Ann P. Hunter

Relationship between in-source and post-source fragment ions in the matrix-assisted laser desorption (ionization) mass spectra of carbohydrates recorded with reflectron time-of-flight mass spectrometers

Abstract The reflectron matrix-assisted laser desorption (ionization) [MALD(I)] mass spectra of many complex carbohydrates (and other compounds) recorded with time-of-flight (TOF) instruments fitted with time-lag focusing (delayed extraction) contain focused fragment ions resulting from fragmentation within the ion source, together with unfocused (metastable) fragments derived from post-source decay. The latter ions are particularly abundant from carbohydrates containing sialic acids as the result of loss of the sialic acid moieties. The relative abundance of these metastable ions was found to decrease with ion-source pulse delay whereas the mass difference between the focused and unfocused ions increased. The mass difference was also found to vary with the instrument used to record the spectra. An equation has been derived that links the masses of the precursor ion with the two fragment ions (focused and unfocused). It contains a term reflecting the geometry of the mass spectrometer and can, thus, be used with any TOF instrument. The formula can be rearranged such that the mass of an absent molecular ion can be predicted from the masses of the two fragment ion peaks. The metastable ions can also be used to confirm fragmentation pathways in the manner similar to that used for spectra recorded with magnetic sector instruments. Metastable ions were also found in spectra recorded with an orthogonal-TOF mass spectrometer using field ionization. The formulae relating the masses of the fragment and precursor ions were also found to apply in this case, further demonstrating their instrument independence.

Water soluble platinum(II) and palladium(II) complexes of alkyl sulfonated phosphines

Abstract The reactions of the alkylsulfonated phosphines LM=Ph 2 P(CH 2 ) n SO 3 Na/K ( n =2, 3, 4) with K 2 PtCl 4 and K 2 PdCl 4 have been studied in homogeneous aqueous solution as a function of pH. In homogeneous acidic solution the protonated phosphines react to give cis - and trans -PtCl 2 (LH) 2 . The biphasic reaction between 1,5-cyclooctadiene platinum(II) chloride in dichloromethane and acidified aqueous LNa/K gives a higher proportion of the cis isomer. In neutral solution the initial reaction to give [PtCl(LNa/K) 3 ] + Cl − is followed by slow formation of cis -PtCl 2 (LNa/K) 2 . K 2 PdCl 4 reacts more rapidly to give PdCl 2 (LNa/K) 2 . In homogeneous alkaline solution rapid oxidation of the phosphine occurs with only small amounts of platinum complex being observable. The biphasic reaction yields phosphine oxide in the aqueous layer and a small amount of the chelate complexes PtL 2 in the organic. Representative complexes have been isolated and characterised and the mechanisms for the reactions discussed. The electrospray mass spectra of solutions of the isolated complexes have been recorded in both positive and negative ionisation modes. The positive ionisation spectra are complicated, but platinum and palladium containing ions derived from loss of chloride, H + and HCl are observed in the negative ionisation spectra.

Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of N-Linked Carbohydrates and Related Compounds

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of N-linked carbohydrates (those linked to asparagine in glycoproteins) was first reported in 1991 by Mock et al. [1] and provides a simple and sensitive method for producing spectra from underivatized N-linked glycans. It has been estimated that MALDI is about 10 to 100 times more sensitive than fast-atom bombardment FAB) for glycan analysis [2], and does not require derivatization by permethylation or peracetylation, as required by FAB, to obtain a signal. When recorded with linear time-of-flight (TOF) mass spectrometers, MALDI mass spectra of neutral N-linked glycans contain few, if any, fragment ions and so the technique provides a rapid method for glycan profiling. The spectrum of N-linked glycans released from chicken ovalbumin, shown in Fig. 1 illustrates the type of result that can be obtained. Structures of the glycans producing the peaks are shown in Table 1. The masses of the peaks can lead directly to the compositions ofthe constituent glycans in terms oftheir isobaric monosaccharide compositions, but determination of further structural details requires production of fragment ions or coupling to other methods such as exoglycosidase digestion as described below. Acidic N-linked glycans produce more fragmentation than neutral glycans and are generally more difficult to handle. Nevertheless, they provide excellent spectra under the right conditions. This paper summarizes work in this area and presents examples of data recorded in our laboratory.

Oxidation of guanine Nucleosides to 4-amidinocarbamoyl-5-hydroxyimidazoles by dimethyldioxirane

Abstract Final oxidation products generated from guanosine and 2′-deoxyguanosine by reaction with dimethyldioxirane have been identified as 4-amidinocarbamoyl-5-hydroxyimidazoles.