Dissociation of the peptide bond in protonated peptides.

The dissociation of the amide (peptide) bond in protonated peptides, [M + H](+), is discussed in terms of the structures and energetics of the resulting N-terminal b(n) and C-terminal y(n) sequence ions. The combined data provide strong evidence that dissociation proceeds with no reverse barriers through interconverting proton-bound complexes between the segments emerging upon cleavage of the protonated peptide bond. These complexes contain the C-terminal part as a smaller linear peptide (amino acid if one residue) and the N-terminal part either as an oxazolone or a cyclic peptide (cyclic amide if one residue). Owing to the higher thermodynamic stability but substantially lower gas-phase basicity of cyclic peptides vs isomeric oxazolones, the N-terminus is cleaved as a protonated oxazolone when ionic (b(n) series) but as a cyclic peptide when neutral (accompanying the C-terminal y(n) series). It is demonstrated that free energy correlations can be used to derive thermochemical data about sequence ions. In this context, the dependence of the logarithm of the abundance ratio log[y(1)/b(2)], from protonated GGX (G, glycine; X, varying amino acid) on the gas-phase basicity of X is used to obtain a first experimental estimate of the gas-phase basicity of the simplest b-type oxazolone, viz. 2-aminomethyl-5-oxazolone (b(2) ion with two glycyl residues).

[1]  D. Mueller,et al.  Hydrogen transfer reactions in the formation of “Y + 2” sequence ions from protonated peptides , 1988 .

[2]  C. Wesdemiotis,et al.  Proton affinities of the N- and C-terminal segments arising upon the dissociation of the amide bond in protonated peptides , 1999, Journal of the American Society for Mass Spectrometry.

[3]  A Whiting,et al.  Role of the site of protonation in the low-energy decompositions of gas-phase peptide ions , 1996, Journal of the American Society for Mass Spectrometry.

[4]  C. Wesdemiotis,et al.  Tandem mass spectrometry of peptides: mechanistic aspects and structural information based on neutral losses. II: Tri- and larger peptides , 1994 .

[5]  C. Wesdemiotis,et al.  Tandem mass spectrometry of peptides: sequence information based on neutral losses. I: Isomeric dipeptides , 1993 .

[6]  P. Kebarle,et al.  Collision-Induced Dissociation Threshold Energies of Protonated Glycine, Glycinamide, and Some Related Small Peptides and Peptide Amino Amides , 1997 .

[7]  I. Papayannopoulos,et al.  The interpretation of collision‐induced dissociation tandem mass spectra of peptides , 1996 .

[8]  C. G. Koster,et al.  The B1-fragment Ion from Protonated Glycine is an Electrostatically-bound Ion/Molecule Complex of CH2=NH+2 and CO , 1996 .

[9]  Sándor Suhai,et al.  Formation of b2+ ions from protonated peptides: an ab initio study , 1999 .

[10]  I. Csizmadia,et al.  The structure and fragmentation of Bn (n≥3) ions in peptide spectra , 1996, Journal of the American Society for Mass Spectrometry.

[11]  W. Lehmann,et al.  Five-membered ring formation in unimolecular reactions of peptides: a key structural element controlling low-energy collision-induced dissociation of peptides. , 2000, Journal of mass spectrometry : JMS.

[12]  C. Wesdemiotis,et al.  The neutral products formed during backbone fragmentations of protonated peptides in tandem mass spectrometry. , 1993, Analytical chemistry.

[13]  E. P. Hunter,et al.  Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An Update , 1998 .

[14]  K. Biemann.  Sequencing of peptides by tandem mass spectrometry and high-energy collision-induced dissociation. , 1990, Methods in enzymology.

[15]  D. G. Morgan,et al.  Linear energy correlation in the low‐energy tandem mass spectra of protonated tripeptides Xxx–Gly–Gly but failure for Gly–Xxx–Gly , 1995 .

[16]  Linear free energy correlations in mass spectrometry , 1999 .

[17]  J. Stults,et al.  Peptide characterization by mass spectrometry. , 1996, Methods in enzymology.

[18]  A. G. Harrison.  Linear free energy correlations in mass spectrometry†Presented, in part, at the 46th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, FL, June 1998. , 1999 .

[19]  C. Wesdemiotis,et al.  Characterization of neutral fragments in tandem mass spectrometry: a unique route to mechanistic and structural information. , 1996, Journal of mass spectrometry : JMS.

[20]  Stephen A. Martin,et al.  Mass spectrometric determination of the amino acid sequence of peptides and proteins , 1987 .

[21]  R. Cooks,et al.  Thermochemical determinations by the kinetic method , 1994 .

[22]  V. Wysocki,et al.  The effect of protonation site on bond strengths in simple peptides: Application of Ab initio and modified neglect of differential overlap bond orders and modified neglect of differential overlap energy partitioning , 1994, Journal of the American Society for Mass Spectrometry.

[23]  C. Wesdemiotis,et al.  Amide bond dissociation in protonated peptides. Structures of the N-terminal ionic and neutral fragments , 1997 .

[24]  V. Wysocki,et al.  Mobile and localized protons: a framework for understanding peptide dissociation. , 2000, Journal of mass spectrometry : JMS.

[25]  J. Holmes.  Assigning structures to ions in the gas phase , 1985 .

[26]  D. G. Morgan,et al.  A linear free-energy correlation in the low-energy tandem mass spectra of protonated tripeptides Gly–Gly–Xxx , 1994 .

[27]  I. Csizmadia,et al.  Why Are B ions stable species in peptide spectra? , 1995, Journal of the American Society for Mass Spectrometry.

[28]  R A O'Hair,et al.  The role of nucleophile--electrophile interactions in the unimolecular and bimolecular gas-phase ion chemistry of peptides and related systems. , 2000, Journal of mass spectrometry : JMS.

[29]  A. G. Harrison,et al.  THE B1 ION DERIVED FROM METHIONINE IS A STABLE SPECIES , 1998 .