Effects of timing and duration of test period and diet type on intake and feed efficiency of Charolais-sired cattle.

Objectives of this experiment were to: 1) determine appropriate test length, timing, and repeatability of DMI, ADG, and efficiency over different biological time points; 2) determine the efficacy of using decoupled performance and intake data to generate accurate feed efficiency measures; and 3) determine the relationship between forage-and grain-feed efficiency measures. Over 2 yr, Charolais crossbred heifers ( = 308) and steers ( = 320) were fed for two 70 d periods and DMI, ADG, and 12th rib fat thickness were recorded. Steers were fed grain-based diets during the growing and finishing periods to determine the effects of test period and timing on DMI and feed efficiency. Heifers were fed forage during the growing period and grain during the finishing period to test the effect of diet type on measures of DMI and feed efficiency. For each 70 d test period, individual DMI was recorded using the GrowSafe (Airdrie, AB) system. Residual feed intake (RFI) was calculated for each test period. Total feeding period ADG (FP_ADG) was calculated for steers by regressing all weights taken from feedlot arrival to final BW, which was calculated by dividing HCW by a standard dressing percentage (63%). Dry matter intake and RFI were correlated (r 0.56; < 0.01, and 0.63; < 0.01, respectively) for the growing and finishing periods of grain-fed steers. Average daily gain was not repeatable ( = 0.11; 0.06) across both test periods for steers. However, growing and finishing ADG were correlated ( = 0.58; < 0.01, and = 0.69; < 0.01, respectively) to FP_ADG. To assess the potential of shortening the intake test, DMI was analyzed in 7 d increments for grain-fed steers during the growing and finishing periods. Regardless of test length, from 7 to 70 d, DMI was strongly correlated ( ≥ 0.87; < 0.01) to total DMI during the growing period. Heifer forage DMI was correlated ( = 0.58; < 0.01) to grain DMI; subsequently, forage and grain RFI were moderately correlated ( = 0.40; < 0.01). This study suggests that DMI is repeatable across varying stages of maturity in cattle, and accurate feed efficiency measures can be obtained in either the growing or finishing period. The relationship of forage and grain DMI and efficiency in heifers suggests that measures of DMI and feed efficiency in heifers are relevant, regardless of diet fed. Intake evaluation periods can be shortened with minimal effects on the accuracy of predicting individual animal DMI.

Survival of rat or mouse ventral mesencephalon neurons after cotransplantation with rat sertoli cells in the mouse striatum.

Transplanting cells across species (xenotransplantation) for the treatment of Parkinson's disease has been considered an option to alleviate ethical concerns and shortage of tissues. However, using this approach leads to decreased cell survival; the xenografted cells are often rejected. Sertoli cells (SCs) are testis-derived cells that provide immunological protection to developing germ cells and can enhance survival of both allografted and xenografted cells. It is not clear whether these cells will maintain their immunosuppressive support of cografted cells if they are transplanted across species. In this study, we investigated the immune modulatory capacity of SCs and the feasibility of xenografting these cells alone or with allografted and xenografted neural tissue. Transplanting xenografts of rat SCs into the mouse striatum with either rat or mouse ventral mesencephalon prevented astrocytic infiltration of the graft site, although all transplants showed activated microglia within the core of the graft. Surviving tyrosine hydroxylase-positive neurons were observed in all conditions, but the size of the grafts was small at best. SCs were found at 1 and 2 weeks posttransplant. However, few SCs were found at 2 months posttransplant. Further investigation is under way to characterize the immune capabilities of SCs in a xenogeneic environment.

Gas-phase basicities for ions from bradykinin and its des-arginine analogues.

