K. Mazeau et al., Predicted influence of N-acetyl group content on the conformational extension of chitin and chitosan chains, J CARB CHEM, 19(9), 2000, pp. 1269-1284
Conformational analysis of chitosan molecules has been performed using the
MM3(92) force field to investigate the role played by the acetamido groups
on the stiffness of these chains. A high dielectric constant value was need
ed to model an aqueous environment and to reproduce the distribution of the
N-acetyl glucosamine group orientation that is observed by NMR. Disacchari
dic fragments, differently substituted at C2, were selected as models for c
hitin and chitosan chains. Their conformational space has been explored by
means of adiabatic mapping of the glycosidic Phi,Psi torsion angles. Althou
gh the overall features of all the potential energy surfaces created appear
similar, the accessible conformational space of a glycosidic bond is affec
ted by the nature of the substituent at C2 on the non-reducing residue of t
he disaccharide unit. This is illustrated by the differences in the calcula
ted partition functions together with the predicted average homonuclear and
heteronuclear coupling constants. Computed maps were used to predict polym
eric unperturbed dimensions, characteristic ratio and persistence length of
idealized chitin and chitosan chains, by Monte Carlo methods. Pure chitosa
n is predicted to be more coiled than pure chitin chains. At low N-acetyl g
roup contents, chain extension appears to be dependent on the degree of sub
stitution. Average chain dimensions increase monotonically for increases in
content up to 60% of N-acetyl groups, but show no significant variation at
higher contents. For molecules consisting of 50% amino and 50% N-acetylate
d residues, random, alternate and block patterns of substitution have been
investigated. It has also been shown that the spatial extension of the poly
mer chains is dependent on the primary structure. Comparison with the liter
ature experimental data is difficult because of the extreme diversity of th
e reported conformationally dependent values. However, such study provides
a unique insight into the dependence of these two factors (degree of acetyl
ation and distribution of acetyl groups) on the stiffness and flexibility o
f different chitin and chitosan chains.