Synopsis
The iodine which is added to an aqueoiis amylose solution is bound only partly by the amylose while forming the blue complex and partly remains free. The eqnilibriiim normality of the free and the bound iodine at half-saturation of amylose by iodine is designated as [I,], and respectively. The stability of the polyiodirie chain formed within the axis of amylose helices depends on its length, i.e., indirectly on the DP of the amylose helices:
The amylose molecule consists of helical segments. Such a molecule may behave as a random coil.
The average length of the helical segments in freshly prepared amylose-iodine complexes depends on temperature, pH, iodide concentration, the presence of other complexforming agents, and the DP of the amylose. This latter factor is investigated in the present paper. By the aid of an automatically recording photometric titrating device the coherent values of [I1] and [II] were determined. Plotting these values against DP, for mechanochemically degraded as well as for periodate-oxidized amyloses resulted in ciirves consisting of two linear sections. The break of the curves occurred between 1>P, 110 and 130. It was concluded that below UP, = 100 the DP of helical segmenk (= sl)P,) is identical to the DP, of the total molecule, i.e., the moleciile consists of only a single, relatively stiff helix. Above this limit the molecule contaiiis sever:d helicd segments.
the greater this stability, the lower the [I,], value.
The IYP of these helical segments can be calciilated as follows: SDP, = 141.1 -10.2 x 105[1,1,
This equation is considered to be valid for 0.5-0.6 mg. arnylose in 100 ml. 0.1N HC1 at 2OoC., X = 650 mp, crivet diamet,er 3.4 cm., the feed rate of the iodate-iodide titrating solution (in acid medium resilltirig in a 5 X 10-3iV Ip solution with a molar iodide to iodine ratio of 1.5) is 0.4 ml./miii. Amylose molecules of, e.g., BP,, = 1380 consist of an average of 11.4 segmeiits hwiiig a of : h i i t , 120 :tiit1 consisting of aii average o f 1.5-1 S helictll tiirris.