The covalently bound diazo group as an infrared probe for hydrogen bonding environments

Abstract

Covalently bound diazo groups are frequently found in biomolecular substrates. The CNN asymmetric stretching vibration (ν_as) of the diazo group has a large extinction coefficient and appears in an uncongested spectral region. To evaluate the solvatochromism of the CNN ν_as band for studying biomolecules, we recorded the infrared (IR) spectra of a diazo model compound, 2-diazo-3-oxo-butyric acid ethyl ester, in different solvents. The width of the CNN ν_as band was linearly dependent on the Kamlet–Taft solvent parameter, which reflects the polarizability and hydrogen bond accepting ability of the solvent. Therefore, the width of the CNN ν_as band could be used to probe these properties for a solvent. We found that the position of the CNN ν_as band was linearly correlated with the density of hydrogen bond donor groups in the solvent. We studied the relaxation dynamics and spectral diffusion of the CNN ν_as band of a natural amino acid, 6-diazo-5-oxo-L-norleucine, in water using nonlinear IR spectroscopy. The relaxation and spectral diffusion time constants of the CNN ν_as band were similar to those of the NNN ν_as band. We concluded that the position and width of the CNN ν_as band of the diazo group could be used to probe the hydrogen bond donating and accepting ability of a solvent, respectively. These results suggest that the diazo group could be used as a site-specific IR probe for the local hydration environments.