The structure of salt bridges between Arg⁺ and Glu⁻ in peptides investigated with 2D-IR spectroscopy: Evidence for two distinct hydrogen-bond geometries

Salt bridges play an important role in protein folding and in supramolecular chemistry, but they are difficult to detect and characterize in solution. Here, we investigate salt bridges between glutamate (Glu⁻) and arginine (Arg⁺) using two-dimensional infrared (2D-IR) spectroscopy. The 2D-IR spectrum of a salt-bridged dimer shows cross peaks between the vibrational modes of Glu⁻ and Arg⁺, which provide a sensitive structural probe of Glu⁻⋯Arg⁺ salt bridges. We use this probe to investigate a β-turn locked by a salt bridge, an α-helical peptide whose structure is stabilized by salt bridges, and a coiled coil that is stabilized by intra- and intermolecular salt bridges. We detect a bidentate salt bridge in the β-turn, a monodentate one in the α-helical peptide, and both salt-bridge geometries in the coiled coil. To our knowledge, this is the first time 2D-IR has been used to probe tertiary side chain interactions in peptides, and our results show that 2D-IR spectroscopy is a powerful method for investigating salt bridges in solution.