The Water-soluble Riboflavin-boron Complex

In the course of attempts to find water-soluble derivatives of riboflavin, a study was made of the complexes formed between riboflavin and compounds of boron. The discovery was made that when proper pH control is exercised stable solutions of riboflavin-boron complex can be prepared of at least 25 times the natural solubility of riboflavin; i.e., a concentration of about 0.3 per cent as against 0.012 per cent. The nature of the riboflavin-boron complex has been characterized in part; however, all phases of the solvent effect are not clear. A brief consideration of the knowledge of organoboron complexes will serve as an introduction to the problem. Boric acid has long been known to form complexes and esters with polyhydroxy compounds (1). Both aliphatic and phenolic compounds have received considerable attention and recently Scudi, Bastedo, and Webb (2) described a complex in which both phenolic and aliphatic groups of pyridoxine are bound by boric acid. Brig1 and Griiner (3) and Vargha (4) have described the preparation under anhydrous conditions of glucoseboric acid esters. Such esters, however, are unstable in water. Brig1 and Griiner state that the mechanism of the boric acid effect with polyhydroxy compounds is not entirely clear but probably occurs in two phases. They picture the first phase of the reaction as that shown in (I).