Apparent gas-phase basicities (GB(app)s) for [M + H]+ of bradykinin, des-Arg1-bradykinin and des-Arg9-bradykinin have been assigned by deprotonation reactions of [M + 2H]2+ in a Fourier transform ion cyclotron resonance mass spectrometer. With a GB(app) of 225.8 +/- 4.2 kcal x mol(-1), bradykinin [M + H]+ is the most basic of the ions studied. Ions from des-Arg1-bradykinin and des-Arg9-bradykinin have GB(app) values of 222.8 +/- 4.3 kcal x mol(-1) and 214.9 +/- 2.3 kcal x mol(-1), respectively. One purpose of this work was to determine a suitable reaction efficiency 'break point' for assigning GB(app) values to peptide ions using the bracketing method. An efficiency value of 0.1 (i.e. approximately 10% of all collisions resulting in a deprotonation reaction) was used to assign GB(app)s. Support for this criterion is provided by the fact that our GB(app) values for des-Arg1-bradykinin and des-Arg9-bradykinin are identical, within experimental error, to literature values obtained using a modified kinetic method. However, the GB(app)s for bradykinin ions from the two studies differ by 10.3 kcal x mol(-1). The reason for this is not clear, but may involve conformation differences produced by experimental conditions. The results may be influenced by salt-bridge conformers and/or by conformational changes caused by the use of a proton-bound heterodimer in the kinetic method. Factors affecting the basicities of these peptide ions are also discussed, and molecular modeling is used to provide information on protonation sites and conformations. The presence of two highly basic arginine residues on bradykinin results in its high GB(app), while the basicity of des-Arg1-bradykinin ions is increased by the presence of two proline residues at the N-terminus. The proline residue in the second position folds the peptide chain in a manner that increases intramolecular hydrogen bonding to the protonated N-terminal amino group of the proline at the first position.

Further studies on the effects of topical lactate on amino acid efflux from the ischemic rat cortex.

A rat four-vessel cerebral occlusion model was used to examine the effects of D-lactate and oxamate, a lactate dehydrogenase inhibitor, on cortical window superfusate levels of amino acids, glucose and L-lactate. Superfusate levels of aspartate, glutamate, taurine, GABA and phosphoethanolamine rose during ischemia and then declined during reperfusion. Glycine and alanine levels tended to increase during reperfusion, whereas glutamine levels were lower. Serine levels were not altered. Glucose levels declined rapidly during ischemia and recovered during reperfusion. Lactate levels were sustained during ischemia and increased during reperfusion. Unlike L-lactate, which attenuated ischemia/reperfusion (I/R) evoked amino acid release (J.W. Phillis, D. Song, L.L. Guyot, M.H. O'Regan, Lactate reduces amino acid release and fuels recovery of function in the ischemic brain, Neurosci. Lett. 272 (1999) 195-198), topical application of D-lactate (20 mM), which is not used as an energy substrate, enhanced the I/R release of aspartate, glutamate, GABA and taurine into cortical superfusates, and also elevated L-lactate levels above those in the controls. Glucose levels were not altered. Oxamate (20 mM) application elevated the pre-ischemia levels of alanine, glycine and GABA and those of GABA during ischemia. Levels of all amino acids, with the exception of phosphoethanolamine, were elevated during reperfusion. Oxamate, an inhibitor of lactate dehydrogenases 1 and 5, did not alter the pattern of efflux of glucose and L-lactate. In the presence of oxamate, L-lactate (20 mM) failed to inhibit amino acid release. The failure of D-lactate to attenuate amino acid release confirms the inability of this isomer to act as a metabolic substrate. The oxamate data indicate that inhibition of lactate dehydrogenase is detrimental to the viability of cortical cells during I/R, even though extracellular lactate levels are elevated. The pre-ischemia increases in alanine and glycine are suggestive of elevations in pyruvate as a result of the block of its conversion to lactate, with transamination reactions converting pyruvate to form these amino acids. In summary, the results further substantiate the concept of a role for L-lactate as a cerebral energy substrate.

Dissociation of multiply charged negative ions for hirudin (54-65), fibrinopeptide B, and insulin A (oxidized).

Collision-induced dissociation (CID) was performed on multiply deprotonated ions from three commercial peptides: hirudin (54-65), fibrinopeptide B, and oxidized insulin chain A. Ions were produced by electrospray ionization in a Fourier transform ion cyclotron resonance mass spectrometer. Each of these peptides contains multiple acidic residues, which makes them very difficult to ionize in the positive mode. However, the peptides deprotonate readily making negative ion studies a viable alternative. The CID spectra indicated that the likely deprotonation sites are acidic residues (aspartic, glutamic, and cysteic acids) and the C-terminus. The spectra are rife with c, y, and internal ions, although some a, b, x, and z ions form. Many of the fragment ions were formed from cleavage adjacent to acidic residues, both N- and C-terminal to the acidic site. In addition, neutral loss (e.g., NH3, CH3, H2O, and CO2) was prevalent from both the parent ions and from fragment ions. These neutral eliminations were often indicative of specific amino acid residues. The fragmentation patterns from several charge states of the parent ions, when combined, provide significant primary sequence information. These results suggest that negative mode CID of multiply deprotonated ions provides useful structural information and can be worthwhile for highly acidic peptides that do not form positive ions in abundance.

