Carbohydrate amphiphiles for supramolecular biomaterials: Design, self-assembly, and applications

Abstract

New concepts that develop supramolecular assemblies from intricate biofunctional carbohydrates are yet to come. Currently, the design of most carbohydrate amphiphiles for supramolecular biomaterials is limited to simple (mono- and di-) saccharides as biofunctional units while the cells are surrounded by complex polysaccharides in their native environment. Moreover, assembly strategies and principles used so far are typically similar to the ones applied for peptide amphiphiles and do not consider specific carbohydrate features such as carbohydrate-carbohydrate interactions. Carbohydrates' structural diversity represented by the chiral and topological abundance, as well as multivalency, is also largely unexplored. Exploitation of this diversity is expected to bring breakthroughs in the field of supramolecular biomaterials, potentially resulting in biomaterials that more closely resemble the key features of the extracellular matrix. Carbohydrate-containing biopolymers such as glycoproteins, proteoglycans, and glycolipids are an incredibly diverse and complex set of natural building blocks used by biological systems to endorse a wide range of molecular functions that are critical to life processes. Simplified synthetic analogs of these polymeric glycoconjugates can capture some of these functions and are increasingly exploited toward the development of supramolecular biomaterials. These carbohydrate amphiphiles can mimic structural aspects through cooperative molecular self-assembly or target distinct signaling pathways through engineered (multivalent) interactions with biological systems. Herein, we discuss the supramolecular principles that regulate the glycome function(s) and the translation of these in the design of supramolecular biomaterials in which carbohydrates are used as information-rich structural elements with huge potential as therapeutic supramolecular biomaterials. Carbohydrate-containing biopolymers such as glycoproteins, proteoglycans, and glycolipids are an incredibly diverse and complex set of natural building blocks used by biological systems to endorse a wide range of molecular functions that are critical to life processes. Simplified synthetic analogs of these polymeric glycoconjugates can capture some of these functions and are increasingly exploited toward the development of supramolecular biomaterials. These carbohydrate amphiphiles can mimic structural aspects through cooperative molecular self-assembly or target distinct signaling pathways through engineered (multivalent) interactions with biological systems. Herein, we discuss the supramolecular principles that regulate the glycome function(s) and the translation of these in the design of supramolecular biomaterials in which carbohydrates are used as information-rich structural elements with huge potential as therapeutic supramolecular biomaterials. IntroductionCarbohydrates are involved in numerous essential processes within the life cycle of cells and organisms—they are a source of energy and important metabolites; they sustain the swelling pressure and mechanical properties of different tissues; and are also responsible for activating cascades of cellular events, ranging from adhesion to differentiation and apoptosis. Their functions are exerted mainly through combinatorial presentation of multiple non-covalent interactions. Because of their structural diversity (Figure 1), carbohydrates can display a vast number of ligand structures with induced fit and cooperativity that render their multivalent interactions specific and selective. The incorporation of carbohydrates in biomedical systems increases the level of molecular complexity, thus giving rise to an exclusive class of bioinformation coding materials that are not accessible using more conventional materials based on peptides, proteins, and other polymers.1Gim S. Zhu Y.T. Seeberger P.H. Delbianco M. Carbohydrate-based nanomaterials for biomedical applications.Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 2019; 11: e1558Crossref PubMed Scopus (13) Google Scholar,2Delbianco M. Bharate P. Varela-Aramburu S. Seeberger P.H. Carbohydrates in supramolecular chemistry.Chem. Rev. 2016; 116: 1693-1752Crossref PubMed Scopus (134) Google ScholarTo capitalize on multivalency and the structural richness of carbohydrates, supramolecular approaches are better suited than covalent synthesis for several reasons: they introduce a dynamic aspect to the system, offering the possibility of more complex communication with its environment through reversible formation, responsiveness, and adaptability; they can generate larger functional and organized structures that would extend the multivalent interactions over longer distances; and their morphology can be tuned to match the targeted biointerface.3Dong R.J. Zhou Y.F. Huang X.H. Zhu X.Y. Lu Y.F. Shen J. Functional supramolecular polymers for biomedical applications.Adv. Mater. 