Molecular Weight Determination of Agar by Sedimentation Equilibrium Measurements

Sedimentation equilibrium and viscosity measurements were carried out on six agar samples in 0.1M KCl at 65 degree C. The sedimentation equilibrium data showed that the weight-average molecular weight, M sub(w), of these agar samples ranged from 3.0x10 super(4) to 1.9xl0 super(5). The ratio of z-average molecular weight, M sub(z), to M sub(w), which indicated the inhomogeneity of molecular weight, ranged from 1.1 to 2.9. The wide moleculare weight distribution suggested the cleavage of agar molecules during alkali treatment. Although the gelation point depended on molecular weight in the case of the same origin sample, the molecular weight dependence of gelation point did not exist in different algal species. From the log-log plots of intrinsic viscosity, [ eta ], versus M sub(w), it was clear that the data plots approximated the following equation: [ eta ] = 8.75x10 super(-4)M super(0.68). From the magnitude of the exponent value, 0.68, we considered that agar molecules were in the form of random coils in 0.lM KCl at 65 degree C. Furthermore, the molecular shape of agar in the case of adding an inhibitor of hydrogen bonding to the solution was also random coils, which supported the view that the sol-gel transition of agar is due to the formation of hydrogen bonds.