Molecular configuration of amylose and its complexes in aqueous solutions. Part IV. Determination of DP of amylose by measuring the concentration of free iodine in solution of amylose–iodine complex

Abstract

Abstract In aqueous solutions of the amylase–iodine complex the concentration of free iodine [I f ] v after reaching equilibrium (or closely approximating it) is determined by the following factors: temperature, pH, concentration of iodide ions and amylose, and DP of amylose. In the present paper the role of temperature, amylose concentration, and DP has been investigated. At half‐saturation of amylose by iodine, the reciprocal value of free iodine defines the equilibrium constant: 1/[I f ] v = K . The relation between [I f ] v , in normality and temperature is the following: 5 + log [I f ] v = −(2.132/ T ) + 8.52, for DP n = 1290, 0.4 mg. amylose in 100 ml. 0.1 N HCl. The value of the energy of activation E a between 2 and 52°C. is 9.72 kcal./mole. The influence of amylose concentration [Am] on photometrically determined [I f ] v , at 20°C, in the range of 0.1–1.2 mg./100 ml. 0.1 N HCl for DP n = 1290 is: 5 + log [I f ] v = 0.209 − 0.047 log [Am]. At [Am] = 0.6 mg. amylose/ 100 ml. 0.1 N HCl and 20°C, the value of [I f ] v depends on DP n as follows: 5 + log [I f ] v = 0.085 = + 0.222 log (10 4 / DP n ). These above equations are summarized by the relation: [I f ] v = exp {16.865 − ( E a / RT )}[Am] 0.047 (10 4 / DP n ) 0.222 ×10 −5 Considering that the determination of [I f ] v by automatic photometric titration can be performed quickly and with appropriate reproducibility, this method is convenient for a rapid empirical and approximate determination of DP of amylose on a microscale. The iodine‐binding capacity [IBC] as well as the value of λ max , have been also investigated as functions of DP n , by photometric and by amperometric titration.