Negative-ion electrospray and fast atom bombardment mass spectrometry of esters of boron acids

Selective detection of carbohydrates and their peptide conjugates by ESI-MS using synthetic quaternary ammonium salt derivatives of phenylboronic acids

We present new tags based on the derivatives of phenylboronic acid and apply them for the selective detection of sugars and peptide-sugar conjugates in mass spectrometry. We investigated the binding of phenylboronic acid and its quaternary ammonium salt (QAS) derivatives to carbohydrates and peptide-derived Amadori products by HR-MS and MS/MS experiments. The formation of complexes between sugar or sugar-peptide conjugates and synthetic tags was confirmed on the basis of the unique isotopic distribution resulting from the presence of boron atom. Moreover, incorporation of a quaternary ammonium salt dramatically improved the efficiency of ionization in mass spectrometry. It was found that the formation of a complex with phenylboronic acid stabilizes the sugar moiety in glycated peptides, resulting in simplification of the fragmentation pattern of peptide-derived Amadori products. The obtained results suggest that derivatization of phenylboronic acid as QAS is a promising method for sensitive ESI-MS detection of carbohydrates and their conjugates formed by non-enzymatic glycation or glycosylation.

Esterification of borate with NAD+ and NADH as studied by electrospray ionization mass spectrometry and 11B NMR spectroscopy

This paper describes for the first time the direct measurement of boric acid (B(OH)(3)) and borate (B(OH)(4) (-)) adduction to NAD(+) and NADH by electrospray ionization mass spectrometry (ESI-MS) and (11)B NMR spectroscopy. The analysis demonstrates that borate binds to both cis-2,3-ribose diols on NAD(+) forming borate monoesters (1 : 1 addition), borate diesters (1 : 2 addition) and diborate esters (2 : 1 addition), whereas, only borate monoesters were formed with NADH. MS in the negative ion mode showed borate was bound to a cis-2,3-ribose diol and not to the hydroxyl groups on the phosphate backbone of NAD(+), and MS/MS showed that the 1 : 1 addition monoester contained borate bound to the adenosine ribose. Boron shifts of borate monoesters and diesters with NAD(+) were observed at 7.80 and 12.56 ppm at pH 7.0 to 9.0. The esterifications of borate with NAD(+) and NADH were pH dependent with maximum formation occurring under alkaline conditions with significant formation occurring at pH 7.0. Using ESI-MS, the limit of detection was 50 micro M for NAD(+) and boric acid (1 : 1) to detect NAD(+)-borate monoester at pH 7.0. These results suggest esterification of borate with nicotinamide nucleotides could be of biological significance.

Structural analysis of diols by electrospray mass spectrometry on boric acid complexes

A method is presented to characterize diols using negative ion electrospray (ES) mass spectrometry in combination with collision-induced dissociation tandem mass spectrometry (MS/MS). The analyte diol is added to a solution containing an ethylene glycol/boric acid [2:1] complex and then subjected to infusion ES. The following boric acid complexes are formed: (i) a complex with two ethylene glycol molecules, (ii) a mixed ethylene glycol/analyte complex, and (iii) a complex with two analyte molecules. The first complex serves as a reference for the assessment of the extent of complex formation with the analyte. The ES mass spectra of acyclic vicinal diols all feature intense mixed complex signals, indicative of efficient complex formation. Chemical fine tuning is achieved by MS/MS experiments. Thus, although the (2R,3R)-(-)-2,3-butanediol and meso-2,3-butanediol stereo-isomers show the same complexation efficiency, MS/MS experiments reveal pronounced structure characteristic differences. By contrast, 1,3- and 1,4-diols are less prone to complex formation as they give only weak signals relative to the reference. For cyclic vicinal diols only the cis isomer produces an intense mixed complex, whose MS/MS spectrum is characteristically different from that of the trans form. The above procedure does not permit an unambiguous differentiation of acyclic polyhydroxy compounds like mannitol and sorbitol. However, structurally related methyl glycosides show characteristic MS/MS spectra. Our findings indicate that the above simple procedure may be useful to probe the presence and structure of diols and other polyols in aqueous solutions. Copyright 1999 John Wiley & Sons, Ltd.

Direct analysis of sugar alcohol borate complexes in plant extracts by matrix-assisted laser desorption/ionization fourier transform mass spectrometry

Matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI/FTMS), operating in the negative ion mode, is used to directly observe sugar alcohol borate complexes in a number of plant fractions. The method involves virtually no sample workup and, in the case of celery phloem sap, requires only 40 nL of sap to observe the borate complex. The isolation and characterization of such soluble borate complexes is important in understanding the distribution of boron in plants. The results show that the complexes are composed of two mannitol or sorbitol ligands (L) complexed to a single borate center (B). In some cases, the boron is complexed to non-alditol monosaccharides. Sustained off-resonance collision irradiation dissociation of the BL2- complex, where L is a mannitol, gives fragments that confirm the proposed structure. Complexes of larger oligosaccharides have also been successfully observed using MALDI/FTMS. Semiempirical molecular orbital calculations (AM1) of the mannitol BL2- complex show that the most favorable configuration is with carbons 3 and 4 of both mannitol residues complexed to the borate. This allows maximum interaction of the remaining hydroxyls with the borate center.

Negative ion formation in electrospray mass spectrometry

Analytical and Chemical Sciences, Research Triangle Institute, Research Triangle Park, North Carolina, USA Negative ion electrospray (ES) operating on a single quadrupole mass spectrometer for the detection of low-molecular-weight molecules is discussed. The ES interface was operated at a positive cylindrical electrode potential to produce negative ions, and the results obtained were compared to the positive ion mode. As in the case of operation in the more common positive mode, negative ions with varying degrees of solvation and structurally relevant fragments can be obtained from a variety of solute species, including β-lactam antibiotics, aminoglycosides, aminocyclitols, tetracyclines, sulfonamides, nucleotides, peptides, and explosives. No fragmentation of parent species, except those from some labile explosives, was provided because low potential differences are applied between the capillary and the first skimmer, and electrical discharge is avoided in the gas phase. An increase in the capillary voltage resulted in collision-induced decomposition to produce structurally relevant fragment ions in both operation modes. An evaluation of representative chromatographic solvents indicated that 2-propanol added with oxygen in the ES bath gas is best suited to suppress electrical (corona) discharge phenomena in negative ion operation, whereas it aids in solution nebulization, desolvation, and transfer of ions in solution to the gas phase. For positive ion mode, no such precaution was necessary. Conditions that promote the formation of ions in solution usually improve ES response. Therefore, an increase in the solvent pH can increase the sensitivity in negative ion ES ionization. Negative ion ES offers the advantage of providing complementary structural information to help in the characterization of an unknown compound or to confirm a certain tentatively proposed structure. Nucleotides and explosives were best characterized in negative ion mode owing to the ease with which they form anions in solution, and they could be detected down to the l-pg /gML level.