2015; 27: 498-526Crossref PubMed Scopus (303) Google Scholar,4Aida T. Meijer E.W. Stupp S.I. Functional supramolecular polymers.Science. 2012; 335: 813-817Crossref PubMed Scopus (2282) Google Scholar However, intermolecular carbohydrate-carbohydrate interactions (CCIs) in simple sugars are not strong enough to generate structured supramolecular scaffolds amenable to biomedical applications. Thus, diverse approaches have been used to display carbohydrates onto functional biomaterials. In this review, we will discuss different types of carbohydrate amphiphiles and strategies employed in their self-assembly, which can generate a plethora of biomaterials with multivalent presentation of carbohydrates that can be tuned in terms of morphology, function, and bioactivity. Finally, we will highlight some of the recent biological applications of supramolecular carbohydrate systems.The role of multivalency in carbohydrate biorecognitionNon-covalent interactions can be up to 100 times weaker than covalent bonds. To compensate for the weakness of these interactions while preserving their reversibility, nature has adopted the concept of multivalency—the ability of a substrate to form multiple individual bonds with a ligand to enhance binding affinity and ensure selectivity and specificity of recognition.5Fasting C. Schalley C.A. Weber M. Seitz O. Hecht S. Koksch B. Dernedde J. Graf C. Knapp E.W. Haag R. Multivalency as a chemical organization and action principle.Angew. Chem. Int. Ed. Engl. 2012; 51: 10472-10498Crossref PubMed Scopus (670) Google Scholar Simultaneously occurring multiple binding events increase the binding contact surface resulting in an interaction that is much stronger and more selective than the individual bonds.Molecular basis of carbohydrate-binding interactionsCarbohydrates interact with their environment through well-defined signatures determined by their sequence and preferred (induced) three-dimensional conformations. Decades of crystallographic and NMR studies have identified the molecular interactions that govern carbohydrate binding and recognition in biological systems.6Solís D. Bovin N.V. Davis A.P. Jiménez-Barbero J. Romero A. Roy R. Smetana K. Gabius H.J. A guide into glycosciences: how chemistry, biochemistry and biology cooperate to crack the sugar code.Biochim. Biophys. Acta. 2015; 1850: 186-235Crossref PubMed Scopus (0) Google Scholar Each carbohydrate, ranging from simple monosaccharide to proteoglycans, has a unique imprint that can be recognized by other biomolecules.7McMahon C.M. Isabella C.R. Windsor I.W. Kosma P. Raines R.T. Kiessling L.L. Stereoelectronic effects impact glycan recognition.J. Am. Chem. Soc. 2020; 142: 2386-2395Crossref PubMed Scopus (11) Google Scholar The position and arrangement of the functional groups determine the formation of hydrophilic and hydrophobic regions through which polar and non-polar interactions can take place, respectively. Despite the structural diversity of the carbohydrate interactome, the nature of interactions at the molecular level are a conserved set: hydrogen bonding is the main driving force, whereas hydrophobic interactions and metal bridges balance and stabilize the complexes (Figure 2).Figure 2Schematic presentation of non-covalent interactions in which carbohydrates can be engagedShow full caption(A) Hydrogen bonding can be either (A1) cooperative or (A2) bidentate.(B) Complexation with cations, in which (B1) proteins’ amino acids, (B2) carbohydrates, and/or (B1) water can also occupy coordination sites.(C) Hydrophobic CH-π stacking that results from the London dispersion forces and stereoelectronic effects (electron-defficient CH bonds are formed via hyperconjugations, as shown in C2).View Large Image Figure ViewerDownload Hi-res image Download (PPT)Hydrogen bondingTwo types of hydrogen bonding have been identified in carbohydrate interactions: (1) cooperative hydrogen bonds in which the carbohydrate’s hydroxyl groups act as both acceptor via the oxygen and donor through the hydrogen (red arrows in Figure 2A1) and (2) bidentate hydrogen bonds (Figure 2A2) in which two adjacent hydroxyl groups of the carbohydrate bind to two different atoms of a planar polar amino-acid side chain (typically aspartate, glutamate, asparagine, glutamine, or arginine).8López de la Paz M.L. Ellis G. Pérez M. Perkins J. Jiménez-Barbero J. Vicent C. Carbohydrate hydrogen-bonding cooperativity − intramolecular hydrogen bonds and their cooperative effect on intermolecular processes − binding to a hydrogen-bond acceptor molecule.Eur. J. Org. Chem. 2002; 2002: 840-855Crossref Scopus (56) Google Scholar,9Gaiser O.J. Piotukh K. Ponnuswamy M.N. Planas A. Borriss R. Heinemann U. Structural basis for the substrate specificity of a Bacillus 1,3–1,4-beta-glucanase.J. Mol. Biol. 2006; 357: 1211-1225Crossref PubMed Scopus (0) Google Scholar Cooperative hydrogen bonding occurs intramolecularly and/or in CCI, while bidentate hydrogen bonds are mainly involved in carbohydrate-protein interactions (CPIs) in which amide groups of asparagine act as donors