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PubMed1998

Oxygen and proton pathways in cytochrome c oxidase.

Cytochrome c oxidase is a redox-driven proton pump, which couples the reduction of oxygen to water to the translocation of protons across the membrane. The recently solved x-ray structures of cytochrome c oxidase permit molecular dynamics simulations of the underlying transport processes. To eventually establish the proton pump mechanism, we investigate the transport of the substrates, oxygen and protons, through the enzyme. Molecular dynamics simulations of oxygen diffusion through the protein reveal a well-defined pathway to the oxygen-binding site starting at a hydrophobic cavity near the membrane-exposed surface of subunit I, close to the interface to subunit III. A large number of water sites are predicted within the protein, which could play an essential role for the transfer of protons in cytochrome c oxidase. The water molecules form two channels along which protons can enter from the cytoplasmic (matrix) side of the protein and reach the binuclear center. A possible pumping mechanism is proposed that involves a shuttling motion of a glutamic acid side chain, which could then transfer a proton to a propionate group of heme alpha 3.

Animals↗
PubMed1999

Apical proton secretion by the inner stripe of the outer medullary collecting duct.

The inner stripe of outer medullary collecting duct (OMCDis) is unique among collecting duct segments because both intercalated cells and principal cells secrete protons and reabsorb luminal bicarbonate. The current study characterized the mechanisms of OMCDis proton secretion. We used in vitro microperfusion, and we separately studied the principal cell and intercalated cell using differential uptake of the fluorescent, pH-sensitive dye, 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Both the principal cell and intercalated cell secreted protons, as identified as Na+/H+ exchange-independent intracellular pH (pHi) recovery from an intracellular acid load. Two proton transport activities were identified in the principal cell; one was luminal potassium dependent and Sch-28080 sensitive and the other was luminal potassium independent and luminal bafilomycin A1 sensitive. Thus the OMCDis principal cell expresses both apical H+-K+-ATPase and H+-ATPase activity. Intercalated cell Na+/H+ exchange-independent pHi recovery was approximately twice that of the principal cell and was mediated by pharmacologically similar mechanisms. We conclude 1) the OMCDis principal cell may contribute to both luminal potassium reabsorption and urinary acidification, roles fundamentally different from those of the principal cell in the cortical collecting duct; and 2) the OMCDis intercalated cell proton transporters are functionally similar to those in the principal cell, raising the possibility that an H+-K+-ATPase similar to the one present in the principal cell may contribute to intercalated cell proton secretion.

Animals↗
PubMed1996

Proton transfer reactivity of large multiply charged ions.

Charge-charge interactions dramatically influence the dissociation and proton transfer reactivity of large multiply protonated ions. In combination with tandem mass spectrometry, proton transfer reactions have been used to determine the charge state of an ion and to increase the effective mass resolution of electrospray ionization mass spectra. A model for the proton transfer reactivity of multiply protonated ions, in which protons are assigned to specific sites in an ion based on the intrinsic reactivity of the site and the sum of point-charge Coulomb interactions between charges, is discussed. In combination with experimentally measured rates of proton transfer to bases of known gas-phase basicity, information about the intramolecular electrostatic interactions, gas-phase ion conformation and maximum charge state of an ion produced by electrospray ionization can be obtained.

Animals↗
PubMed2000

Do amines react with protonated peptides in the gas phase via transacylation reactions to induce peptide bond cleavage?

The proposal that protonated peptides react with NH(3) in the gas phase via transacylation reactions (Tabet et al., Spectros. Int. J. 5: 253 1987) has been investigated by studying the reactions of the fixed charge derivatives [RC(O)NMe(2)CH(2)CO(2)H](+) (R=Me and Ph) with pyridine and triethylamine and the reactions of protonated glycine oligomers and leucine enkenphalin with butylamine. Under the near thermal conditions of the quadrupole ion trap, both the fixed charge derivatives as well as the protonated peptides react with the amines via either proton transfer or proton bound dimer formation. Collision induced dissociation of protonated peptides in the presence of butylamine yields b(n) and y(n) sequence ions as well as [b(n) + BuNH(2)](+) and [y(n) + BuNH(2)](+) ions. MS(3) experiments reveal that a major route to these [b(n) + BuNH(2)](+) and [y(n) + BuNH(2)](+) ions involves ion-molecule reactions between the b(n) and y(n) sequence ions and butylamine. MS(4) experiments, carried out to determine the nature of the [b(n) + BuNH(2)](+) ions, reveal that they correspond to a mixture of hydrogen bonded (i.e. proton bound dimer) and covalent amide bond structures.

