Deprotonated Species

Abundant one or two intense fragment ions around tn/z 300 correspond to FA carboxylate ions depending on whether the FA substituents are identical or not, respectively. The peak intensity of the carboxylate anion resulting from the sn-2 FA substituent is approximately three times more intense due to its sterically favorable loss than that from the sn-l FA chain of deprotonated dPE species. 

There also exists a fragment ion at m/z 196, corresponding to a glycerophos-phoethanolamine anion derivative, in the product-ion spectra of all PE species in low abundance, which is characteristic of the phosphoethanolamine head group. 

It should be noted that, in general, the product-ion ESI-MS spectra of deprotonated PE species consisting of polyunsaturated FA substituents are readily distinguishable from those consisting of saturated ones. This is due to a fact that the polyunsaturated FA carboxylate anions formed after CID in a QqQ-type mass spectrometer undergo vigorous secondary dissociation, while the saturated FA carboxylate anions undergo minimal secondary dissociation after formation [41,42]. Therefore, the peak intensity of the polyunsaturated FA carboxylate anion is lower than expected. 

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Sometimes we need to know how the concentrations of the ions present in a solution of a polyprotic acid vary with pH. This information is particularly important in the study of natural waters, such as rivers and lakes (Box 10.1). For example, if we were examining carbonic acid in rainwater, then, at low pH (when hydronium ions are abundant), we would expect the fully protonated species (H2C03) to be dominant at high pH (when hydroxide ions are abundant), we expect the fully deprotonated species (C032 ) to be dominant at intermediate pH, we expect the intermediate species (HC03, in this case) to be dominant (Fig. 10.20). We can verify these expectations quantitatively. 

Different orientations for the protonated and deprotonated species as well as a change of the surface coverage with the bulk aqueous pH can further complicate the problem. 

Cu -amikacin complex at pH 7.4 (the protonated and deprotonated species at B5 oxygen exist in equimolar equilibrium)." ... 

The ion formation may occur in the bulk solution before the electrospray process takes place or in the gas phase by protonation or salt adduct formation, or by an electrochemical redox reaction. Polar compounds already exist in solution as ions therefore, the task of the electrospray is to separate them from their counterions. This is the case of many inorganic and organic species and all those compounds that show acidic or basic properties. Proteins, peptides, nucleotides, and many other bio- and pharmaceutical analytes are typical examples of substances that can be detected as proto-nated or deprotonated species. 

For ESP ionization, the analytes must be ionic, or have an ionizable functional group, or be able to form an ionic adduct in solution the analytes are commonly detected as deprotonated species or as cation adducts of a proton or an alkali metal ion. When using positive ion ESP ionization, use of ammonium acetate as a mobile-phase modifier is generally unsuitable. Instead, organic modi-... 

To find the overall goethite-water distribution coefficient for benzoic acid (HBz), assume that only adsorption of the deprotonated species (benzoate, Bz") is important ... 

Reaction with nucleophiles by deprotonation at a CH bond occurs in pyridine much more readily than in benzene. The reactivity order is y > 3 > a rather than y, a > (3 because of lone pair lone pair repulsion in the a-deprotonated species (c/. discussion in Section 3.2.1.8). 

The principal species of a monoprotic or polyprotic system is found by comparing the pH with the various pKa values. For PH < pKh the fully protonated species, H A, is the predominant form. For pA) < pH < pK2, the form H , A is favored and, at each successive pK value, the next deprotonated species becomes principal. Finally, at pH values higher than the highest pK, the fully basic form (A"-) is dominant. The fractional composition of a solution is expressed by a, given in Equations 10-17 and 10-18 for a monoprotic system and Equations 10-19 through 10-21 for a diprotic system. 

Deprotonation of the methyl group was achieved by the action of Bu"Li/TMEDA on 2,5-dimethylthiophene (74TL2373). A dimetallated derivative was formed in 15-20% yield, carbonation of which gave a mixture of the two acids (320) and (321) (Scheme 93). An exo ring-opening of the deprotonated species can take place under appropriate conditions ... 

For the complex /ac,/ac-(H20)2(NH3)3Cr(0H)Cr(NH3)3(H20)25+ the first acid dissociation constant clearly shows that the singly deprotonated species must be hydrogen bond stabilized (Table XIX). However, the fact that the difference between the first and the second acid dissociation constants for this system is relatively small is consistent with stabilization also of the doubly deprotonated species (by two intramolecular hydrogen bonds) as shown in Fig. 15. A similar effect is expected for the cations (H20)5Cr(0H)Cr(H20)55+ and (H20)5Ir(0H)Ir(H20)55+. 

The kinetics of the condensation of the Cr(H20)63+ ion and its corresponding deprotonated species have been studied in the pH region 3.5-5.0 [25°C, I = 1.0 M(NaC104)] (201). The study of this reaction is complicated by the formation of higher oligomers. Chromatographic analysis of the products as a function of time established the dinuclear species to be the main product for the first 5% of reaction, and the initial-rate kinetics of condensation were studied by a pH-stat technique. The observed pH dependence of the rate was interpreted in terms of the second-order rate constants defined by Eq. (44), and values for... 

Aza-analogues of oxygen compounds frequently show related reactivity patterns, and this is certainly seen in a comparison of the chemistry of imines and carbonyl compounds. For example, 1,3-diimines are readily deprotonated to yield the 1,3-diketonate analogues. The most frequent consequence of this is that reactions which are expected to yield 1,3-diimine complexes often lead to those of the deprotonated species. This is seen in the formation of the gold(m) complex of a deprotonated macrocycle in the reaction of 1,2-diaminoethane with 2,4-pentanedione in the presence of Na[AuCl4] (Fig. 5-4). The exact sequence of events in this reaction is not known, but note that the product is a square-planar gold(in) complex of the doubly-deprotonated macrocycle, rather than a gold(i) species ... 

Metal-ion coordination at N(7) acidifies the proton at N(l) by up to two log units [36], Metal-ion binding at N(7) weakens both the basicity and met-al-ion binding capability at N(l). For inosine and guanosine and their nucleotides in neutral solutions, the decrease in metal-ion stability at N(l) is more than offset by the greater fraction of N(l) deprotonated species. 

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