Effects of cysteic acid groups on the gas-phase reactivity and dissociation of [M + 4H]4+ ions from insulin chain B.

Gas-phase ion/molecule reactions and collision-induced dissociation (CID) were conducted on [M + 4H]4+ of insulin chain B. This Fourier transform mass spectrometry work involved ions from the oxidized peptide (with two cysteic acid residues) and its reduced form (with two cysteine residues). Kinetic behavior during deprotonation and hydrogen/deuterium exchange reactions indicates that insulin B (ox) ions have two distinct structural types. In contrast, insulin B (red) ions have only one major reacting population, which has a more compact structure than the oxidized ions. No significant differences in fragmentation patterns for the two insulin B (ox) populations were observed when CID was performed as a function of deprotonating reaction time. However, markedly different fragmentation was found between [M + 4H]4+ of insulin B (ox) and (red). Therefore, the presence of cysteic acid groups in insulin B (ox) significantly impacts dissociation and presumably structure. This suggests that some insulin B (ox) ions are zwitterionic, with the five basic sites protonated and one cysteic acid group deprotonated. Molecular dynamics calculations revealed several viable structures that are consistent with the experimental results. For example, the most stable form of the reduced ion, which is unprotonated at the His10, is very compact and has lost the alpha-helix of native insulin. Low energy structures for the oxidized ions include a zwitterion with an intraionic interaction between anionic Cyx7 and cationic His10, as well as a nonzwitterionic conformer that lacks a proton at Phe1; both structures retain the alpha-helix. These structures may account for the two experimentally observed isomers, although others are possible. In addition, experiments on oxidized insulin B were conducted from methanolic solution, which may denature the conformation, and pure aqueous solution, which may leave a native conformation. These differences in solvent composition had no effect on the gas-phase results.

Negative ion postsource decay time-of-flight mass spectrometry of peptides containing acidic amino acid residues.

Acidic peptides have been studied by negative ion postsource decay (PSD) matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The peptides contained from 5 to 16 residues and were chosen on the basis of their patterns of the acidic residues. Using typical MALDI sample preparation techniques employing an acidic matrix, gastrin I (1-14), and epidermal mitosis inhibiting pentapeptide yielded much larger deprotonated ion signals, [M - H]-, than protonated ions, [M + H]+. This may be due to their absence of basic residues, coupled with their arrays of acidic residues. The PSD fragmentation of the peptide negative ions showed that an array of acidic residues, as in gastrin I (1-14), yielded simple spectra containing mainly backbone cleavage ions from the C-terminus. Hirudin (54-65), which contains two sets of two consecutive Glu residues, and fibrinopeptide A and fibrinopeptide B, with isolated acidic residues, also showed backbone cleavages as common fragment ions. In addition, the two sets of isolated consecutive amino acid residues in Cys(Bzl)84-CD4 (81-92) and hirudin (54-56) yielded internal ions from the cleavages at the (O=C)-NH bond between the acidic residues. Also observed were ions with unique side chain losses, such as the loss of C6H4O from a tyrosine residue and SCH2C6H5 and CH2C6H5 from a benzylated cysteine residue. Compared to the positive mode, the negative-ion PSD yielded fewer fragments which usually involved only one type of backbone cleavage (e.g., [Yn - H2O]-). These simple spectra aided interpretation. Overall, the acidic peptides studied yielded negative ion PSD spectra that were useful for peptide sequencing.

Negative ion matrix-assisted laser desorption/ionization time-of-flight post-source decay calibration by using fibrinopeptide B.

Fibrinopeptide B (M(r) 1552.58) was employed as a calibration compound for matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) post-source decay (PSD) fragment ion analysis in the negative mode. Experiments were performed by using both continuous and delayed extraction, with the maximum reflectron voltages being 30 and 21 kV, respectively. For comparison, a common positive ion PSD calibrant, ACTH(18-39) (M(r) 2466.7), was also employed with positive ion calibration constants being applied to negative ion spectra. Using fibrinopeptide B as the calibrant, the negative ion PSD results for angiotensin II (M(r) 1046.2), renin substrate tetradecapeptide (horse) (M(r) 1759.0), and the custom-synthesized peptide (K2G4)2 (M(r) 987.1) showed a factor of 1.5-2 improvement in absolute mass accuracy. Typical absolute mass-to-charge ratio accuracies were within +/- 1 Thompson and were achieved even when the peptide being analyzed was more massive than fibrinopeptide B. In addition, both calibrants showed increased accuracy when experiments were conducted in the delayed extraction mode. Other advantages of using fibrinopeptide B are its moderate cost and the ability to perform calibration and sample analysis for negative ion PSD under the same instrumental conditions.