and the acidic side chains of aspartate and glutamate as acceptors, thus providing a specific arrangement, selective for adjacent equatorial/equatorial or equatorial/axial hydroxyl groups. (In the biological milieu, water plays an important role in hydrogen bonds networks: the loss of bound water in protein binding pockets at the expense of carbohydrates comes with an entropic cost. For more details on this issue, the reader is referred to Lemieux’s work on the role of water in carbohydrate biorecognition.)10Lemieux R.U. How water provides the impetus for molecular recognition in aqueous solution.Acc. Chem. Res. 1996; 29: 373-380Crossref Google ScholarInteractions with cationsThe X-ray crystal structures of some lectins reveal the presence of divalent cations near the carbohydrate-binding site. The metal ions either fix the positions of certain amino acids that interact with the carbohydrate or form bridges between the carbohydrate ligand and the binding pocket (Figure 2B1). Among different metals, calcium ions (Ca2+) are the most common coordination centers with the carbohydrates’ hydroxyl groups or the ring oxygen as ligands, and the other coordination sites can be occupied either by amino-acid residues or water molecules (Figure 2B1).11Drickamer K. Ca2+-dependent sugar recognition by animal lectins.Biochem. Soc. Trans. 1996; 24: 146-150Crossref PubMed Scopus (0) Google Scholar In CCIs, the coordination sites are occupied by hydroxyl groups or water molecules (Figure 2B2). In complex carbohydrates (e.g., glycoconjugates, glycosaminoglycans), such coordination can result in formation of bridges between different carbohydrate chains—an indispensable feature in cell-cell adhesion, which increases the adhesion force. Cations can also compensate the negative charge of carbohydrates that contain acidic groups. Thus, the concentration of the cations plays a crucial role not only in balancing the adhesive forces versus the charge repulsion but also in modulating the cell surface charge.Hydrophobic interactionsDespite being highly polar and solvated molecules, carbohydrates can engage in hydrophobic interactions through non-polar regions that are created by the localization of several axial protons on the same face of the carbohydrate ring. Hydrogen bonds provide a structural framework that enables molecules approaching to close proximities where van der Waals forces contribute to the stability of carbohydrate-protein complexes. The additional contribution of CH-π interactions was demonstrated experimentally and theoretically relatively recently.12del Carmen Fernández-Alonso M.D. Cañada F.J. Jiménez-Barbero J. Cuevas G. Molecular recognition of saccharides by proteins. Insights on the origin of the carbohydrate-aromatic interactions.J. Am. Chem. 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While London dispersion forces are the main attractive forces of CH-π interactions, the weaker electrostatic interaction between the electron-rich aromatic ring and electron-deficient C–H bonds of the carbohydrate (Figure 2C2) controls the directionality of the bond. A comparison between different aromatic amino acids shows that all four of them, namely, tryptophan, tyrosine, phenylalanine, and histidine, engage in CH-π interactions with carbohydrates but there is a strong preference for tryptophan followed by tyrosine, phenylalanine, and histidine.13Hudson K.L. Bartlett G.J. Diehl R.C. Agirre J. Gallagher T. Kiessling L.L. Woolfson D.N. Carbohydrate-aromatic interactions in proteins.J. Am. Chem. Soc. 2015; 137: 15152-15160Crossref PubMed Scopus (180) Google Scholar This order reflects the electrostatic surface potential and electron-richness of their respective π-systems.Multivalent carbohydrate-binding modesA key feature of carbohydrate interactions is the combinatorial impact of the above interactions that must be multiplied and organized in a cooperative fashion to enhance and modulate the strength of binding and render it biologically functional (Figure 3). Of note, multiple simultaneous interactions have unique collective properties that are different than the simple arithmetic sum of the respective monovalent interactions. Multivalent ligands usually bind to their receptors in a sequential manner so that the entropic cost is paid in the initial binding. The entropic barrier of subsequent interactions is therefore lower, resulting in favorable binding.5Fasting C. Schalley C.A. Weber M. Seitz O. Hecht S. Koksch B. Dernedde J. Graf C. Knapp E.W. Haag R. Multivalency as a chemical organization and action principle.Angew. Chem. Int. Ed. Engl. 2012; 51: 10472-10498Crossref PubMed Scopus (670) Google ScholarFigure 3Examples of multivalencyShow full caption(A and B) (A) Carbohydrate-carbohydrate interactions that take place during (A1) cell-to-cell interactions or (A2) alginate chains in the presence of metal ions (gray) and (B) protein-carbohydrate interactions between: (B1) glycan-binding proteins and glycosaminoglycans; (B2) lectins and glycoconjugates with multivalent carbohydrates presentation can promote lectin clustering; and (B3) multiple lectins and multivalent carbohydrate forming cross-linked nets. Cells are presented in blue, receptors in light green, and the metal ions in dark gray.