained from Golden Jomalina Food Industries Sdn. Bhd. (Malaysia), and the main components after reduction were lauryl (49 %), myristyl (16 %) and oleyl (7 %) alcohols,

Phase behaviour and structure, POM, DSC and SWAXS

The phase behaviour of αManPKO and its complexes was assessed by polarised optical microscopy, POM, and confirmed by differential scanning calorimetry, DSC. αManPKO forms a monotropic smectic A phase below ca. 146 °C, assessed by the appearance of battonêtes under the polarised microscope, which further coalesce into a focal conic fan texture, in coexistence with homeotropic regions. The PEG⋅αManPKO, P4VP⋅αManPKO and PEG45-b-P4VP18⋅αManPKO complexes, also develop battonêtes on cooling from the

Conclusions

We have prepared complexes of the so-called αManPKO mannoside with different polymeric substrates, resulting in three new supramolecular polymers with smectic A mesomorphism, following a facile method to yield new formulations containing natural-based liquid crystal carbohydrates, and their lyotropic properties in water solutions are under current evaluation. The polymeric segments are located at the interface of the glycoside bilayers, stabilised by specific interactions with the αManPKO

CRediT authorship contribution statement

Nurul Fadhilah Kamalul Aripin: Conceptualization, Data curation, Writing - original draft, Writing - review & editing, Funding acquisition. Jonathan Maclean Heap: Data curation, Formal analysis. Rafael Piñol: Data curation, Methodology. Vijayan Manickam-Achari: Data curation, Formal analysis, Writing - original draft. Alfonso Martinez-Felipe: Conceptualization, Investigation, Writing - original draft, Writing - review & editing, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Ministry of Education of Malaysia [FRGS/1/2019/TK05/UITM/02/9, 2019]; Royal Academy of Engineering, U.K., and Academy of Science, Malaysia [NRCP1516/4/61, 2016]; University of Aberdeen [SF10192, 2018] and University Malaya [UMRG grant RP038B-17AFR].

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