Abstract
Three alginates with fundamentally different block structures, poly-M, poly-G, and poly-MG, have been investigated upon ionic crosslinking with chitosan oligosaccharides (CHOS), using circular dichroism (CD), rheology, and computer simulations, supporting the previously proposed gelling principle of poly-M forming zipper-like junction zones with chitosan (match in charge distance along the two polyelectrolytes) and revealing a unique high gel strength poly-MG chitosan gelling system. CD spectroscopy revealed an increased chiroptical activity exclusively for the poly-M chitosan gelling system, indicative of induced conformational changes and higher ordered structures. Rheological measurement revealed gel strengths (G′ < 900 Pa) for poly-MG (1%) CHOS (0.3%) hydrogels, magnitudes of order greater than displayed by its poly-M analogue. Furthermore, the ionically crosslinked poly-MG chitosan hydrogel increased in gel strength upon the addition of salt (G′ < 1600 at 50 mM NaCl), suggesting a stabilization of the junction zones through hydrophobic interactions and/or a phase separation. Molecular dynamics simulations have been used to further investigate these findings, comparing interaction energies, charge distances and chain alignments. These alginates are displaying high gel strengths, are known to be fully biocompatible and have revealed a broad range of tolerance to salt concentrations present in biological systems, proving high relevance for biomedical applications.
Highlights
Alginate is a linear polysaccharide predominantly occurring in nature in the class of brown seaweed Phaeophyta,[1] various bacteria such as Pseudomonas and Azotobacter vinelandii incorporate alginate as exocellular polymeric material.[2,3] Alginates form a polysaccharide family composed of (1 / 4)-linked b-D-mannuronic acid (M-unit) in 4C1 conformation and its C5epimer, a-L-guluronic acid (G-unit) in the 1C4 conformation with a pKa-value of circa 3.5
The three samples were selected based on their molecular weight average (Mw) and intrinsic viscosity ([h]), to mitigate the in uence of their molecular size on the conducted gelling experiments.[58,59]
An increase in chitosan oligomer DP led to a higher relative increase in gel strength for poly-MG compared to poly-M, suggesting a different underlaying crosslinking mechanism for the two different types of alginate block structures
Summary
Alginate is a linear polysaccharide predominantly occurring in nature in the class of brown seaweed Phaeophyta,[1] various bacteria such as Pseudomonas and Azotobacter vinelandii incorporate alginate as exocellular polymeric material.[2,3] Alginates form a polysaccharide family composed of (1 / 4)-linked b-D-mannuronic acid (M-unit) in 4C1 conformation and its C5epimer, a-L-guluronic acid (G-unit) in the 1C4 conformation with a pKa-value of circa 3.5 These unbranched polyanionic block copolymers are composed of homopolymeric regions of M-units (M-blocks), G-units (G-blocks), and regions of alternating epimers (MG-blocks) of various lengths.[4,5]. CHOS have attracted increasing attention in recent years as they have been associated with numerous biological effects.[11,12,13,14] Chitosan is known for its high biocompatibility, biodegradability[15,16] and low toxicity.[17,18]
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