AGGREGATION STATE OF CHITIN AND CHITOSAN NANOPARTICLES AT DIFFERENT DEGREES OF ACETYLATION

FQ3

Eduardo de Faria Franca (1,2), Dr. Luiz Carlos Gomide Freitas (2) and Dr. Roberto D. Lins (1)

(1) Pacific Northwest National Laboratory, Richland, WA, USA,  (2) Departamento de Química, Universidade Federal de São Carlos, São Carlos, SP, Brazil

Chitin, a (1®4)-linked N-acetyl-b-D-glucosamine is widely distributed in nature as the main component in the exoskeleton of crustaceans, insects and fungi. Chitosan, its main derivative, is generally produced by alkaline deacetylation of chitin. These biopolymers have been widely developed for use as antimicrobials, biomedical materials, cosmetics, agricultural materials and principally as decontaminant of residual waters containing pesticides and heavy metals. These various applications require information about the degree of acetylation (DA) and distribution of acetyl groups along the biopolymer chain.  The possible effect of the DA on chitosan solubility has been discussed previously, although still a lot of contradiction in the literature. Despite the experimental efforts during the last decade, the behavior of chitosan at molecular level remains elusive due to the intrinsic nature of these polymers. In order to overcome this limitation, a variety of molecular simulation techniques have been employed to evaluate the influence of the number and the distribution of the acetyl groups on the aggregation state of chitin and chitosan nanoparticles in aqueous environment. In this study, a set of explicit-solvent 20-ns molecular dynamics simulations of chitosan nanoparticles were performed at different degrees of acetylation (0%, 40%, 60% and 100%) at neutral pH. Two setups were considered for the systems with 40% and 60% of acetylation, random and block where acetyl groups were evenly distributed or aggregated in a confined region, respectively. The starting filaments in all nanoparticles were modeled as hydrated crystals with filaments in a 2-fold helix conformation. Analyses of these simulations reveal that the presence of acetyl groups in chitin and chitosan filaments contribute to the intra-chain hydrogen bonds stabilization between adjacent sugars monomers, decreasing the solubility of these biopolymers in water and stabilizing a crystal like configuration. The nanoparticles up to 40% of acetylation are moderately soluble in neutral pH, showing more stable aggregates when acetyl groups are in block distribution. This happens due to the increase of inter-chain hydrogen and hydrophobic interaction between chitosan fibers. Chitosans with 60% or more of acetyl groups are insoluble and present similar degrees of swelling, in agreement with experimental observations. The spatial distribution of the N-acetyl and the hydroxylmethyl seem to be responsible to trap water molecules in a well-defined orientation resulting in a peculiar microsolvation environment around the polysaccharides filaments. The simulations also suggest that neither the inter-chain hydrogen bonds nor the hydrophobic interactions, created by the acetyl groups, were responsible for the chitosan aggregation. On the other hand, the solubility of chitin and chitosan are strongly affected by the solvent orientation around the polysaccharide filament and this probably is the main cause of aggregation of these biopolymers.