ALKALI-MODIFICATION OF CARRAGEENANS - MECHANISM AND KINETICS IN THE KAPPA-IOTA-SERIES, MU-NU-SERIES AND LAMBDA-SERIES

The cyclization reaction (formation of 3,6-anhydro-alpha-D-galactose units from alpha-D-galactose 6-sulfate) of carrageenans follows a pseudo first-order kinetics, being 20-60 times faster for carrageenans of the kappa-family than for those of the lambda-family. In lambda-carrageenans the clustering of the sulfate groups around the hydroxyl on C-3 of the alpha-unit shields it from polarization or ionization, reducing the cyclization reaction rate. Furthermore, molecular models of lambda-carrageenans suggest an adequate geometry for the interaction between the hydroxyl group on C-3 of the alpha-unit and the sulfate group on C-2 of the beta-unit. These observations also explain the lack of cyclized derivatives of lambda-carrageenans in nature. The ease with which the cyclization reaction occurs for carrageenans of the kappa-family indicates that the alkaline treatments used industrially could be carried out under milder conditions, giving products of high gel strength.