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Selectivity in fragmentation of N-methylacetamide

The fragmentation pattern of the peptide model system, N-methylacetamide, after resonant K-shell excitation is investigated using ion time-of-flight spectroscopy. A tendency for site-specific fragmentation, in particular preferential peptide bond cleavage occurring upon resonant excitation of N1s electrons is observed.
P Salen et al.  PCCP 16 (2014) 15231

The fragmentation of peptides has been intensively investigated in the past using various activation methods. Many measurements using unspecific excitation typically display cleavage of the weak peptide bond, as a consequence of a statistical fragmentation where the internal energy is randomized before dissociation occurs. In contrast, for example, the use of intense femtosecond near-infrared laser beams with feedback-optimized pulse shapes has revealed preferential cleavage of specific peptide bonds.
Soft X-rays are, to some extent, suitable for site-specific bond breaking in molecules, due to the localized nature of K-shell excitation. By tuning the wavelength to the K-edge corresponding to a particular atomic element one may choose which atomic species to excite. Consequently, if some memory of the localized excitation is conserved after the Auger process and maintained through the dissociation, one could expect to selectively break the bonds around the core excited site.
Here an investigation into the fragmentation pattern of N-methylacetamide (CH3NHCOCH3) and a deuterated version of it using resonant K-shell excitation is presented. High resolution NEXAFS spectra were recorded in the vicinity of the C, N and O edges; subsequently the fragmentation patterns associated with these resonances was studied using the ion time-of-flight (TOF) coincidence spectroscopy technique using a 3D ion-momentum imaging spectrometer.
A comparison of fragmentation patterns upon excitation of several NEXAFS resonances reveals pronounced differencesand indications of site-selective bond breaking. In particular the excitation of the strongest N1s resonance significantly increases the probability of breaking the bonds connected with the N-atom site, the N–Caand the peptide bond. Further evidence for site selective bond breaking was found in an increased probability of removing the OH group at the O1s resonance. Selectivity in fragmentation is found for simple bond breaking with <400 fs lifetime, but, surprisingly also in metastable ion decays on a >50 ns time scale.

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Selectivity in fragmentation of N-methylacetamide after resonant K-shell excitation
Salén P, Kamińska M, Squibb R J, Richter R, Alagia M, Stranges S, van der Meulen P, Eland J H D, Feifel R, Zhaunerchyk V 
PCCP 16-29 (2014) 15231
Last Updated on Monday, 13 February 2017 21:47