Acylation↗
PubMed2006

Proton transfer pathways in the mutant His-64-Ala of human carbonic anhydrase II.

We have investigated the possible proton transfer pathways from the surface of the protein to the zinc-bound water molecule in the mutant His-64-Ala of human carbonic anhydrase II. Starting with an input of known crystallographic structures of the mutant, we model the proton pathways as hydrogen-bonded networks of proton conducting groups and bound solvent molecules. No proton path is detected in the mutant, in close agreement with the experimental observation of a 20-fold decrease in its catalytic efficiency compared to the wild-type enzyme. We also investigate in detail changes in hydration structure at the active site of the mutant and the resulting proton paths in the presence of an exogenous proton donor 4-methylimidazole (4-MI). The proton transfer pathways thus detected are correlated to the observed chemical rescue of catalytic activity by 4-MI.

Amino Acid Substitution↗
PubMed1991

Premelting thermal fluctuational interbase hydrogen-bond disrupted states of a B-DNA guanine-cytosine base pair: significance for amino and imino proton exchange.

Modified self-consistent phonon theory when applied to the DNA double helix indicates the existence of fairly long-lived states in which single interbase H bonds are disrupted. One can then postulate a number of situations in which particular disrupted H bonds can enhance particular proton exchange. In this paper we postulate a number of such partially open states for a B-conformation GC base pair and calculate the probability of each of these states for a B-conformation poly(dG).poly(dC). We compare these probabilities to those probabilities needed to explain various observed proton exchange rates. We propose that, for a GC base pair in B conformation, there are two amino proton exchangeable states--a cytosine amino proton exchangeable state and a guanine amino proton exchangeable state; both require the disruption of only the corresponding interbase H bond. The imino proton exchange, however, requires the disruption of all three interbase H bonds and this defines a third open state. Our calculated probabilities for a GC base pair in these three states are in fair agreement with available experimental estimates from measurements of amino and imino proton exchange.

Base Composition↗
PubMed2003

Could redox-switched binding of a redox-active ligand to a copper(II) centre drive a conformational proton pump gate? A synthetic model study.

A proposal for a redox-linked conformational gate to proton translocation--a proton pump gate--based upon a transition-metal redox-switchable hemilabile ligand (RHL) system is made. Consideration of the requirements for such a system reveals copper(II) to be the ideal metal centre. To test the proposal and, thereby, to provide an artificial proton pump gate, the copper coordination chemistry of three tris(pyridylmethyl)amine (tpa) ligands with one "leg" (PY*) substituted at the 6-position of the pyridine ring by a dimethoxyphenyl (L(1)), a hydroquinone (H(2)L(2)) or a quinone (L(3)) substituent has been investigated. Crystal structures of sp-[Cu(kappa(4)N-L(1))Cl]Cl.3 H(2)O (sp=square pyramidal), sp-[Cu(kappa(3)N-H(2)L(2))Cl(2)] and tbp-[Cu(kappa(4)N,kappaO-HL(2))][PF(6)] (tbp=trigonal bipyramidal) have been determined. The Cu(I) complexes [Cu(L)(MeCN)(n)](+) (L=L(1), H(2)L(2)) display physicochemical properties consistent with a "dangling" PY* leg; from the NMR spectra, the barriers to inversion of the ligand amine donor for the Cu(I) complexes are estimated to be within the range of about 30-45 kJ mol(-1). In the Cu(II) complexes, coordination of the PY* leg is finely balanced and critically depends on the nature of the PY* substituent and the availability of potential co-ligand(s). For example, tbp-[Cu(kappa(4)N-L(1))Cl](+) reacts cleanly with Cl(-) ions to afford sp-[Cu(kappa(3)N-L(1))Cl(2)]; Vis/NIR spectrophotometric titrations suggest the affinity of tbp-[Cu(kappa(4)N-L(1))Cl](+) for Cl(-) ion in dichloromethane is 9.7 x 10(2) and is at least 10(4)-fold greater than that of tbp-[Cu(kappa(4)N-L(3))Cl](+). The complex sp-[Cu(kappa(3)N-H(2)L(2))Cl(2)] has a "dangling" PY* leg, in which an intramolecular OH(hydroquinone).N(pyridine) hydrogen bond "ties-up" the pyridyl nitrogen atom, and reacts with Brønsted bases to give tbp-[Cu(kappa(4)N,kappaO-HL(2))](+). Two-electron oxidation of sp-[Cu(kappa(3)N-H(2)L(2))Cl(2)] is linked to loss of two protons and a conformational change, and affords tbp-[Cu(kappa(4)N-L(3))Cl](+). The [Cu(kappa(3)N-H(2)L(2))Cl(2)]-[Cu(kappa(4)N-L(3))Cl](+) system provides a first demonstration of the critical step in the proposed proton pumping cycle, namely a redox-driven and proton-linked conformational change. The possible biological relevance of this work to proton pumping in cytochrome c oxidase is mentioned.

