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Proton mobility in protonated peptides: a joint molecular orbital and RRKM study

✍ Scribed by István Pál Csonka; Béla Paizs; György Lendvay; Sándor Suhai

Publisher
John Wiley and Sons
Year
2000
Tongue
English
Weight
411 KB
Volume
14
Category
Article
ISSN
0951-4198

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✦ Synopsis

The mobile proton model was critically evaluated by using purely theoretical models which include quantum mechanical calculations to determine stationary points on the potential energy surface (PES) of a model compound, and Rice-Ramsperger-Kassel-Marcus (RRKM) calculations to determine the rate constants of various processes (conformational changes, proton transfer reactions) which occur during mass analysis of protonated peptides. Extensive mapping of the PES of protonated N-formylglycinamide resulted in various minima which were stabilized by one or more of the following types of interaction: internal hydrogen bond, charge transfer interaction, charge delocalization, and ring formation. The relative energies of most of the investigated minima are less then 20 kcal mol À1 compared with the most stable species. More importantly, the relative energies of the transition structures connecting these minima are fairly low, allowing facile transitions among the energetically low-lying species. It is demonstrated that a path can be found leading from the energetically most stable species, protonated on an amide oxygen, to the structure from which the energetically most favorable fragmentation occurs. It is also shown that the added proton can sample all protonation sites prior to fragmentation. The RRKM calculations applied the results of ab initio computations (structures, energetics, vibrational frequencies) to the reactions (internal rotations, proton transfers) occurring in protonated N-formylglycinamide, and clearly lend additional evidence to the mobile proton model. Based on the results of the PES search on protonated N-formylglycinamide, we also comment on the mechanism proposed by Arnot et al. (

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