Photodegradation of carbohydrates. Part IV. Direct photolysis of D-glucose in aqueous solution

Abstract

Radioactive tracer methods have been used to study the degradation of D-glucose on u.v. irradiation in oxygenated aqueous solution. After ca. 20% decomposition of the original D-glucose, the main products are gluconic acid, a tetrose, and a reducing product which has λ_max. 2650 Å. There is considerable secondary decomposition of initial products even in the early stages of the reaction. After 9–10% decomposition of D-glucose, the major decomposition can be accounted for by the arabinose and gluconic acid formed.

D-[1 -¹⁴C] Glucose and D-[1 -¹⁴C₁]glucono-γ-lactone were used to elucidate the initial site of chemical change. Of the products from D-[1 -¹⁴C]glucose, only gluconic acid contains ¹⁴C; the remaining products, including arabinose, are inactive. The D-glucose which disappears is equivalent to the sum of the gluconolactone and the volatile products produced. When D-[1 -¹⁴C] glucono-γ-lactone is irradiated, carbon dioxide is released exclusively from C-1. The arabinose and other aldehydic fragments containing fewer carbon atoms are inactive.

The site of radiation action is, therefore, C-1; this supports the view that the ‘acetal chromophore’ at C-1 is the primary absorbing site where photochemical reaction subsequently occurs. Filter experiments demonstrate that the spectral region near 2300 Å is the most active in D-glucose irradiations. It appears, therefore, that chemical change results from excitation and dissociation at the lactol ring oxygen.