Copper↗
PubMed2005

Experimental and computational studies of hydrogen bonding and proton transfer to [Cp*Fe(dppe)H].

The present contribution reports experimental and computational investigations of the interaction between [Cp*Fe(dppe)H] and different proton donors (HA). The focus is on the structure of the proton transfer intermediates and on the potential energy surface of the proton transfer leading to the dihydrogen complex [Cp*Fe(dppe)(H2)]+. With p-nitrophenol (PNP) a UV/Visible study provides evidence of the formation of the ion-pair stabilized by a hydrogen bond between the nonclassical cation [Cp*Fe(dppe)(H2)]+ and the homoconjugated anion ([AHA]-). With trifluoroacetic acid (TFA), the hydrogen-bonded ion pair containing the simple conjugate base (A-) in equilibrium with the free ions is observed by IR spectroscopy when using a deficit of the proton donor. An excess leads to the formation of the homoconjugated anion. The interaction with hexafluoroisopropanol (HFIP) was investigated quantitatively by IR spectroscopy and by 1H and 31P NMR spectroscopy at low temperatures (200-260 K) and by stopped-flow kinetics at about room temperature (288-308 K). The hydrogen bond formation to give [Cp*Fe(dppe)H]HA is characterized by DeltaH degrees =-6.5+/-0.4 kcal mol(-1) and DeltaS degrees = -18.6+/-1.7 cal mol(-1) K(-1). The activation barrier for the proton transfer step, which occurs only upon intervention of a second HFIP molecule, is DeltaH(not equal) = 2.6+/-0.3 kcal mol(-1) and DeltaS(not equal) = -44.5+/-1.1 cal mol(-1) K(-1). The computational investigation (at the DFT/B3 LYP level with inclusion of solvent effects by the polarizable continuum model) reproduces all the qualitative findings, provided the correct number of proton donor molecules are used in the model. The proton transfer process is, however, computed to be less exothermic than observed in the experiment.

Computer Simulation↗
PubMed2005

Decomposition of neutral, singly and doubly protonated benzoquinone in the gas phase.

The unimolecular fragmentations of singly and doubly protonated ortho-, meta-, and para-benzoquinones (BQH(+) and BQH(2)(2+), respectively) are studied by tandem mass spectrometry. The dominant fragmentation pathways lead to the elimination of a neutral CO molecule from BQH(+) and, by charge separation, to the expulsion of protonated CO from BQH(2)(2+). Reaction mechanisms are elucidated based on labeling experiments and UB3 LYP calculations. These results reveal that the respective reactions proceed in an analogous fashion to the decarbonylation of neutral benzoquinones, which decompose into carbon monoxide and cyclopentadienone. Single protonation facilitates all steps on the reaction pathway with neutral CO and O-protonated cyclopentadienone as final products. In contrast, double protonation leads to an increase of the barriers for the decomposition yielding CO.H(+) and O-protonated cyclopentadienone. This major process of BQH(2)(2+) is accompanied by two minor channels, which lead to the elimination of neutral carbon monoxide and water, respectively. The proton affinity of the para-BQH(+) monocation is estimated as 3.6+/-0.3 eV.

Algorithms↗
PubMed2006

NMR investigation of the dihydrogen-bonding and proton-transfer equilibria between the hydrido carbonyl anion [HRe2(CO)9]- and fluorinated alcohols.

