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One of the primary goals of spectroscopic studies of small, gas-phase peptides is to understand the driving forces for the conformational preferences in proteins, particularly the formation of secondary structural features such as α or turns, or ß sheets, often using the aromatic amino acids as conformational probes. Beginning with the first electronic spectrum of jet-cooled tryptophan, 1 which contains features from six different stable conformers, the field has blossomed as infrared-ultraviolet (IR-UV) double resonance techniques combined with quantum chemical calculations often lead to unambiguous assignment of the secondary structure. Most of these studies are carried out in supersonic expansions using either sample heating or laser desorption to bring neutral peptides into the gas phase together with detection schemes based on fluorescence or ionization. 2–6 Most peptides are closed-shell ions in solution …