Infrared probing of 4-Azidoproline conformations modulated by azido configurations

4-Azidoproline (Azp) can tune the stability of the polyproline II (P _II) conformation in collagen. The azido group in the 4R and 4S configurations stabilizes and destabilizes the P_II conformation, respectively. To obtain insights into the dependence of the conformational stability on the azido configuration, we carried out Fourier transform (FT) IR experiments with four 4-azidoproline derivatives, Ac-(4R/S)-Azp-(NH/O)Me. We found that the amide I and azido IR spectra are different depending on the azido configuration and C-terminal structure. The origin of such spectral differences between 4R and 4S configurations and between C-terminal methylamide and ester ends was elucidated by quantum chemistry calculations in combination with ¹H NMR and time- and frequency-resolved IR pump-probe spectroscopy. We found that the azido configurations and C-terminal structures affect intramolecular interactions, which are responsible for the ensuing conformational and thereby IR spectral differences. Consequently, 4-azidoproline conformations modulated by azido configurations can be probed by IR spectroscopy. These findings suggest that 4-azidoproline can be both a structure-control and -probing element, which enables the infrared tracking of proline roles in protein structure, function, and dynamics.