ESI-MS studies

Zhu, J., Shi, Z. (2003). ESI-MS studies of polyether surfactant behaviors in reversed-phase HPLC system. Int. J. Mass Spectrom. 226(3), 369-378. 

The kinetics of iron(III) dissociation from a series of dihydrox-amate siderophores and siderophore mimics, including rhodo-torulic acid (3) and alcalagin, have been investigated (52,127,128, 177,178). ESI-MS studies show that these systems form multiple species as a function of pH and siderophore/iron ratio (128). The lability of these systems and the resultant multiple species leads to several parallel paths to iron(III) dissociation (177). Both the distribution of structures and kinetics of dissociation were shown to be dependent on the length of the spacer chain between the dihydroxamate donor groups (52,127). 

In the ESI MS studies some care must be taken due to the possible formation of false peaks (73). The time-of-flight (TOF) ESI MS equipment of a new generation with an orthogonal interface design allows one to avoid somewhat experimental artifacts of this kind. 

As discussed above, the development of mild MS techniques has led to further progress in the determination of proanthocyanidin size distribution. In particular, ESI-MS studies have demonstrated that prodelphinidin and procyanidin units coexist within the polymers, where they seem distributed at random. A list of mass signals attributed to proanthocyanidins detected in grape or wine extracts is given in Table 5.2. 

Olefin isomerization, with Claisen rearrangement, 1, 365 Olefin metathesis with alkyllead, 9, 415 in aqueous media, 1, 834 ESI—MS studies, 1, 812 in high-throughput catalyst discovery, 1, 365 in ionic liquids, 1, 869 for polymerization characteristics, 1, 149 Grubbs catalysts, 1, 151 Schrock catalysis, 1, 150... 

The dimerization was established also by the observation of upfield shifted signals (A8 = -2.5-3.0 ppm) for the included guest,270 by ESI-MS studies on dimers... 

In 1998 evidence for the existence of a commonly invoked intermediate for a variety of oxygen transfer reactions involving the ]0 = Mn ] moiety was obtained by interception of an ]0 = Mn (salen)(OIPh)] complex (Fig. lA) and a binu-clear [pi-0(Mn (salen)(0IPh))2] complex (Fig. IB) [36]. Later, an ESI-MS study on the Mn-catalyzed oxidative kinetic resolution of secondary alcohols by PhI(OAc)2 reported the observation of a similar manganese salen intermediate [MnV(salen)(PhIO)(OCH(CH3)Ph]+ (Fig. 1C) [6]. This, along with the observation of [Mn "(salen)(PhI(OAc)2)] (Fig. ID), allowed the proposal of a possible catalytic cycle for the kinetic resolution of secondary alcohols by this system. 

Scheme 1. Proposed catalytic cycle forthe decomposition of hydroperoxide by Mn-MeTACN based on ESI-MS studies.

There has been some Interest In the study of ruthenium-catalyzed systems. For example, hydrogenation reactions catalyzed by Ru(ll)-arene complexes have been the focus of a few ESI-MS studies, the first of which appeared in 2000. In this report, three species (Eig. lOA-C) were detected in a mixture of the... 

B.N. Green, S.N. Vinogradov, An ESI-MS study of the subunit structure of the giant hemoglobin from the leech Nephelopsis oscura, J. Am. Soc. Mass Spectrom., 15 (2004) 22. 

In early negative-ion ESI-MS studies, halogenated solvent additives were applied to reduce the risk of discharge formation (Ch. 6.3.2). The use of 1,1,1,3,3,3,-hexafluoro-2-propanol (HFIP) to the mobile phase was proposed for oligonucleotides [22]. This is for instance applied in the rapid characterization of synthetic oligonucleotides by microcapillary LC-MS on a quadrupole-time-of-flight hybrid (Q-TOF) instrument [20]. 

Przybylski, P., Pospieszny, T., Huczynski, A., and Brzezinski, B. (2008b). El MS and ESI MS studies of the bisesquiterpene from cotton seeds Gossypol and its aza-derivatives. J. Mass Spectrum. 43, 680-686. 

Interestingly, the self-assembly of catenanes was also observed from a three-component system. Three components, la, 2, and 34, self-assembled in water in a 2 1 1 stoichiometry to give catenane 35 in very high quantitative yield of 94% (Scheme 12). The structure of 35 was conformed by X-ray crystallography (Figure 8), ESI-MS, and NMR. It is noteworthy that the thermodynamic stability of 35 overcomes the combination problem which arises in the self-assembly of larger sets of components. At least, the formation of the three component macrocycles 3a, 36, and 37 is possible because their thermodynamic stability is comparable with that of 35. Further, the formation of more flexible and expanded catenane 40 was also observed when the components la, 38, and 39 were combined in water in 2 1 1 ratio (Scheme 13). The structure of 34 was deduced from an ESI-MS study. 

A different approach is based on the Cj-symmetrical triol amines introduced by Nugent [30]. Together with titanium tetraisopropoxide they afford titanium species of type 17 which react further with cumyl hydroperoxide to give a rigid monomeric titanium peroxide 18 [31].Thelatter (withR=Ph) was characterized by NMR spectroscopy and ESI-MS studies [32]. 

In summary, through the use of rapid chemical quench techniques, multiple studies demonstrated the formation of a single tetrahedral intermediate in the reaction pathway of EPSP synthase (Scheme 4, pathway a) which is formed by an attack of the 5-OH group of shikimate-3-phosphate on C-2 of PEP. A complete kinetic and thermodynamic description of this enzyme reaction pathway could be demonstrated, including the isolation and structural elucidation of a tetrahedral enzyme intermediate as originally proposed by Sprinson. This work established the catalytic mechanism and definitively showed that no covalent enzyme—PEP adduct is formed on the reaction pathway. Subsequent work using rapid mixing pulsed-flow ESI—MS studies and solution phase NMR " provides additional support for the catalytic pathway in Scheme 4, pathway a. 

Figure 7 ESI-MS studies of tetramolecular quadruplexes. The TG5T oligonucleotide was incubated in 0.15 M pH 7.0 ammonium acetate buffer for various amounts of time, then analysed by electrospray mass spectrometry (negative mode). From the initial peak corresponding to the TG5T single-strand (SS), one can see the gradual conversion to a quadruplex (G4), as well as the presence of dimeric (Di) and trimeric (Tri) species...

Beyond the characterisation of primary structures, the direct analysis tertiary structure states and even non-covalent supramolecular complexes by mass spectrometry have not been considered feasible in previous work. In a few cases tertiary structure-dependences have been found, e. g. specific fragmentations in FAB mass spectra of a-helical polypeptides and some MALDI and PD mass spectra of proteins suggesting some native-like structure of macromolecular ions [106, 107]. This situation has changed drastically recently with the analytical development of ESI-MS. A substantial number of ESI-MS studies have demonstrated the identification of supramolecular complexes of biopolymers, as well as specific non-covalent complexes with low-molecular weight constituents [15—18, 31]. In contrast to other ionisation methods in which, predominantly, singly charged ions are produced (EAB,... 

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