The interaction of fluorinated alcohols with the anionic hydrido complex [HRe2(CO)9]- (1) has been investigated by NMR spectroscopy. According to the acidic strength of the alcohols, the interaction may result not only in the formation of dihydrogen-bonded ROH...[HRe2(CO)9]- adducts 2, but also in proton transfer to give the neutral species [H2Re2(CO)9] (3). With the weaker acid trifluoroethanol (TFE) evidence for the occurrence of the dihydrogen-bonding equilibrium was obtained by 2D 1H NOESY. The dependence of the hydride chemical shift on TFE concentration at different temperatures provided values for the constants of this equilibrium, from which the thermodynamic parameters were evaluated as deltaH(degrees) = -2.6(2) kcal mol(-1), deltaS(degrees) = -9.3(2) cal mol(-1) K(-1). This corresponds to a rather low basicity factor (E(j) = 0.64). Variable-temperature T1 measurements allowed the proton-hydride distance in adduct 2 a to be estimated (1.80 angstroms). In the presence of hexafluoroisopropyl alcohol (HFIP) simultaneous occurrence of both dihydrogen-bonding and proton-transfer equilibria was observed, and the equilibria shifted versus the protonated product 3 with increasing HFIP concentration and decreasing temperature. Reversible proton transfer between the alcohol and the hydrido complex occurs on the NMR timescale, as revealed by a 2D 1H EXSY experiment at 240 K. For the more acidic perfluoro-tert-butyl alcohol (PFTB) the protonation equilibrium was further shifted to the right. Thermal instability of 3 prevented the acquisition of accurate thermodynamic data for these equilibria. The occurrence of the proton-transfer processes (in spite of the unfavorable pK(a) values) can be explained by the formation of homoconjugated RO...HOR- pairs which stabilize the alcoholate anions.

Anions↗
PubMed2002

Epidermal growth factor stimulates proton efflux from chondrocytic cells.

Proton efflux from chondrocytes alters the extracellular pH and ionic composition of cartilage, and influences the synthesis and degradation of extracellular matrix. Epidermal growth factor (EGF) promotes chondrocyte proliferation during skeletal development and accumulates in the synovial fluid in rheumatoid arthritis. The purpose of this study was to investigate the effect of EGF on proton efflux from chondrocytes. When monitored using a Cytosensor microphysiometer, EGF was found to rapidly activate proton efflux from CFK2 chondrocytic cells and rat articular chondrocytes. The actions of EGF were concentration-dependent with half-maximal effects at 0.3-0.7 ng/ml. Partial desensitization and time-dependent recovery of the response were observed following repeated exposures to EGF. EGF-induced proton efflux was dependent on extracellular glucose, and inhibitors of Na(+)/H(+) exchange (NHE) markedly attenuated the initial increase in proton efflux. The response was diminished by inhibitors of phosphatidylinositol 3-kinase and phospholipase C, but not by inhibitors of MEK (MAPK/ERK kinase) or protein kinase A or C. Thus, EGF-induced proton efflux involves glucose metabolism and NHE, and is regulated by a discrete subset of EGF-activated signaling pathways. In vivo, proton efflux induced by EGF may lead to an acidic environment, enhancing turnover of cartilage matrix during development and in rheumatoid arthritis.

Animals↗
PubMed2006

An unprecedented rearrangement in collision-induced mass spectrometric fragmentation of protonated benzylamines.

The collision-induced dissociation (CID) mass spectra of several protonated benzylamines are described and mechanistically rationalized. Under collision-induced decomposition conditions, protonated dibenzylamine, for example, loses ammonia, thereby forming an ion of m/z 181. Deuterium labeling experiments confirmed that the additional proton transferred to the nitrogen atom during this loss of ammonia comes from the ortho positions of the phenyl rings and not from the benzylic methylene groups. A mechanism based on an initial elongation of a C--N bond at the charge center that eventually cleaves the C--N bond to form an ion/neutral complex of benzyl cation and benzylamine is proposed to rationalize the results. The complex then proceeds to dissociate in several different ways: (1) a direct dissociation to yield a benzyl cation observed at m/z 91; (2) an electrophilic attack by the benzyl cation within the complex on the phenyl ring of the benzylamine to remove a pair of electrons from the aromatic sextet to form an arenium ion, which either donates a ring proton (or deuteron when present) to the amino group forming a protonated amine, which undergoes a charge-driven heterolytic cleavage to eliminate ammonia (or benzylamine) forming a benzylbenzyl cation observed at m/z 181, or undergoes a charge-driven heterolytic cleavage to eliminate diphenylmethane and an immonium ion; and (3) a hydride abstraction from a methylene group of the neutral benzylamine to the benzylic cation to eliminate toluene and form a substituted immonium ion. Corresponding benzylamine and dibenzylamine losses observed in the spectra of protonated tribenzylamine and tetrabenzyl ammonium ion, respectively, indicate that the postulated mechanism can be widely applied. The postulated mechanisms enabled proper prediction of mass spectral fragments expected from protonated butenafine, an antifungal drug.

