Probing the formation of isolated cyclo-FF peptide clusters by far-infrared action spectroscopy

Small cyclic peptides containing phenylalanine residues are prone to aggregate in the gas phase into highly hydrophobic chains. A combination of laser desorption, mass spectrometry and conformational selective IR-UV action spectroscopy allows us to obtain detailed structural insights into the formation processes of the cyclic L-phenylalanyl-L-phenylalanine dipeptide (named cyclo-FF) aggregates. The rigid properties of cyclo-FF result in highly resolved IR spectra for the smaller clusters (n ≤ 3) and corresponding conformational assignments. For the higher order clusters (n > 3) the spectra are less resolved, however the observed ratios, peak positions and trends in IR shifts are key to make predictions on their structural details. Whereas the mid-IR spectral region between 1000-1800 cm^-1 turns out to be undiagnostic for these small aggregates and the 3 μm region only for specific calculated structures, the far-IR contains valuable information that allows for clear assignments.