Chasing Weakly Bound Biological Water Near Peptide Backbone in Aqueous Environment by Ultrafast 2D IR Infrared Spectroscopy

Abstract

Abstract There has been a long-standing debate as to how many hydrogen bonds a peptide backbone amide can form in aqueous solutions. In this work, the hydrogen-bonding structural dynamics of N-ethylpropionamide (NEPA, a b-model peptide) in water was examined using linear and nonlinear infrared (IR) spectroscopy. The results showed two sub bands in the amide-I mode in heavy water (D2O), which were found to arise from a weakly hydrogen-bonded (WHB) dynamical water molecule in the vicinity of the amide C=O group on the basis of a commonly known nearby water molecule that is strongly hydrogen bonded (SHB). This picture is supported by quantum calculations, molecular dynamics simulations and NMR spectroscopy. Further, the thermodynamics and kinetics of the WHB species, whose amide-I frequency is 13 cm-1 higher than the SHB state (with two strongly H-bonded water molecules on the amide C=O side), are examined by waiting-time and temperature dependent chemical-exchange 2D IR spectroscopy. While the activation energy for the change from the SHB state to the SHB state is about 13.25 kJ/mol, the breaking or weakening the WHB with the amide occurs with a time constant of half picosecond at room temperature. Our results provided experimental evidence of a mobile water molecule nearby the peptide backbone, allowing us to gain more insights into the dynamics of the backbone hydration of both a- and b-peptides.