CHEMISTRY

V. O. MOKHNACH and N. M. RUSAKOVA

ON THE MECHANISM OF THE IODINE–STARCH REACTION

(Presented by Academician I. I. Chernyaev, May 7, 1962)

An enormous number of works have been devoted to the study of the color reaction of starch with iodine over the century and a half that have passed since its discovery ($^{1,2}$). However, the results of these studies are wholly out of proportion to the effort expended. If the structure of the iodine–starch complex may with some probability be spoken of as an inclusion compound, then at present nothing is known about the mechanism of the iodine–starch reaction.

Meanwhile, this question is not only of theoretical interest, but also has great practical significance. As early as 100 years ago ($^3$) the medicinal properties of iodized starch were known, but because it was used as an external agent, its advantages over other iodine preparations remained unnoticed. In 1942 ($^4$), iodized starch was for the first time successfully administered internally for the treatment of human dysentery. Thus its non-toxicity with this method of administration and its obvious advantages in comparison not only with other iodine preparations, but also with antibiotics and sulfanilamides, were first demonstrated.

To understand the biological activity of iodized starch, it is important to identify the form of iodine that is incorporated into the macromolecule. In this connection, the study of the mechanism of the iodine–starch reaction is of special interest.

For this purpose we carried out a spectrophotometric study of the interaction of starch with iodine in aqueous solutions.

Experimental Part

Solutions of iodized starch were prepared in twice-distilled water from ordinary starch of the “Goslaborsnabzhenie” grade by the action of a calculated amount of iodine. The iodine preparation, chemically pure grade, was subjected to double sublimation. Spectrophotometric studies were carried out at room temperature on an SF-4 spectrophotometer in the range 220–800 mμ.

As shown by literature data ($^{5—8}$) and by our studies ($^{9,10}$), in the absorption spectra of blue iodine solutions in the presence of starch and certain other macromolecules, an absorption band is observed in the visible region with $\lambda_{\max} 580—620$ mμ. This band is often accompanied by another absorption band in the ultraviolet part of the spectrum with $\lambda_{\max} 350—360$ mμ.

We studied the appearance of these bands in the absorption spectrum of an aqueous solution of iodized starch against the background of the absorption spectra of the individual components of the complex.

As can be seen in Fig. 1, curve 1 represents the absorption spectrum of a 0.008% aqueous solution of iodine, in which there are three absorption bands with $\lambda_{\max} 288—290$, $\lambda_{\max} 350—354$, and $\lambda_{\max} 460$ mμ, belonging respectively to $\mathrm{J}_3^-$, $\mathrm{JO}^-$, and $\mathrm{J}_2$ ($\mathrm{J}_2$ here and below is an abbreviated expression for $n \cdot \mathrm{H}_2\mathrm{O}\,[\mathrm{J}^+ \ldots \mathrm{J}^-]$). Curve 2, characterizing

absorption in the UV region of a 0.5% aqueous starch solution, reveals only a continuous absorption edge with a small optical-density value in the far ultraviolet. After the iodine solution had been mixed with the starch solution, we investigated the absorption spectrum of the resulting blue solution. As curve 3 shows, the absorption bands at \(\lambda\) 288—290 and \(\lambda\) 460 mµ disappeared, while the band at \(\lambda\) 350—360 mµ showed a sharp increase in optical density. At the same time, in the visible part of the spectrum a pronounced absorption maximum appeared at \(\lambda\) 600 mµ, characteristic of the iodine-starch complex. Thus, the addition of starch to an aqueous iodine solution leads to the disappearance of iodine in the form of \(J_3^-\) and \(J_2\), with formation of the system: \(JO^- — H_2O\) — iodine-starch. It follows from this that, in the presence of starch, \(J_2\) molecules dissociate with formation of complex ions \(J_3^-\) and \(JO^-\), of which only the former participates in formation of the complex. The complex anion \(JO^-\) does not

Fig. 1. Absorption spectra of aqueous solutions: 1 — \(J_2 — H_2O\); 2 — starch — \(H_2O\); 3 — \(J_2 — H_2O\) — starch. In all solutions the concentration of \(J_2\) is 0.008%, starch 0.5%; layer thickness 0.2—1 cm

interact with the polymer, as a result of which its concentration increases as \(J_2\) is split, as indicated by the sharp rise in optical density at \(\lambda\) 350—360 mµ.

If this interpretation of the results of our experiment is correct, the following assumption may be made. At a relatively low concentration of starch in an aqueous iodine solution, when its amount is insufficient for complete absorption of the \(J_3^-\) anion formed as a result of the dissociation of \(J_2\), the spectrum of the solution should contain 4 maxima: at \(\lambda\) 288—290 mµ (\(J_3^-\)), \(\lambda\) 350—360 mµ (\(JO^-\)), \(\lambda\) 460 mµ (\(J_2\)) and \(\lambda\) 600 mµ (iodine-starch complex). If, further, we gradually increase the starch concentration, while retaining in the solution the initial concentration of \(J_2\), the absorption bands at \(\lambda\) 288—290 and \(\lambda\) 460 mµ should disappear from the spectrum of the solution, with a simultaneous increase in optical density at \(\lambda\) 350—360 and \(\lambda\) 600 mµ.