Ammonia↗
PubMed1991

Water-proton nuclear magnetic relaxation in heterogeneous systems: hydrated lysozyme results.

Spin-lattice relaxation rates of water protons in hydrated immobilized lysozyme are measured as a function of magnetic field strength. The dependence of water relaxation versus hydration is presented from 35 to 55% by weight water content. The water-proton relaxation is directly coupled to that of the protein and the coupling exists in the absence of chemical exchange. A model is applied where relaxation within the two proton phases is coupled through a dipolar cross-relaxation mechanism as well as chemical exchange. The observed amplitudes of the water-proton relaxation profiles scale with the ratio of protein to water protons as well as the protein-proton relaxation rate. The field dependence of the protein-proton spin-lattice relaxation is presented in the presence of D2O where a cross-relaxation coupling is absent. The coupled relaxation model accounts well for the NMR relaxation data as a function of magnetic field strength which is similar to measurements on other heterogeneous systems such as tissues.

Magnetic Resonance Spectroscopy↗
PubMed2006

Molecular theory of field-dependent proton spin-lattice relaxation in tissue.

A molecular theory is presented for the field-dependent spin-lattice relaxation time of water in tissue. The theory attributes the large relaxation enhancement observed at low frequencies to intermediary protons in labile groups or internal water molecules that act as relaxation sinks for the bulk water protons. Exchange of intermediary protons not only transfers magnetization to bulk water protons, it also drives relaxation by a mechanism of exchange-mediated orientational randomization (EMOR). An analytical expression for T1 is derived that remains valid outside the motional-narrowing regime. Cross-relaxation between intermediary protons and polymer protons plays an important role, whereas spin diffusion among polymer protons can be neglected. For sufficiently slow exchange, the dispersion midpoint is determined by the local dipolar field rather than by molecular motions, which makes the dispersion frequency insensitive to temperature and system composition. The EMOR model differs fundamentally from previous models that identify collective polymer vibrations or hydration water dynamics as the molecular motion responsible for spin relaxation. Unlike previous models, the EMOR model accounts quantitatively for 1H magnetic relaxation dispersion (MRD) profiles from tissue model systems without invoking unrealistic parameter values.

Magnetic Resonance Spectroscopy↗
PubMed1996

The pH dependence of hydrogen-deuterium exchange in trp repressor: the exchange rate of amide protons in proteins reflects tertiary interactions, not only secondary structure.

The pH dependence of amide proton exchange rates have been measured for trp-repressor. One class of protons exchanges too fast to be measured in these experiments. Among the protons that have measurable hydrogen-deuterium exchange rates, two additional classes may be distinguished. The second class of protons are in elements of secondary structure that are mostly on the surface of the protein, and exchange linearly with increasing base concentration (log kex versus pH). The third class of amide protons is characterized by much higher protection against exchange at higher pH. These protons are located in the core of the protein, in helices B and C. The exchange rate in the core region does not increase linearly with pH, but rather goes through a minimum around pH 6. The mechanism of exchange for the slowly exchanging core protons is interpreted in terms of the two-process model of Hilton and Woodward (1979, Biochemistry 18:5834-5841), i.e., exchange through both a local mechanism that does not require unfolding of the protein, and a mechanism involving global unfolding of the protein. The increase in exchange rates at low pH is attributed to a partial unfolding of the repressor. It is concluded that the formation of secondary structure alone is insufficient to account for the high protection factors seen in the core of native proteins at higher pH, and that tertiary interactions are essential to stabilize the structure.

Amides↗
PubMed2004

Reorganization in apo- and holo-beta-lactoglobulin upon protonation of Glu89: molecular dynamics and pKa calculations.