Gas-phase reactivity and molecular modeling studies on triply protonated dodecapeptides that contain four basic residues.

Gas-phase deprotonation and hydrogen/deuterium (H/D) exchange reactions for ions from three model dodecapeptides were studied by Fourier transform ion cyclotron resonance mass spectrometry. Molecular dynamics calculations were employed to provide information on conformations and Coulomb energies. The peptides, (KGG)4, (K2G4)2, and K4G8, each contain four high basicity lysine residues and eight low basicity glycine residues; however, in the present work only three lysine residues were protonated. Proton transfer reactions with a series of reference amines revealed apparent gas-phase acidities in a narrow range of 207.3-209.6 kcal/mol, with deprotonation efficiencies following the order [K4G8 + 3H]3+ > [(KGG)4 + 3H]3+ > [(K2G4)2 + 3H]3+. The three ions also react similarly with d4-methanol: each exchanged a maximum of 23-25 of their 25 labile hydrogens, with the first 15-17 exchanges occurring at rate constants of (1.6-2.6) x 10(-11) cm3 molecule-1 s-1. The experimental results agree with molecular modeling findings of similar conformations and Coulomb energies for the three peptide ions. The [M + 3H]3+ data are compared to data obtained previously in our laboratory for the "fully" protonated [M + 4H]4+ (Zhang, X.; Ewing, N. P.; Cassady, C. J. Int. J. Mass Spectrom. Ion Phys., in press). For (KGG)4 and (K2G4)2, there is a marked difference in H/D exchange reactivity between 3+ ions and 4+ ions. The 4+ ions, which have diffuse conformations, slowly exchange only 14 hydrogens, whereas their more compact 3+ counterparts exchange 23-25 hydrogens at a 5-times greater rate. In contrast, the 3+ and 4+ ions of K4G8 have similar compact conformations and exchange reactivity. The results indicate that a multiply hydrogen-bonded intermediate between the deuterating reagent and the peptide ion is necessary for facile H/D exchange. The slower, incomplete H/D exchange of [(KGG)4 + 4H]4+ and [(K2G4)2 + 4H]4+ is attributed to the inability of their protonated lysine n-butylamino groups (which extend away from the peptide backbone) to form this intermediate.

Reactivity and gas-phase acidity determinations of small peptide ions consisting of 11 to 14 amino acid residues.

Small peptides ions consisting of a comparable number of amino acid residues but varying in composition and sequence were allowed to undergo gas-phase deprotonation reactions. These multiply protonated ions were generated by electrospray ionization and analyzed in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The peptides studied contain 11-14 amino acid residues and included adrenocorticotropic hormone (ACTH) fragment (11-24), fibrinopeptide B (human), gastrin I fragment (1-13) (human), renin substrate tetra-decapeptide (horse), somatostatin, substance P and tyrosine protein kinase. Rate constants were determined for the deprotonation reactions of the peptide ions with a series of reference compounds of known gas-phase basicities ranging from 190.0 to 232.6 kcal mol-1. From these values, apparent gas-phase acidities (GAapp) were assigned to [M + nH]n+ (n > or = 2), of each peptide. All of the multiply charged peptide ions were sequentially deprotonated to the +1 charge state by ion-molecule reactions. The GAapps ranged from 193.3 kcal mol-1 (for [M + 4H]4+ of renin substrate, the ion most readily deprotonated) to > 232.6 kcal mol-1 (for [M + 2H]2+ of ACTH (11-24), the ion most difficult to deprotonate). The proximity of intrinsically basic sites (and therefore potential protonation sites) has an effect on the observed deprotonation rates. Ions experiencing Coulomb repulsion resulting from adjacent protonation sites often show more facile deprotonation. However, the intrinsic basicity of a protonation site also plays a role in determining the case of deprotonation. As a result, some lower charge state peptide ions deprotonate more readily than other peptides with higher charges but with more basic protonation sites. In addition, conformation and the influence of intramolecular hydrogen bonding may affect the reactivity of some peptide ions. Also observed was non-linear kinetic behavior that indicates multiple isomers at certain charge states for some peptides, e.g. [M + nH]n+, (n = 2 and 3) for ACTH 11-24 and [M + 3H]3+ for somatostatin.

Determination of the gas-phase basicities of proline and its di- and tripeptides with glycine: the enhanced basicity of prolylproline.

