Physicochernical behavior of homogeneous series of acetylated chitosans in aqueous solution: Role of various structural parameters

Physicochemical properties of four different homogeneous series of chitosans with degrees of acetylation (DA) and weight-average degrees of polymerization (DPw) ranging from 0 to 70% and 650 to 2600, respectively, were characterized in an ammonium acetate buffer (pH 4.5). Then, the intrinsic viscosity ([eta](0)), the root-mean-square z-average of the gyration radius (R-G,R-z), and the second virial coefficient (A(2)) were studied by viscometry and static light scattering. The conformation of chitosan, according to DA and DPw, was highlighted through the variations of alpha and nu parameters, deduced from the scale laws [eta](0) = KwMwalpha and R-G,R-z = K'M-w(nu), respectively, and the total persistence length (L-p,L-tot). In relation with the different behaviors of chitosan in solution, the conformation varied according to two distinct domains versus DA with a transition range in between. Then, (i) for DA <25%, chitosan exhibited a flexible conformation; (ii) a transition domain for 25 <DA < 50%, where the chitosan conformation became slightly stiffer and, (iii) for DA > 50%, on increasing DPw and DA, the participation of the excluded volume effect became preponderant and counterbalanced the depletion of the chains by steric effects and long-distance interactions. It was also highlighted that below and beyond a critical DPw,c (ranging from 1300 to 1800 for DAs from 70 to 0%, respectively) the flexibility of chitosan chains markedly increased then decreased (for DA > 50%) or became more or less constant (DA < 50%). All the conformations of chitosan with regards to DA and DPw were described in terms of short-distance interactions and excluded volume effect.