Molecular dynamics (MD) simulations starting from crystallographic data allowed us to directly account for the effects of the protonation state of Glu89 on the conformational stability of apo- and holo-beta-lactoglobulin (BLG). In apo-BLG simulations starting from the protonated crystal structure, we observe a long-lived H-bond interaction between the protonated Glu89 and Ser116. This interaction, sequestering the proton from the aqueous medium, explains a pK(half) value evaluated at pH 7.3 by continuum electrostatics/Monte Carlo computation on MD data, using linear response approximation. A very large root-mean-square deviation (RMSD; 5.11 A) is observed for the EF loop between protonated and unprotonated apo-BLG. This results from a quite different orientation of the EF loop that acts either as a closed or as an open lid above the protein calyx. Proton exchange by Glu89 in apo- but not in holo-BLG is associated with a reorganization energy of 4.7 kcal/mol. A 3-ns MD simulation starting from the crystal structure of protonated apo-BLG, but considering the Glu89 as unprotonated, shows the progressive opening of the lid giving rise to the Tanford transition. In both holo-BLG forms, the lid is most probably held in place by hydrophobic interactions of amino acid side-chains of the EF loop with the palmitate hydrocarbon tail.

Animals↗
PubMed2005

Water dynamics simulation as a tool for probing proton transfer pathways in a heptahelical membrane protein.

The proton transfer pathway in a heptahelical membrane protein, the light-driven proton pump bacteriorhodopsin (BR), is probed by a combined approach of structural analysis of recent X-ray models and molecular dynamics (MD) simulations that provide the diffusion pathways of internal and external water molecules. Analyzing the hydrogen-bond contact frequencies of the water molecules with protein groups, the complete proton pathway through the protein is probed. Beside the well-known proton binding sites in the protein interior-the protonated Schiff base, Asp85 and Asp96, and the H(5)O(2) (+) complex stabilized by Glu204 and Glu194-the proton release and uptake pathways to the protein surfaces are described in great detail. Further residues were identified, by mutation of which the proposed pathways can be verified. In addition the diffusion pathway of water 502 from Lys216 to Asp96 is shown to cover the positions of the intruding waters 503 and 504 in the N-intermediate. The transiently established water chain in the N-state provides a proton pathway from Asp96 to the Schiff base in the M- to N-transition in a Grotthus-like mechanism, as concluded earlier from time-resolved Fourier transform infrared experiments [le Coutre et al., Proc Nat Acad Sci USA 1995;92:4962-4966].

Aspartic Acid↗
PubMed2002

Combined quantum chemical and RRKM modeling of the main fragmentation pathways of protonated GGG. II. Formation of b(2), y(1), and y(2) ions.

Quantum chemical and RRKM calculations were performed on protonated GGG in order to determine the atomic details of the main fragmentation pathways leading to formation of b(2),y(1), and y(2) ions. Formation of y(1) ions on the "diketopiperazine" pathway is initiated from relatively high-energy C-terminal amide nitrogen protonated species for which the N-terminal amide bond is in the cis isomerization state. The reaction goes through a transition structure which is only slightly less favored than the reactive configuration itself. RRKM calculations indicate that this reaction is extremely fast as soon as the fragmenting species have more internal energy than the reaction threshold. The calculated energetics suggests that y(1) ions are formed on the "diketopiperazine" pathway with a non-negligible (6-10 kcal/mol) reverse activation barrier. Investigation of species occurring during the formation of b(2) ions having an oxazolone structure indicates that y(1) ions can be formed also from intermediates previously thought to result in only b(2) ions. As the first step of the "b(x)-y(z)" pathway proposed here the extra proton must reach the nitrogen of the C-terminal amide bond. Attack of the N-terminal amide oxygen on the carbon center of the C-terminal amide bond results in formation of the oxazolone ring while the detaching G leaves the precursor ion. Under low-energy collision conditions the complex of protonated 2-aminomethyl-5-oxazolone and G can rearrange to form a proton-bonded dimer of these species. In such circumstances the extra proton is shared by the two monomers and dissociation of the dimer will be determined by the thermochemistry involved. Based on the "b(x)-y(z)" pathway one can easily explain the linear relationship between the logarithm of the y(1)/b(2) ion abundance ratio and the proton affinity of the C-terminal amino acid substituent for the series of H-Gly-Gly-Xxx-OH tripeptides where Xxx was varied (Morgan DG, Bursey MM. Org. Mass. Spectrom. 1994; 29: 354). The calculated energetics indicates that both y(1) and b(2) ions are formed with no reverse activation barrier on the "b(x)-y(z)" pathway.

Diketopiperazines↗