Peptides proton transfer reactions

Modified Marcus Parameters for Proton-Transfer Reactions with Deprotonated Metal Peptide Complexes... 

More recent work revealed the importance of gas phase proton transfer reactions. [91-94] This implies that multiply charged peptide ions do not exist as preformed ions in solution, but are generated by gas phase ion-ion reactions (Chap. 11.4.4). The proton exchange is driven by the difference in proton affinities (PA, Chap. 2.11) of the species encountered, e.g., a protonated solvent molecule of low PA will protonate a peptide ion with some basic sites left. Under equilibrium conditions, the process would continue until the peptide ion is saturated with protons, a state that also marks its maximum number of charges. 

Aminyl radicals have also been detected indirectly during the reaction of hydroxyl radicals (HO ) or their conjugated base ( 0 ) with the free amino group of amino acids (Reactions (3.9) and (3.10)) [40-43], and identified by time-resolved EPR experiments [44]. Similar reactions may be expected for peptides. While Reactions (3.9) and (3.10) show a net hydrogen transfer, they likely proceed via a stepwise electron-transfer and proton-transfer (Reaction (3.11)), a reaction commonly referred to as proton-coupled electron transfer (PCET). Proton transfer from the ami-nium radical cation to the base (OH ) will likely occur within the solvent cage. 

Ion trap instruments also provide the possibility to study ion-ion reactions, e g., reactions of multiple-charge peptide or protein ions with ions of opposite polarity. Mostly, proton-transfer reaction are performed, but electron transfer, fluoride transfer reactions, and even attachment reactions may occur as well [81]. Ion traps, either three-dimensional or linear ones, with multiple ion sources are appUed in such experiments [82]. 

Combined with electrospray ionization or MALDI, FT-ICR-MS is a very attractive tool for gas-phase studies of biomolecules such as peptides and proteins, oligonucleotides, and oligosaccharides. The possibihty to trap ions for prolonged periods of time, even up to thousands of seconds, can be applied in the study of gas-phase ion-molecule reactions. Application of proton-transfer reactions in ICR cells in the study of biomolecules has been reviewed [97]. Detailed structural as well as conformational studies on biomolecules rely on H/D-exchange experiments, for which FT-ICR is an excellent tool, e.g., [98, 99]. 

Finally, the proton transfer reactions of multiply-charged anions recently have been reported in mass spectrometric studies. These systems are of great interest, particularly in the analysis of complex substrates such as peptides, and computational chemistry can play an important role in the interpretation of experimental results. Special considerations... 

Ogorzalek Loo, R. R. Smith, R. D. Proton transfer reactions of multiply charged peptide and protein cations and anions. J. Mass Spectrom. 1995, 30, 339-347. 

The most important multiply charged polyatomic positive ions are compounds with two or more basic groups which when protonated lead to doubly or poly-charged ions. Typical examples are diamines such as the double protonated a, to alkyldiamines, H3N(CH2)pNH2+, and the most important class, the polyprotonated peptides and proteins, which have multiple basic residues. Charge reduction for these systems occurs through proton transfer from one of the protonated basic sites to a solvent molecule. Such a reaction is shown below for the monohydrate of a doubly protonated diamine ... 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

Murakami et al. studied alternative pyridoxamine-surfactant systems [23]. These authors synthesized hydrophobic pyridoxamine derivatives (30 and 31) and peptide lipid molecules (32-35). Catalyst 30 or 31 and the peptide lipids formed bilayer membranes in water, which showed transamination reactivity in the presence of metal ions such as Cu(ii). It was proposed that the pyridoxamine moiety was placed in the so-called hydrogen-belt domain interposed between the polar surface region and the hydrophobic domain that is composed of double-chain segments within the bilayer assembly. The basic group (such as imidazole) in the peptide lipid molecules could catalyze the proton transfer involved in the transamination reaction. In addition, marked substrate discrimination by these bilayer membrane systems was performed through hydrophobic interactions between substrates and the catalytic site. 

Substitution Kinetics of Copper(II)—Peptide Complexes. Three main reaction pathways have been found for the displacement of copper from peptide complexes—(1) proton transfer to the peptide group, (2) nucleo-... 

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