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.
Bibliographical noteFunding Information:
We gratefully acknowledge the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) for the support of the FELIX Laboratory. We thank the FELIX team for their experimental support and helpful discussions. This work was sponsored by NWO Exact and Natural Sciences for the use of supercomputer facilities (Grant no. 2019.062) at the SurfSARA Cartesius cluster. Furthermore, Alexander Lemmens is particularly acknowledged for his help on the OPO measurements, and helpful discussions in general.
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