As Fig. 2 shows, the experiment fully confirms this assumption. Curve 1 demonstrates four clearly expressed absorption bands with maxima at \(\lambda\) 288—290, \(\lambda\) 350—360, \(\lambda\) 460 and \(\lambda\) 600 mµ. On curve 2 the band with \(\lambda_{\max}\) 288—290 mµ is already absent, which indicates the disappearance of the complex anion \(J_3^-\) as a result of an increase in starch concentration. Curve 3 demonstrates the complete absence in the spectrum also of the band with \(\lambda_{\max}\) 460 mµ, i.e. the disappearance of \(J_2\) from the solution. At the same time, curves 2 and 3 reveal a gradual increase in the optical density of the absorption bands at \(\lambda\) 350—360 and 600 mµ, which indicates an increase in the concentration of \(JO^-\) and of the iodine-starch complex in connection with the increase in starch concentration. The rise of the ordinates at \(\lambda\) 280—290 and \(\lambda\) 460 mµ with increasing concentration

starch in solution occurs through the growth of broad absorption bands upon formation of the anion \(\mathrm{JO^-}\) and of the iodine–starch complex, which extend into these ordinates. This is also confirmed by the chemistry of the reaction: the increase in ordinates at \(\lambda 350\) and \(\lambda 600\ \mathrm{m\mu}\) at a strictly constant iodine concentration can occur exclusively through absorption by \(\mathrm{J_3^-}\) (corresponding to \(280\)—\(290\ \mathrm{m\mu}\)) and \(\mathrm{J_2}\) (corresponding to \(460\ \mathrm{m\mu}\)).

As is known, when iodine dissolves in water a reversible reaction takes place:

\[ \mathrm{J_2 + H_2O \rightleftarrows HJ + HJO}, \]

the equilibrium of which is strongly shifted to the left.

Fig. 2. Absorption spectra of aqueous iodine–starch solutions at constant \(\mathrm{J_2}\) concentration of 0.02% and varying starch concentration: 1 — at 0.03%; 2 — at 0.05%; 3 — at 0.07%; 4 — at 0.1%. Layer thickness 0.4—0.5 cm

In this case the formation of the complex anion \(\mathrm{J_3^-}\) is observed through attraction of the \(\mathrm{J_2}\) molecule to \(\mathrm{J^-}\), in the same way as the latter attracts water molecules. The \(\mathrm{J_3^-}\) ion is in equilibrium with the products of its dissociation:

\[ \mathrm{J_3^- \rightleftarrows J^- + J_2}\quad (^{11}). \]

When starch is added to an iodine solution, interaction of the latter with the \(\mathrm{J_3^-}\) anion begins, with formation of the iodine–starch complex. This leads to disturbance of the equilibrium, further splitting of \(\mathrm{J_2}\), and an increase in the concentration of \(\mathrm{JO^-}\). What has been said is in full agreement with the spectrophotometric data obtained by us.

Fig. 3. Effect of starch on an aqueous iodine solution subjected to dialysis: 1 — absorption curve of the aqueous iodine solution; 2 — the same after addition of starch. Initial concentration of \(\mathrm{J_2}\) solution 0.002 \(N\). Layer thickness 0.4—5.005 cm

We also demonstrated the formation of the iodine–starch complex by inclusion of the \(\mathrm{J_3^-}\) anion with the aid of dialysis. An aqueous \(0.002\,N\) \(\mathrm{J_2}\) solution was subjected to dialysis through a cellophane membrane \(40\ \mu\) thick. Nine hours after the start of dialysis, only an absorption band with \(\lambda_{\max} 280\ \mathrm{m\mu}\) was observed in the absorption spectrum of the solution (Fig. 3, 1). After starch was added to this solution, \(\lambda_{\max} 570\ \mathrm{m\mu}\) was found in the absorption spectrum, while \(\lambda_{\max} 280\ \mathrm{m\mu}\) disappeared (Fig. 3, 2). These changes in the absorption spectra are direct proof of the formation

iodine-starch complex by direct interaction of starch with the anion \(J_3^-\).

Comparison of our spectrophotometric data with the chemistry of aqueous iodine solutions makes it possible to represent the mechanism of the iodine-starch reaction in the form:

\[ \mathrm{J}_2 + \mathrm{H}_2\mathrm{O} \to 2\mathrm{H}^{+} + \mathrm{J}^{-} + \mathrm{JO}^{-}, \tag{1} \]

\[ \mathrm{J}_2 + \mathrm{J}^{-} \to \mathrm{J}_3^{-}, \tag{2} \]

\[ \mathrm{J}_3^{-} + \text{starch} \to \text{iodine-starch}. \tag{3} \]

Komarov Botanical Institute
Academy of Sciences of the USSR

Received
28 XII 1961

CITED LITERATURE

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