Comparative study on the self-assembly of pectin and alginate molecules regulated by calcium ions investigated by atomic force microscopy

Atomic force microscopy investigation and viscosity measurements were applied on pectin solution. Single fiber lengths of both high-methoxyl and low-methoxyl pectin in 0.005 mg/ml were in the range of 19.22-312.11 nm while width was reduced 9.94 % after base catalyzed de-esterification. With adding calcium ions, the viscosity of 0.5 mg/ml low-methoxyl pectin continued growing from 1.30mPa.s while high-methoxyl pectin solution viscosity stabled at the initial level of 1.26mPa.s. Comparation of binding behaviors of calcium ions with pectin and alginate revealed the structural influence on inducing force. The formation of dimers was followed by lateral aggregation of dimers when calcium ions were added to low-methoxyl pectin. The lack of third step occurred in alginate where aggregations broken by excessive calcium ions were attributed to the side chains of pectin. Large numbers of esterified galacturonic acid residues limited aggregation of high-methoxyl pectin in intra-molecular interactions which induced by combination of nonspecific hydrophic interactions and hydrogen bonds.