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

Covalently bound diazo groups are frequently found in biomolecular substrates. The Cdouble bond, length as m-dashNdouble bond, length as m-dashN asymmetric stretching vibration (νas) of the diazo group has a large extinction coefficient and appears in an uncongested spectral region. To evaluate the solvatochromism of the Cdouble bond, length as m-dashNdouble bond, length as m-dashN νas 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 Cdouble bond, length as m-dashNdouble bond, length as m-dashN νas 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 Cdouble bond, length as m-dashNdouble bond, length as m-dashN νas band could be used to probe these properties for a solvent. We found that the position of the Cdouble bond, length as m-dashNdouble bond, length as m-dashN νas 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 Cdouble bond, length as m-dashNdouble bond, length as m-dashN νas 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 Cdouble bond, length as m-dashNdouble bond, length as m-dashN νas band were similar to those of the Ndouble bond, length as m-dashNdouble bond, length as m-dashN νas band. We concluded that the position and width of the Cdouble bond, length as m-dashNdouble bond, length as m-dashN νas 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.