On the Origin of the Blue Color in The Iodine/Iodide/Starch Supramolecular Complex

Abstract

The nature of the blue color in the iodine-starch reaction is still a matter of debate. Some textbooks still invoke charge-transfer bands within a chain of neutral I2 molecules inside the hydrophobic channel defined by the interior of the amylose helical structure. However, the consensus is that the interior of the helix is not altogether hydrophobic—and that a mixture of I2 molecules and iodide anions reside there and are responsible for the intense charge-transfer bands that yield the blue color of the “iodine-starch complex”. Indeed, iodide is a prerequisite of the reaction. However, some debate still exists regarding the nature of the iodine-iodine units inside the amylose helix. Species such as I3-, I5-, I7- etc. have been invoked. Here, we report UV-vis titration data and computational simulations using density functional theory (DFT) for the iodine/iodide chains as well as semiempirical (AM1, PM3) calculations of the amylose-iodine/iodide complexes, that (1) confirm that iodide is a pre-requisite for blue color formation in the iodine-starch system, (2) propose the nature of the complex to involve alternating sets of I2 and Ix- units, and (3) identify the nature of the charge-transfer bands as involving transfer from the Ix- σ* orbitals (HOMO) to I2 σ* LUMO orbitals. The best candidate for the “blue complex”, based on DFT geometry optimizations and TD-DFT spectral simulations, is an I2-I5-I2 unit, which is expected to occur in a repetitive manner inside the amylose helix.

The Iodine/Iodide/Starch Supramolecular Complex

“…It is now generally accepted that iodide ions are also required in this process. Thus, the iodine atoms would align inside the amylose helix as a mixture/combination of I2 and I − [21,22,[24][25][26][27] with an unusual metallic-like structure [23]. This mixture would not entail isolated I − ions, but rather In − polyiodides (n = 3, 5, 9…) [28][29][30][31][32].…”

“…According to semiempirical calculations, the binding of water or of interspersed water and iodine molecules inside the amylose helix results in slight steric distortions of the polymer [22]. The water molecules were found to adhere closely to the walls of the internal channel of amylose and to exclude I 2 from interactions, suggesting that I 2 molecules alone would be unable to dislocate water molecules from inside the amylose helix.…”

“…Recent DFT calculations on iodine/iodide chains have been interpreted as evidence that without iodide, the blue color cannot be formed in the starch-iodine system [22]. These simulations propose that the nature of the complex consists of alternating sets of I 2 and I x − units, where the nature of the charge transfer bands responsible for the blue color involves transfer from the I x − σ* orbitals (HOMO) to the I 2 σ* LUMO orbitals (cf.…”

“…Figure 3 illustrates the proposed structures of amylose-I 2 and amylose-tri-iodide species based on electronic structure calculations. Figure 4 illustrates the representative structures proposed for the iodine/iodide arrangement inside the amylose helix [22].…”

Implementación de una secuencia didáctica a distancia para cuantificación de almidón en alimentos por espectrofotometría

“…En continuación con las actividades de la secuencia de aprendizaje, el almidón obtenido fue utilizado como patrón para la construcción de una curva de calibración, utilizando el método de acomplejamiento con yodo, en el que se mide la absorbancia de un complejo de coloración azul a una longitud de onda de 600 a 630 nm y que se forma, según los más recientes trabajos, debido a que en el interior de la hélice de amilosa, se acomodan moléculas de I 2 y iones yoduro, generando intensas bandas de transferencia de carga que son las responsables de la coloración azul intensa (Pesek et al, 2022). Para las mediciones se seleccionó el filtro verde porque corresponde al color de luz que absorbe el complejo de almidón soluble que se buscaba cuantificar, el cual tiene una coloración púrpura-azulada, de esta forma es posible intensificar el color del compuesto analizado, mejorando la sensibilidad del espectrofotómetro casero.…”