Stearns, Jaime A.Guidi, MoniaBoyarkin, Oleg V.Rizzo, Thomas R.2007-08-242007-08-242007-08-24200710.1063/1.2798111https://infoscience.epfl.ch/handle/20.500.14299/10424WOS:000250295900040We present the spectroscopy and photofragmentation dynamics of two isomeric protonated dipeptides, H+AlaTyr and H+TyrAla, in a cold ion trap. By a combination of infrared-ultraviolet double resonance experiments and density functional theory calculations, we establish the conformations present at low temperature. Interaction of the charge at the N-terminus with the carbonyl group and the tyrosine π-cloud seems to be critical in stabilizing the low-energy conformations. H+AlaTyr has the flexibility to allow a stronger interaction between the charge and the aromatic ring than in H+TyrAla, and this interaction may be responsible for many of the differences we observe in the former: a significant red-shift in the ultraviolet spectrum, a much larger photofragmentation yield, fewer stable conformations, and the absence of fragmentation in excited electronic states.spectroscopyMass spectrometrybiophysicstext::journal::journal article::research article