The gas-phase basicity (GB) of proline, which is the only imino acid and is reported to play a significant role in protein folding, was determined by deprotonation reactions in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Protonated peptide ions were generated by fast atom bombardment in an external ion source. The GB of proline was found to be 213.3 kcal/mol. Among the dipeptides studied, the GB of glycylproline (GlyPro) was determined to be 214.8 kcal/mol, while prolyglycine (ProGly) was 4.2 kcal/mol more basic (GB = 219.0 kcal/mol). The basicity of the tripeptide prolylglycylglycine (ProGlyGly, GB = 219.0 kcal/mol) is 2 kcal/mol higher than the basicities of glycylglycylproline (GlyGlyPro) and glycylprolylglycine (GlyProGly), both of which have GB = 217.0 kcal/mol. The enhanced basicity of ProGlyGly is consistent with the protonation site being the terminal amino nitrogen with enhanced stabilization of the charge by the nearby bulkier residue. Interestingly, prolyproline (ProPro), which has a GB of 223.3 kcal/mol, is more basic than the other di- and tripeptides studied. Consequently, semi-empirical AM1 calculations were employed to probe the structural characteristics and intramolecular hydrogen bonding interactions for ProPro, GlyPro, ProGly and glycylglycine (GlyGly). ProPro was found to have a unique structure with two potential hydrogen bonds between amino hydrogens and both carbonyl oxygens.

Elucidation of isomeric structures for ubiquitin [M + 12H]12+ ions produced by electrospray ionization mass spectrometry.

Gas-phase deprotonation reactions, hydrogen-deuterium exchange reactions and collision-induced dissociation (CID) were used to distinguish between two isomeric forms of [M + 12H]12+ produced from the protein ubiquitin. Ions were generated by electrospray ionization and studied in a Fourier transform ion cyclotron resonance mass spectrometer. For [M + 12H]12+ formed directly from the electrospray process, deprotonation reactions with ammonia and 2-fluoropyridine yield non-linear pseudo-first-order kinetic behavior that indicates the presence of two ion structures. The fraction of ions that undergo the fastest deprotonation reactions, and is presumably the least energetically stable isomer, accounts for approximately 60% of the [M + 12H]12+ produced by electrospray. In reactions with D2O and CD3OD, the [M + 12H]12+ which are deprotonated faster exchange the first 11 +/- 1 hydrogens more readily that the remaining [M + 12H]12+ population. Results from CID experiments, obtained as a function of reaction time with the amines, also indicate the existence of more than one [M + 12H]12+ structure. The CID fragmentation patterns provide information about the general locations of the charge sites. Surprisingly, evidence for only one structure (the slow-reacting, more stable species) is found for [M + 12H]12+ that is produced by gas-phase deprotonation of [M + 13H]13+, which is the "fully protonated' form of ubiquitin. These results are discussed in terms of ubiquitin isomers related to protonation site and three-dimensional conformation.

Gas-phase basicities of histidine and lysine and their selected di- and tripeptides.

The gas-phase basicities (GB) of histidine, lysine, and di- and triglycyl peptides containing either one histidine or one lysine residue have been determined. In all, 12 compounds were examined in a Fourier transform ion cyclotron resonance mass spectrometer. The GBs of the biomolecules were evaluated by proton transfer reactions employing a range of reference compounds with varying gas-phase basicities. In addition, the GBs were determined by using the kinetic method of collision-induced dissociation on a proton-bound dimer containing the peptide and a reference compound. The GBs of histidine and lysine were both found to be 220.8 kcal/mol via proton transfer reactions. The kinetic method experiments, including dissociation of a proton-bound dimer containing both histidine and lysine, also suggest equivalent GBs for these amino acids. However, the small peptides containing lysine are generally more basic than the corresponding histidine-containing peptides. For the peptides, the data suggest that the protonation site is on the basic side chain functional group of the histidine or lysine residues. The GBs of the di- and tripeptides are dependent upon the location of the basic residue. For example, the GBs of the tripeptides glycylglycyl-L-lysine (GlyGlyLys) and L-lysylglycylglycine (LysGlyGly) were both determined to be 230.7 kcal/mol while a GB of kcal/mol was obtained for glycyl-L-lysylglycine (GlyLysGly). A similar GB trend is seen with the histidine-containing tripeptides. Generally, the GBs obtained by using the kinetic method are slightly higher than those obtained by deprotonation reactions; however, the trends in relative GB values are essentially the same with the two techniques.

