N-nitrosation of N-acetyltryptophan probed by IR spectroscopy of the gaseous anion

A stable N1-nitroso derivative of N-acetyltryptophan, [NANT-H]−, has been assayed by infrared multiple photon dissociation (IRMPD) in the ‘fingerprint’ range. IRMPD spectra, interpreted by DFT calculations, display features characteristic of the nitrosation motif, which lack in the native N-acetyltryptophan anion, [NAT-H]−. The most stable [NANT-H]− isomers, nicely accounting for the experimental features, present the carboxylic group interacting with the amide and benzene ring hydrogens. The side chain is oriented gauche with respect to the indole plane, while the NNO group may adopt either a syn (1ds, global minimum) or an anti (2da, 2.7 kJ mol−1 higher in energy) configuration.