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 center dot center dot center dot O and N-H center dot center dot center dot O bonds involved in the folded conformers of O2 '-protonated D-(+)-biotin in the gas phase which, therefore, depends exclusively on the corresponding geometric parameters.