Infrared analogs of heteronuclear nuclear magnetic resonance coherence transfer experiments in peptides

Using short pulses whose bandwidth covers only one amide band and that are resonant with either the amide I or the amide II band, we propose two-color multiple pulse infrared experiments that are analogs of heteronuclear nuclear magnetic resonance (NMR) experiments for probing the structure of peptides and proteins. These pulse sequences yield simpler spectra of nonoverlapping vibrational bands with fewer overlapping peaks than one-color techniques, and open new ways to more sophisticated control over coherence transfer pathways. Differences between NMR and IR techniques stemming from the different initial conditions and the pulse field strengths are taken into account. We find that in several cases, π pulses commonly used in NMR to simplify the spectra can be avoided in vibrational spectroscopy due to the low temperature initial condition (i.e., vibrational energies are large compared to kT).