Proton-ionizable families

The main advantage of the zirconium family of columns is their stability from pH 1 to 10 and at temperatures from ambient to 200°C. Their separating character also differs from silica-based columns due to the lack of ionizable surface molecules. Silica above pH 3.0 loses a proton to form anionic silicate moieties, giving the bonded-phase silica column some anionic as well as nonpolar organic column characteristics (Fig. 6.8a). 

Biomolecules such as proteins and oligonucleotides present mass spectra that are complicated by the presence of multiply charged families of ions. The parent species, for example, generated by a soft ionization method such as FAB of ESI, will yield several m/z peaks in which m is equal to a constant plus (protein) or minus (nucleic acid) a variable number of proton masses, while z is a variable. Egg white lysozyme, for example, yields ESI-mass spectra with fine parent peaks between m/z values of 1194 and 1791 the corresponding z values are 12-8 [see Fig. 15.9(a)]. 

Larger analytes with more protonation-deprotonation sites yield larger families of peaks for each fragment, and the overlap of mlz ranges for different fragment families can further complicate the spectra of biomolecules. For this reason, soft ionization methods, that produce few fragments, are particularly useful for biomolecule MS. 

For a given family of compounds, there exists a certain relationship between the proton affinity and the core ionization energy of the atom which is protonated . The latter could be approximated by the Is-orbital energy, e(ls), of the relevant atom of the conjugate anion, which is relatively stable with respect to the small variations in the basis sets. Calculations demonstrated that for phenol derivatives, a linear relationship (equation 30) equally exists ... 

CA provides the best-defined example of an ionized Zn -bound water mechanism. " Thrl99 and orientation residues such as Glul06, and proton shuttle residues such as His64, are needed to make CA the fastest biological catalyst. 6-Pyruvoly-tetrahydropterin synthase,E. coli fuculose-1-phosphate aldolase, " " and E. coli fructose-1,6-bisphosphate aldolase belong to the CA family, although the reactions these enzymes are involved in are different. 

The gibberellin family of compounds, derived from the olefin (-)-kaurene, have been identified in some fungi, and all plants where they function as phytohormones. In plants, (-)-kaurene is formed by two diterpene cyclases working in sequence, copalyl diphosphate synthase (enf-kaurene synthase A or (-)-CPP synthase) and enf-kaurene synthase (enf-kaurene synthase B or (-)-KS). (-)-CPP synthase catalyzes the protonation-initiated cyclization of GGPP to (-)-CPP, while (-)-KS cyclizes (-)-CPP into (-)-kaurene via an ionization-initiated mechanism (see Scheme 18). 

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