Suppression of the selective cleavage at N‐terminal of proline is observed in the peptide cleavage by proteolytic enzyme trypsin and in the fragment ion mass spectra of peptides containing Arg‐Pro sequence. An insight into the fragmentation mechanism of the influence of arginine residue on the proline effect can help in prediction of mass spectra and in protein structure analysis. In this work, collision‐induced dissociation spectra of singly and doubly charged peptide AARPAA were studied by ESI MS/MS and theoretical calculation methods. The proline effect was evaluated by comparing the experimental ratio of fragments originated from cleavage of different amide bonds. The results revealed that the backbone amide bond cleavage was selected by the energy barrier height of the fragmentation pathway although the strong proton affinity of the Arg side chain affected the stereostructure of the peptide and the dissociation mechanism. The thermodynamic stability of the fragment ions played a secondary role in the abundance ratio of fragments generated via different pathways. Fragmentation studies of protonated peptide AACitPAA supported the energy‐dependent hypothesis. The results provide an explanation to the long‐term arguments between the steric conflict and the proton mobility mechanisms of proline effect.
The two-step photodissociation mechanism of 1,3-cyclohexane dinitrite is confirmed by observation of the laser-induced fluorescence spectrum of the intermediate 3-nitrosooxy cyclohexoxy radical.
Due to nearly diffusion-limited radical–radical coupling, synthetically useful, selective radical–radical cross-coupling reactions remain challenging. However, different radical lifetimes and various radical initiation approaches now provide the possibility for radical–radical cross-coupling. In this chapter, recent advances in radical–radical cross-coupling reactions are described. In the first part, a basic kinetic phenomenon called the persistent radical effect is briefly reviewed and explained. The remainder of the chapter presents a series of case studies, illustrating several types of radical–radical cross couplings in a variety of disparate settings.
Alkyl dinitrites have attracted attention as an important type of nitrosating agent and a pollution source in atmosphere. The reactivity and chemistry of alkyl dinitrites induced by the two ONO functional groups are relatively unknown. In this work, decompositions of 1,3- cyclohexane dinitrite and 1,4-cyclohexane dinitrite are studied by electron impact ionization mass spectroscopy (EI-MS). Apart from NO+ (m/z=30), fragment ions m/z=43 and 71 are the most abundant for the 1,3-isomer. On the other hand, fragments m/z=29, 57, 85, and 97 stand out in the EI-MS spectrum of 1,4-isomer. Possible dissociation mechanisms of the two dinitrites are proposed by theoretical calculations. The results reveal that the ring-opening of 1,3-cyclohexane dinitrite mainly starts from the intermediate ion (M-NO)+ by cleavage of two αC-βC bonds. For 1,4-cyclohexane dinitrite, in addition to the decomposition via intermediate (M-NO)+, cleavage of βC-βC bonds can occur directly from the parent cation M+. The results will help to understand the structural related chemistry of alkyl dinitrites in atmosphere and in NO transfer process.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.