Ionization properties

In a series of organic acids of similar type, not much tendency exists for one acid to be more reactive than another. For example, in the replacement of stearic acid in methyl stearate by acetic acid, the equilibrium constant is 1.0. However, acidolysis in formic acid is usually much faster than in acetic acid, due to higher acidity and better ionizing properties of the former (115). Branched-chain acids, and some aromatic acids, especially stericaHy hindered acids such as ortho-substituted benzoic acids, would be expected to be less active in replacing other acids. Mixtures of esters are obtained when acidolysis is carried out without forcing the replacement to completion by removing one of the products. The acidolysis equilibrium and mechanism are discussed in detail in Reference 115. 

Quinoxalin-2-ones are in tautomeric equilibrium with 2-hydroxy-quinoxalines, but physical measurements indicate that both in solution and in the solid state they exist as cyclic amides rather than as hydroxy compounds. Thus quinoxalin-2-one and its A -methyl derivative show practically identical ultraviolet absorption and are bases of similar strength. In contrast, the ultraviolet spectra of quinoxalin-2-one and its 0-methyl derivative (2-methoxyquinoxaIine) are dissimilar. The methoxy compound is also a significantly stronger base (Table II). Similar relationships also exist between the ultraviolet absorption and ionization properties of 3-methylquinoxalin-2-one and its N- and 0-methyl derivatives. The infrared spectrum of 3- (p-methoxy-benzyl)quinoxalin-2-one (77) in methylene chloride shows bands at 3375 and 1565 cm" which are absent in the spectrum of the deuterated... 

Let us begin by contrasting the ionization properties of sodium and chlorine ... 

Koumanov A, Karshikoff A, Friis EP, Borchert TV (2001) Conformational averaging in pK calculations Improvement and limitations in prediction of ionization properties of proteins. J Phys Chem B 105 9339-9344. 

We selected liquid ammonia because of its pronounced solubilizing characteristics and powerful ionizing properties. At -33°C and atmospheric pressure, the pKa-value for auto-ionization of liquid ammonia [2NH3 = NH2 + NH4 ] is 34 and since the equivalent value for water is only 14, many substances (with pKa-values between 14 and 34) which are neutral in water should be capable of splitting off protons in liquid ammonia. Acidic... 

According to the electrostatic model the solvation is due to electrostatic interaction between the charged ions and the dipolar solvent molecules. Thus the solvating and ionizing properties of a solvent are considered as being due primarily to the dipole moment of the solvent molecules. Thus, ionic compounds such as sodium chloride are insoluble in non-polar solvents such as carbon tetrachloride. Actually, rather than the dipole moment the field action of the dipoles should be considered. This approach might explain why acetonitrile (p = 3.2) is poor in its ionizing properties compared to water (p = 1.84). However, no numerical values are available for this quantity. 

Thus it is apparent that the ionizing properties of a solvent are determined by the sum of its coordinating properties, e.g. a factor must be added to its dono-city which accounts for the solvation of the anion under consideration. 

For the consideration of the ionizing properties of a solvent it is therefore imperative to consider both the coordinating and the dielectric properties 101. An illustrative example is given in Table 8, in which three different donor solvents Dj.Da and D3 are considered. Dt and D3 are assumed to have the same dielectric constant while Dj and D2 are assumed to have the same coordinating properties. The values for e and A Ion have arbitrarily been selected. From Aion and ATdiss the sum of associated and dissociated ions can readily be calculated and the result shows that in D3 the overall process of ionization is progressing further than in D2, which has a much higher dielectric constant, but has somewhat smaller coordinating properties than D3. 

A. Chattopadhyay and E. London, Spectroscopic and ionization properties of V-(7-nitro-2,1,3-benzoxadiazol-4-yl)-labeled lipids in model membranes, Biochim.Biophys.Acta 938, 24-34 (1988). 

Another type of non-spectral matrix effect, associated with the oxidation state of the analyte, was proposed by Zhu et al. (2002). Figure 14 plots the relative Fe(II) to total Fe ratio of ultra pure Fe standard solutions versus the difference between the 8 Fe value of the mixed valence state standard and the 5 Fe value of the Fe(III) only standard. The oxidation state of these standards was not quantified by Zhu et al. but based on colorimetric methods using 2,2 -bipyridine the relative Fe(ll) to total Fe ratios of these standards are well known. This matrix effect appears to exert a signihcant control on isotope accuracy, where for example if a reduced ferrous solution was compared to an oxidized ferric standard, the accuracy of the 5 Fe value could be affected by up to l%o. This matrix effect associated with oxidation state is unlikely to be a result of space charge effects because the mass of an electron is unlikely to produce a large change in the mass of the ion beam. Perhaps this matrix effect may be associated with ionization properties in the plasma. 

The general method for ASMS is shown in Fig. 4.1. In ASMS, the target concentration is generally set at 5-10 xM, so that at equilibrium, ligands with affinities of no weaker than Ku 10 xM will be significantly bound and, therefore, retained in the ultrafiltration steps. The minimal concentration of each small molecule is dictated by the eventual need to detect ligands by mass spectrometry after several cycles of ultrafiltration and subsequent extraction. In order to ensure detection just above baseline for the vast majority of compounds, which vary in inherent ionization properties and efficiency of mass spectrometric visibility, the starting compound concentration is set at 1.5 pM per compound. The mixture... 

One has to emphasize that MS also is associated with several drawbacks when it comes to bioactivity screening. First of all, the optimum, native conditions for bioactivity screening (pH 7.2, addition of 150 mM sodium chloride) are entirely incompatible with optimum conditions for MS detection which, for ESI-MS, typically require acidic pH values and the presence of organic modifiers to enhance ionization properties of the analytes. Assay development for MS-based assays therefore mainly requires the testing of different assay conditions, particularly the replacement of nonvolatile buffers with MS-compatible volatile buffers. Furthermore, it is essential to monitor ion suppression effects, for example, by the... 

The ionization properties of side-chain substituents will usually carry through into the peptide or protein and influence the behaviour of the polymer. However, the actual pATa values of the amino acid side-chains in the protein are modified somewhat by the position of the amino acid in the chain, and the environment created by other substituents. Typical pATa values are shown in Table 13.2. 

Nevertheless, the addition of a small amount of TFA (0.1%) or 1% phosphoric acid [45], which is necessary in many cases to guarantee proper ionization, again makes the matrices acidic. Here, a tradeoff between the need for smooth conditions within the matrix on the one hand and on the ionization properties on the other hand has to be found for a particular analytical problem. 

Fig. 6.2 Stark structure and field ionization properties of the m = 1 states of the H atom. The zero field manifolds are characterized by the principal quantum number n. Quasidiscrete states with lifetime r > 10-6 s (solid line), field broadened states 5 x 10 10 s < x < 5 x 10-6 s (bold line), and field ionized states r < 5 x 10 10 s (broken line). Field broadened Stark states appear approximately only for W > ITC. The saddle point limit Wc = -2 /E is shown by a heavy curve (from ref. 3).

Berger, R., Friebe, M., Pietzsch, H.J., Scheunemann, M., Noll, B., Fietz, T., Spies, H., and Johannsen, B. (1997). Lipophilicity and ionization properties of some amine-bearing technetium and rh aiiprirf mixed ligand chelates of the same ligand structlrsfitute for Bioinorganic and Radiopharmaceutical Chemistry, Annual Report, pp. 43-47. 

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