Structure and Conformation of Protonated <scp>d</scp>-(+)-Biotin in the Unsolvated State

A combined computational and infrared multiphoton dissociation (IRMPD) spectroscopic investigation shows that protonated d-(+)-biotin, formed in the gas phase by ESI-MS, acquires a folded structure with proton bonding between the ureido and valeryl carbonyls, and that only a single conformer of such a structure predominates. A uniform frequency vs distance correlation function is proposed for the O(+)-H&#183;&#183;&#183;O and N-H&#183;&#183;&#183;O bonds involved in the folded conformers of O2&#39;-protonated d-(+)-biotin in the gas phase which, therefore, depends exclusively on the corresponding geometric parameters.