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Multivalent CCIsIn biological systems, CCIs draw attention owing to their involvement in cell-cell adhesion and recognition processes.15Bucior I. Scheuring S. Engel A. Burger M.M. Carbohydrate-carbohydrate interaction provides adhesion force and specificity for cellular recognition.J. Cell Biol. 2004; 165: 529-537Crossref PubMed Scopus (97) Google Scholar,16Hakomori S. Carbohydrate-to-carbohydrate interaction, through glycosynapse, as a basis of cell recognition and membrane organization.Glycoconj. J. 2004; 21: 125-137Crossref PubMed Scopus (124) Google Scholar The weak CCI are advantageous during the first step of cell surface screening and together with their induced fit, as compared with protein-protein interactions (e.g., integrins, cadherins), allow fine-tuning for primary connections between the cells’ glycocalyx through the formation of low-affinity and reversible bonds (Figure 3A1).16Hakomori S. Carbohydrate-to-carbohydrate interaction, through glycosynapse, as a basis of cell recognition and membrane organization.Glycoconj. J. 2004; 21: 125-137Crossref PubMed Scopus (124) Google Scholar,17Lai C.H. Hütter J. Hsu C.W. Tanaka H. Varela-Aramburu S. De Cola L. Lepenies B. Seeberger P.H. Analysis of carbohydrate-carbohydrate interactions using sugar-functionalized silicon nanoparticles for cell imaging.Nano Lett. 2016; 16: 807-811Crossref PubMed Google Scholar These are then reinforced, if the fit is right, through multiple, simultaneous, and specific interactions, before less reversible processes can take place to form dynamic but stable complexes. In most biological systems, bivalent ions, particularly Ca2+, promote CCIs by locking the carbohydrate chains in their complementary arrangement to provide an optimal fit between cells and/or by crosslinking the carbohydrate chains from interacting cells. Such cross-links might only be involved as a driving force in the initial cell-to-cell approach or provide additional adhesion forces per binding site, leading to the formation of the so-called carbohydrate zippers (Figure 3A2). The zippers are usually formed by the systematic repetition of membrane proteoglycans at the cell surface and their adhesion strengths are within the piconewton range. While this binding mechanism is considered to be general for CCI, other binding modes may also occur, illustrating the tremendous value of multivalency in creating specificity and modulating binding forces.Multivalent CPIsMost of the specific biological roles of carbohydrates are mediated by CPIs. If we exclude the glycan-specific antibodies, the proteins that interact with carbohydrates can be categorized into two major groups: lectins and glycosaminoglycan-binding proteins (GBPs). Their monovalent interactions, i.e., protein binding to a monosaccharide at a single site, are generally weak (Kd in the micromolar range) but the binding affinities can be enhanced dramatically by adopting protein conformations that facilitate multivalent binding. The spatial arrangement of the binding sites can modulate the affinity of the interaction and thus allows selectivity. The carbohydrate density is also important because of the avidity (collective affinity) effect, but too high density can cause steric hindrance and compromise the selectivity.Various binding modes exist and depend on the protein structure, the nature and complexity of carbohydrate, the environment, and the particular function (Figure 3B). GBPs interact with sulfated glycosaminoglycans via clusters of positively charged amino acids, e.g., Cardin-Weintraub sequences (Figure 3B1). On the other hand, lectins recognize specific carbohydrate termini and bind them into structurally defined pockets (Figures 3B2 and 3B3). Multivalent carbohydrates can either bind to subsites of single or multiple lectins. In the case of single lectins, the subsites are generally clustered to face the carbohydrate, facilitating face-to-face binding through multiple identical interactions. The clustering of receptor subsites increases the valency on the protein side, thereby strengthening the interaction. This phenomenon is known as the “cluster glycoside effect” and the overall interaction can be stronger than the sum of the individual interactions in some cases. Lectins that are involved in signal transduction processes use different binding modes with cross-linked nets between multiple lectins and one or more multivalent glycoconjugates (Figure 3B3). These cross-links create networks of non-covalent interactions that can be either linear, two dimensional, or three dimensional. One-dimensional cross-links result from interactions between bivalent carbohydrates and bivalent lectins, where each carbohydrate site is bound to a different lectin. 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