Self-assembly of pulp derivatives as amphiphilic compounds: Verification of molecular association and complexation with low and high molecular mass compounds

Acetic acid pulps (AAPs) have been converted to amphiphiles by hydroxypropylation. The resulting hydroxypropyl derivatives (HP-AAPs) formed self-aggregates in water. The interaction of HP-AAP molecules was investigated by a viscometric method. The reduced viscosity (eta(sp)/c) of HPA-AP in both water and chloroform was increased remarkably above a concentration of 0.1% This implies that HP-AAP molecules strongly interact with each other, caused by the hydrophobic interaction of residual lignins in water and by hydrogen bonding of polysaccharides in chloroform. The self-aggregates adsorb water-soluble fluorescent agents. The highest adsorption capacity was observed at the lowest pH among three pH conditions investigated. They also solubilized sparingly water-soluble fluorescent agents in water in larger amounts than did sodium dodecyl sulfate when used as a surfactant. These results suggest that the self-aggregates of HP-AAP adsorb low molecular mass compounds as inclusion compounds. To clarify the interaction of HP-AAP with biopolymers as high molecular mass compounds, the change in the activity of papain, a protease, in phosphate buffer (pH 6.2) was examined in the presence and absence of HP-AAP. HP-AAP acted as an inhibitor of papain at the initial stage of mixing. After mixing for 24 hours, however, the papain activity was revived and preserved for 6 days. In contrast, the papain activity vanished in the absence of HP-AAP after 24 hours because of autolysis. Therefore, HP-AAP protects papain against autolysis, resulting from tight complexation with the biopolymer.