title: Covalently bound azido groups are very specific water sensors, even in hydrogen-bonding environments.
abstract: Covalently bound azido groups are found in many commercially available biomolecular precursors and substrates, and the NNN asymmetric stretching band of these groups is a strong infrared absorber that appears in a spectral region clear of other signals. In order to evaluate comprehensively the solvatochromism of the asymmetric azido NNN stretching band for site-specific use in biomolecular contexts, infrared spectra of the model compounds 5-azido,1-pentanoic acid and 3-(p-azidophenyl),1-propanoic acid were acquired in a large variety of nonpolar, polar, and hydrogen-bond-donating solvents, as well as mixed aqueous-organic solvents. Spectra in pure solvents indicated that the aliphatic NNN stretching frequency maximum does not depend on solvent polarity, while the aromatic NNN frequency displays a weak but nonzero sensitivity to polarity. In both cases, the NNN frequency exhibits a blue-shift in H-bond-donating solvents, but the frequency in water is higher than in any other H-bond-donating solvent including solvents that are stronger H-bond donors. In nonfluorinated H-bond donor solvents, the frequency blue shift scales with the density of H-bond donors. This sensitivity to the presence of water was further explored in several mixed solvent environments, with the conclusion that this vibrational mode is a highly specific sensor of hydration, even in environments containing other H-bond donors like amides and alcohols, due to the very high local density of H-bond donors in water. The relatively uncomplicated (compared to nitriles, for example), water-specific response of this vibrational mode should lead to its adoption as a site-specific probe of hydration in many different possible systems in which the presence and role of molecular water is of primary interest.
authors.affiliation: Department of Chemistry, Haverford College, 370 Lancaster Avenue, Haverford, Pennsylvania 19041-1392, USA.
authors.forename: Marta P
authors.initials: MP
authors.lastname: Wolfshorndl
authors.forename: Rachel
authors.initials: R
authors.lastname: Baskin
authors.forename: Ishita
authors.initials: I
authors.lastname: Dhawan
authors.forename: Casey H
authors.initials: CH
authors.lastname: Londergan
mesh_terms: D001386:Azides / Q000737:chemistry*; D006860:Hydrogen Bonding; D015394:Molecular Structure; D012997:Solvents / Q000737:chemistry; D013055:Spectrophotometry, Infrared / Q000379:methods; D014867:Water / Q000737:chemistry*
chemical_list: D001386:Azides; D012997:Solvents; D014867:Water