Boroacetylation of carbohydrates. Correlations between structure and mass spectral behavior in monoacetylhexose cyclic boronic esters

Abstract

The interaction of an acetyl moiety with boron accompanied by the loss of alkyl radical is shown to occur following electron beam ionization in the mass spectrometer. This is similar to the interaction of phosphate with boron previously shown to occur in alkaneboronic esters of certain sugar phosphate dimethyl esters. I n the case of acetates, the interacting moiety can frequently be introduced into a hexose in an equilibrium reaction in which treatment with an alkaneboronic acid is followed by acetylation of the remaining hydroxyl. The interaction, following ionization, is shown to be localized to the adjacent cyclic boronate in the case of D-glucose and D-galactose alkaneboronate acetates. The use of this interaction as a stereochemically sensitive probe of structure is demonstrated. In the course of this work, the cyclic bis(a1kaneboronates) of monoacetylated Dglucose, D-galactose, D-mannose, D-idose, D-allOSe, D-altrose, D-talose, D-glucose, D-fructose, D-sorbose, D-tagatose, and Dpsicose were examined by mass spectrometry. High-resolution data, as well as specific deuterium labeling of the six carbon atoms of glucose, reveal the origin of characteristic classes of ions.The preparation of a substance for mass spectral analysis by gas chromatography-mass spectrometry (GC-MS) is a n opportunity to exercise some control over the fragmentation process of the ionized molecule. Cyclic boronic esters of carbohydrates are useful because they have steric requirements for their formation and thus confer an additional degree of specificity on stereoisomers. In this respect they resemble isopropylidene derivatives. The boronic esters, however, are rapidly formed in reactions which give quantitative conversions with many sugars.The isopropylidene ketals2 have a further similarity to the alkaneboronates in that both are capable, on electron beam ionization, of losing an alkyl side chain from the derivative moiety to produce a high abundance even electron ion. This similarity is only a formal one, however, because of the different electronic structures of boron and carbon. In the isopropylidene case, the radical loss from carbon produces an ion that is readily stabilized by the 1,3-dioxolane structure. The situation with boron is very different in that the cyclic boronate is incapable of stabilizing the charge left by the departing radical without the intervention of another functionality. W e have reported on this interaction in the case of cyclic alkaneboronates of sugar phosphates3 In the latter the phosphate is the interacting moiety which, when favored by proximity, supports the formation of high abundance [M -alkyl]+ ions.The principal subject of the present paper is the nature of this process in monoacetates of sugar alkaneboronates. The interaction of the acetate with boron appears weaker than that of phosphate, thus more stereoselective, and therefore a more useful probe of structure. Furthermore, the single acetyl moiety is easily introduced in equilibrium reactions with sugars and alkaneboronic acids. The reactio...

The postprandial rates of glycogen and lipid synthesis of lean and obese female Zucker rats

“…The reasons for these apparently con¯icting results may relate to the amount and composition of the substrates administered and to the timing of the measurements. Thus in one study the incorporation of 14 C from a 14 C-alanine load into hepatic glycogen was found to be higher in obese Zucker rats in both the fed and 24 h fasted state, 2 whereas in another the incorporation of 14 C-alanine into hepatic glycogen was lower in obese rats 3 ± 4 h after a meal while the incorporation of 14 C-glycerol and 3 H-glucose into hepatic glycogen was similar between obese and lean rats. 1 Hence it is necessary to make simultaneous measurements of glycogen and lipid synthesis in response to a ®xed test meal in order to determine whether the partitioning of substrates between these two processes is different from normal in obese Zucker rats.…”

Diurnal rhythm in endogenous glucose production is a major contributor to fasting hyperglycaemia in type 2 diabetes. Suprachiasmatic deficit or limit cycle behaviour?

“…The lactate was removed from the resin using formic acid (0.5 N), which was then evaporated to dryness, dissolved in water and scintillation fluid added. The radioactivity was determined in a liquid scintillation [17]. The butaneboronyl derivative was analysed on a GC/MS (Agilent Technologies, Palo Alto, CA, USA) using a Zebron 1701 column (Phenomenex, Torrence, CA, USA) and selective ion monitoring at 297 and 303 m/z, and a standard curve of known enrichments.…”

Determination of Complex Isotopomer Patterns in Isotopically Labeled Compounds by Mass Spectrometry

“…The 13 C natural abundance that is lost by the incorporation of 13 C labels into the molecule will be the difference between eq 12 and 13. We can subtract this difference between eq 12 and 13 from the observed (M+2)/M to get the corrected (M+2)/M: (14) Equation 14 can be simplified to give the useful form of the (M+2)/M 13 C correction: (15) The 13 C correction for the (M+2)/M isotope can be carried out in our matrix equation by adjusting the a 0 (2) values for each isotopomer containing 13 C labels: (16) The above 13 C correction is carried out on the measured isotopic distribution of the unlabeled isotopomer's M+1 and M+2 abundances using eq 11 and 16, respectively. The unlabeled isotopomer's isotopic distribution with the 13 C correction is used in the A matrix in eq 9:…”