Regioselective chitosan end-group activation: the triskelion approach

Abstract

Chitosan (CS) end-group conjugation methods are rarely reported in the literature, mainly since the CS terminal aldehyde moiety produced by nitrous acid depolymerization is only present in trace amounts in its reactive form. In a previous study, our group proposed an intermolecular thioacetylation process that allowed terminal conjugation of thiol-reactive species to chitosan with 50% efficiency. However, this reaction is incompatible with acid-labile substituents and the conversion efficiency of CS end-groups could be limited by the size of the thiol-reactive species engaged in the reaction, mainly by steric hindrance since two substituents are required to obtain the stabilized thioacetal derivative. In the present study, we developed a novel CS end-group thioacetylation approach relying on a new regioselective linker that bears three thiol moieties. This trivalent linker, referred to as triskelion here, was specifically designed for activation of the CS 2,5-anhydro-D-mannose (M-Unit) end-group and consists of a thiolhook for efficient aldehyde conjugation through an intramolecular reaction and a thiol-tail that remains available for subsequent end-group functionalization with any thiol-reactive species. The chemical synthesis of this linker provided the desired material with high yields over three steps. The in situ intramolecular thioacetylation process between the triskelion linker and 2,5-anhydro-D-mannose (MUnit, monomeric) was assessed by semi-quantitative LC-MS studies, revealing that the corresponding intramolecular thioacetal largely predominated ($90%). This regioselective derivatization was also performed onto M-Unit CS aldehydes and the desired CS-b-triskelion conjugates were obtained with functionalization degrees over 85%, as confirmed by NMR spectroscopy ( 1 H and DOSY). As a final assessment of the CS-b-triskelion thiol-tail reactivity, these conjugates were successfully engaged with thiol-reactive magnetic beads into disulfide bond displacement with 50% efficiency. The proposed CS terminal activation with the triskelion linker opens new perspectives for biomedical applications, especially brush-like surface modifications and other copolymer formation through disulfide linkages or Michael-type additions.

Reducing-End Functionalization of 2,5-Anhydro-d-mannofuranose-Linked Chitooligosaccharides by Dioxyamine: Synthesis and Characterization

“…COS take advantage of various interesting physico-chemical and biological properties, including principally water-solubility, biodegradability, biocompatibility, antibacterial, antiviral, and antifungal activities [5][6][7][8][9]. In order to develop new COS-based materials with advanced and significant added-value applications, the functionalization of COS with chemically reactive groups is currently being explored intensively [9][10][11][12][13][14][15]. In this context, we have recently described a powerful and versatile method for the introduction of "clickable" chemical groups such as alkene, alkyne, azide, hydrazide, and thiol at the reducing end of COS synthesized by nitrous acid depolymerization of chitosan [16].…”

Advances in chitooligosaccharides chemical modifications

“…They also developed a novel approach allowing a more efficient one‐step activation of the aldehyde by thioacetylation (Scheme 13C). [141] This process relies on the use of a trivalent linker bearing a thiol‐hook and a thiol‐tail. At first, the thiol‐hook reacted with the aldehyde through hemithioacetal intermediate followed by water elimination to result in thioacetal formation.…”

“…Scheme summing up the possible reaction on 2,5 anhydro‐ d ‐mannose end chain oligomers: A, Schiff base formation followed by reduction (NaBH 3 CN, NaBH 4 ), [138] B, oximation with formation of isomer (E) and (Z), [139,140] C, thioacetylation with trivalent linker, [141] D, thioacetylation, [142] E, reduction, [139] F, oxidation in carboxylic acid, [143] and G, addition of hydrazine followed by reduction [144,145] …”