Linear and two-dimensional infrared spectroscopy of the multifunctional vibrational probe, 3-(4-azidophenyl)propiolonitrile. Deperturbing a Fermi triad by isotopic substitution.

Abstract

Infrared probes are chemical moieties whose vibrational modes are used to obtain spectroscopic information about structural dynamics of complex systems; in particular, of biomacromolecules. Here, we explore the vibrational spectroscopy and dynamics of a reagent, 3-(4-azidophenyl)propiolonitrile (AzPPN), for selectively tagging thiols in protein environments with a multifunctional infrared probe containing both, an azide and a nitrile chromophore. The linear infrared spectrum of AzPPN is heavily perturbed in the antisymmetric azide stretching region as a result of accidental Fermi resonances. Isotopically labeling the azide group at the β-position deperturbs the spectrum considerably and reveals two combination tones that mix with the antisymmetric stretching fundamental into a Fermi triad of hybrid vibrational excitations. Moreover, two-dimensional infrared (2DIR) spectra were recorded for 15Nβ-labeled AzPPN, which reveal waiting-time-dependent spectral shifts of diagonal peaks and dynamic buildups of cross peaks. The 2DIR-spectrotemporalevolution is indicative of irreversibleintramolecular redistribution within 2-3 psof the pump-induced excess vibrational energy into low-frequency modes of the molecule that are coupled to either the azide or the nitrile stretching transition dipoles. Finally, energy dissipation into the solvent occurs with a time constant of18 ps.