Protein bonds between amino acids
are one of the most important
biological linkages that create life. The detection of amino acids
in the interstellar environments and in meteorites may lead to the
suggestion that amino acids came from outer space and that peptides
bonds may have been created in the gas phase. Here we show experimentally
the creation of covalent bonds, most likely peptide bonds, between
serine dipeptides in the gas phase. More specifically, we show that
spraying a solution of Ser-Ser dipeptides results, in addition to
dipeptide clusters, in a peak with the same mass as the serine tetrapeptide,
which also has the same fragmentation pattern. Moreover, we show that
this mass is formed upon collision induced dissociation of clusters
containing four serine dipeptides. Thence, if the dipeptide can be
generated abiotically the polymerization process may occur spontaneously.
Previous studies have shown that the gas-phase fragmentation of the retinal chromophore after S0-S1 photoexcitation results in a prominent fragment of mass 248 which cannot be explained by the cleavage of any single bond along the polyene chain. It was therefore theorized that the fragmentation mechanism involves a series of isomerizations and cyclization processes, and two mechanisms for these processes were suggested. Here we used isotope labeling MS-MS to provide conclusive support for the fragmentation mechanism suggested by Coughlan et al. (J. Phys. Chem. Lett. 2014, 5, 3195).
The barrier energies for isomerization and fragmentation were measured for a series of retinal chromophore derivatives using a tandem ion mobility spectrometry approach. These measurements allow us to quantify the effect of charge delocalization on the rigidity of chromophores. We find that the role of the methyl group on the C13 position is pivotal regarding the ground state dynamics of the chromophore. Additionally, a correlation between quasi-equilibrium isomer distribution and fragmentation pathways is observed.
Betaine (Bet) is a pure zwitterion with an extraordinarily large dipole moment, which allows it to form stable clusters in the gas phase of the form X±BetN, where X± is a positive or negative ion. We show here that such clusters have a prominent magic number at N = 4 for all X± ions used in this work. Nevertheless, we observe a marked difference in the fragmentation pattern of anionic and cationic clusters: while cationic clusters fragment by evaporating one betaine monomer at a time, fragmentation of anionic clusters is through fission resulting in the emission of one or several betaine molecules. Theoretical calculations show that charged betaine tetramers have a square like structure with the central ion lying above the cluster plane and explain the difference in fragmentation patterns as a result of the charge distribution within the betaine molecule.
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