“…Rapidly, several conformers of the three natural amino-acids Phe [16], Trp [20] and Tyr [56], and sometimes their hydrates [57,58], were identified through their vibrational fingerprint, refining former pioneering studies based on UV spectroscopy only [48,[59][60][61][62]. Numerous studies on peptides containing one of the three natural aromatic residues followed [46,47,[51][52][53][54][55], the presence of a near-UV chromophore being the tribute to pay to gain the conformer-selectivity brought by the IR/UV double resonance technique. Non-natural residues derived from the natural UV chromophores have also been widely employed [52,111,[119][120][121][122][123][124][125][126][127][128][129].…”