Association of biotin with silver (I) in solution: a circular dichroism study

A strong polymeric helical complex of Ag(I) with biotin, a water-soluble vitamin also called vitamin H or B₇, has been described in the crystal phase, yet with missing spatial characteristic of its solution structure. In this work, the complexation of d-(+)-biotin with Ag⁺ in aqueous solution has been studied by means of circular dichroism spectroscopy and mass spectrometry with electrospray ionization in positive mode (MS-ESI+). For comparison, the spectra of the Nujol suspension of studied systems in the solid state were also measured. The results indicate that the Ag(I)–biotin complex has a helical arrangement of the biotin molecules in solution. Our results show that the binding process involves the thioether sulfur atom and ureido carbonyl oxygen atom, while the carboxylate side group is not coordinated with the silver. The MS-ESI experiments have revealed the composite structures of Ag⁺ with biotin of various constitutions, which represent the products of the polymeric helical Ag(I)–biotin complex.