Patterns matter part 1: Chitosan polymers with non-random patterns of acetylation

Abstract Chitosans are a family of binary co-polymers of glucosamine and N-acetylglucosamine residues with superb material properties and versatile biological functionalities. Structurally, they are characterised by three parameters, namely the degree of polymerisation (DP), fraction of acetylation (FA), and pattern of acetylation (PA). Both the physico-chemical properties and the biological activities of chitosans are influenced by these structural parameters. Moreover, chitosan samples are invariably mixtures of molecules differing in these properties so that the values determined for DP and FA are mean values. Consequently, to fully describe a chitosan polymer sample, determination of the dispersities Đ for both DP and FA need to be included, too, but the likely influence of these parameters on properties and bioactivities of chitosans is still mostly ignored. In this review, we summarise the state of the art concerning the production and analysis of defined chitosan polymers as well as our current knowledge on the structure-function relationships of their properties and bioactivities. Progress in this field was mostly driven by the availability of increasingly sensitive analytical techniques, such as HPSEC-RID-MALLS for determination of DP and ĐDP, and NMR for determination of FA and PA. More recently, capillary electrophoresis has been suggested for the analysis of ĐFA, and enzymatic-mass spectrometric fingerprinting methods are revolutionizing FA and PA analyses. Heterogeneous and homogeneous chemical methods of partial de-N-acetylation of chitin and partial N-acetylation of polyglucosamine yield chitosans with known FA, but yet unknown ĐFA, while different chemical and physical methods of partial depolymerisation yield chitosans with known DP and ĐDP. Today, the availability of increasing numbers of well-defined recombinant chitin deacetylases is beginning to give access to chitosan polymers with different non-random PA. In the recent past, this has allowed systematic investigations of the influence of DP and FA on material properties and biological activities, it currently allows us to begin investigating the roles of ĐDP and PA, and it will hopefully soon enable us to also study the role of ĐFA.