Quantifying Biomolecular Recognition with Site-Specific 2D Infrared Probes.

Azidohomoalanine (Aha) is an unnatural amino acid containing an infrared active azido side chain group that can, through frequency shifts of the azido stretch vibration, act as a probe of local structure. To realize the potential of such structural probes for protein science, we have developed a two-dimensional infrared spectrometer employing fast mechanical scanning and intrinsic phasing of the resulting spectra, leading to a lower sensitivity limit of ∼100 μOD level samples. Using this approach, we quantify the biomolecular recognition between a PDZ2 domain and two Aha-mutated peptides. It is shown that this method can distinguish different binding modes and that the energetics of binding can be determined.