Five structures of the Pro-Gly dipeptide unveiled by laser ablation rotational spectroscopy

Herein, for the first time, solid samples of Pro-Gly have been vaporized by laser ablation (LA), and a chirped pulse Fourier transform microwave spectrometer (CP-FTMW) has been employed to explore the broadband rotational spectrum in the 3.0-8.0 GHz range. By integrating experimental data with quantum-chemical computations, we accurately characterized the conformational landscape of this flexible dipeptide, identifying up to five distinct conformers. The N-H⋯N-H hydrogen bond between the amine group of the glycine residue and the amine in the proline ring is highly stabilizing and is present in all conformers. Furthermore, the four most stable conformers exhibit additional stabilizing O-H⋯OC hydrogen bonds between the hydroxyl group and the carbonyl group of proline. We analyzed the key differences between Pro-Gly and Gly-Pro, providing insights into Pro-Gly dipeptide's greater tendency to form β-turn configurations in proteins, in contrast to the Gly-Pro dipeptide's preference for extended conformations. We have illustrated how collisional relaxation distorts the equilibrium conformational distribution, giving rise to missing conformers in the conformational landscape.