Apparent gas-phase acidities of multiply protonated peptide ions: Ubiquitin, insulin B, and renin substrate.

The gas-phase deprotonation reactions of multiply protonated bovine ubiquitin, insulin chain B, and renin substrate tetradecapeptide ions have been studied in a Fourier transform ion cyclotron resonance mass spectrometer coupled with an external electrospray source. Rate constants were measured for the reactions of these peptide ions with a series of reference compounds of known gas-phase basicities ranging from 195.6 to 232.6 kcal/mol. The apparent gas-phase acidities (GAapp) of the multiply protonated peptide ions [M + nH](n+) were determined with deprotonation reactions. The deduced values of GAapp show a strong dependence on the charge states of the multiply protonated peptide ions. In general, the values decrease as the charge states of the peptide ions increase. For ubiquitin ions, the determined GAapps values decrease from >232.6 to 205.0 kcal/mol for n=4-13; for insulin B ions, the GAapps decrease from >232.6 to 198.2 kcal/mol for n=2-5; for renin substrate ions, the GAapps decrease from 221.6 to <195.6 kcal/mol for n=2-4. Interestingly, at a given mass-to-charge ratio, the GAapps of these peptide ions agree within 10 kcal/mol despite large differences in their mass and charge. The ubiquitin and insulin B ions generated under the present conditions reveal multiple isomers at certain charge states, n=4, 5, 6, 12 for ubiquitin and n=4, 5 for insulin B, as evidenced by the fact that the isomers display distinctively different deprotonation reaction rates with certain reference compounds.

An electrospray ionization mass spectrometry study of copper adducts of protonated ubiquitin.

A study of the addition of Cu(II) to a ubiquitin electrospray solution shows that the copper ion in the ubiquitin remains doubly charged and displaces two protons on the protonated protein molecule. This observation indicates a chelating bond between the protein and the Cu(II) species. The addition of Cu(I) also was studied and significant intensity was observed for adducts with up to four Cu(I) species attached, with each Cu(I) bonded to one basic site on the protein.

Deprotonation reactions of multiply protonated ubiquitin ions.

The gas-phase deprotonation reactions of multiply protonated ubiquitin ions have been studied in a Fourier-transform ion cyclotron resonance mass spectrometer. Electrospray ionization was used to generate ubiquitin ions with attachment of 7-13 protons. Rate constants were measured for the reactions of these protein ions with four amines: n-propylamine, di-n-propylamine, tri-n-propylamine, and N,N,N',N'-tetramethyl-1,4-diaminobutane. The gas-phase basicities of the amines ranged from 210.1 kcal/mol to 232.6 kcal/mol. The rate constants were found to increase as the charge state of the ion increased and as the basicity of the amine increased. Several reactions proceed at near the collision rate and have rate constants in excess of 10(-8) cm3 molecule-1 s-1. With the more basic reactants, multiple protons could be stripped sequentially from ubiquitin ions at roughly equivalent rates, suggesting that these protons are attached to sites with similar basicities. In general, deprotonation occurs if the gas-phase basicity of the amine is within 10 kcal/mol of the intrinsic gas-phase basicity of the amino acid residue being deprotonated. For [M+nH]n+, n = 4-6, nonlinear pseudo-first-order kinetic behavior indicated the presence of multiple ion structures. Kinetic, structural and thermodynamic aspects of these reactions are discussed.

Gas-phase basicities of serine and dipeptides of serine and glycine.

The gas-phase basicities of serine and dipeptides containing amino acid residues of serine and glycine were determined by proton transfer reactions in a Fourier transform ion cyclotron resonance mass spectrometer. The gas-phase basicity (GB) of L-serine was found to be 205.9 kcal/mol, with addition of a hydroxymethyl group (-CH2OH) increasing the basicity by 4.5 kcal/mol relative to the simplest amino acid glycine (GB = 201.4 kcal/mol). This is attributed to a combination of intramolecular hydrogen bonding, induction, and symmetry effects. For the dipeptides, addition of a hydroxymethyl group does not result in a large increase in basicity relative to the basicity of glycylglycine (GB = 208.0 kcal/mol). The gas-phase basicities determined for glycyl-L-serine, L-serylglycine, and L-sery-L-serine are 209.3,210.6, and 210.9 kcal/mol, respectively. In comparison to glycylglycine, addition of the hydroxymethyl group at the N terminus has a greater impact on basicity than its placement at the C terminus. These data suggest that the protonation site for these dipeptides is the N-terminal amino nitrogen.