Determination of Bond Dissociation Energies Using Electrospray Tandem Mass Spectrometry and a Derived Effective Reaction Path Length Approach

A new approach for calculating bond dissociation energies (BDEs) from ES-MS/MS measurements has been developed. The new method features a "derived effective reaction path length" that has been applied to measure BDEs of alkali metal (Li+) adducts and halide (Cl-) adducts of monoacylglycerol, 1,2-diacylglycerol, and 1,3-diacylglycerol lipids. Also studied were lithium-bound dimers of monoacylglycerols, 1,2-diacylglycerols, and 1,3-diacylglycerols. BDEs for the adducts and dimers of the lipids were derived from collision-induced dissociation experiments using a triple quadrupole mass spectrometer with electrospray as the ionization source. Mass spectral data were used to empirically derive a single-exponential growth equation that relates product cross section to collision energy. From these single-exponential equations, a general second-order polynomial was derived using a multivariate growth curve model that enables prediction of BDEs of unknown complexes. Mass spectral results were compared to computer-generated bond dissociation energies using Becke-style three-parameter density functional theory (B3LYP, employing the Lee-Yang-Parr correlation functional), with excellent agreement between experimental and theoretical energy values. The newly developed method is general in nature and can be used for the measurement of metal or halide ionic adduct bond dissociation energies and for the measurement of bond energies of noncovalent interactions such as dimer dissociation energies. The validity of the method has been rigorously established using a triple quadrupole, but it may also be applied to other mass spectrometers that allow user control of the collision cell potential.

Identification, quantification and comparison of major non-polar lipids in normal and dry eye tear lipidomes by electrospray tandem mass spectrometry

Millions of individuals suffer from a health condition known as keratoconjunctivitis sicca (KCS, also known as 'dry eye'). Studies have indicated that the lipids in the tear film layer, which covers the outer portion of the eye, may be directly correlated with the existence of dry eye syndrome. By identifying and comparing the major, non-polar lipids in normal eye tears with a dry eye model, it may be possible to identify a symptom of, or a contributing factor to, dry eye. Electrospray tandem mass spectrometry (ES-MS/MS) was used to identify and compare the non-polar lipids, detected as lithium adducts, from normal and dry eye tear samples obtained from rabbits. A limited number of normal human tear samples were also examined for lipid content, and a close resemblance to rabbit was observed. Three distinct regions were delineated in the ES mass spectra of the non polar lipids, m/z 20-500, 500-800 and 800-1100. A common feature noted among identified lipid components was a glycerol backbone with fatty acyl substituents attached. Product ion spectra were obtained for lithiated monoacyl-, 1,2- and 1,3-diacyl- and triacylglyceride standards. Newly proposed structures and fragmentation pathways for the major product ions are presented for the 1,2- and 1,3-diglycerides, and also for the monoglyceride. New approaches to distinguishing asymmetric 1,2-diglycerides and 1,2- from 1,3-diglycerides are proposed. For the rabbit tear samples, the m/z 20-500 range contains monoester diols with empirical formulas C(n)H(2n)O(4), the m/z 500-800 range includes diesters with empirical formulas C(n)H(2n-2)O(5) and the m/z 800-1100 range contains triesters with empirical formulas C(n)H(2n-4)O(6). Also found in the extracts were three isoprene acetals (terpenoids).

Formation and decompositions of chloride adduct ions,

The ability to promote chloride-attachment ions of the form [M + Cl]- in negative ion electrospray ionization mass spectrometry (ESI-MS) has been developed using chlorinated solvents such as chloroform and carbon tetrachloride. This approach expands the current capabilities of negative ion ESI-MS by enabling detection of analytes that lack acidic sites and thus exhibit weak [M - H]- signals. In contrast to the remote-site collision-induced dissociation (CID) often observed in positive ion ESI-MS/MS for alkali metal cation adducts, the decomposition of chloride adducts usually proceeds via competitive dissociations to form Cl-, which is not structurally informative, or [M - H]-. The latter can provide structural information via consecutive decompositions. For compounds having higher gas-phase acidities than HCl, a low CID collision energy can promote the formation of [M - H]-, whereas for the majority of compounds with lower gas phase acidities than HCl, higher collision energies generally improve the relative yield of [M- H] . Because chloride attachment occurs primarily at electrophilic hydrogens, the daughter ion ratio, Cl-/[M - H]-, depends primarily upon the difference in gas phase acidity between the analyte molecule and HCl. At higher collision energies, entropic factors take on increased importance in determining the product ratio. The difference between the deltaS(0) terms for formation of Cl and formation of [M - H]- has been estimated for a series of substituted phenols and a series of acetic acid analogs. Finally, a novel neutral loss of CH3Cl from glycerophosphocholine and from ganglioside GM3 methyl ester is reported.

Stereospecificity of monoacylglycerol and diacylglycerol acyltransferases from rat intestine as determined by chiral phase high-performance liquid chromatography

Using chiral phase high-performance liquid chromatography of diacylglycerols, we have redetermined the ratios of 1,2-/2,3-diacyl-sn-glycerols resulting from acylation of 2-monoacylglycerols by membrane bound and solubilized triacylglycerol synthetase of rat intestinal mucosa. With 2-oleoyl[-3H]glycerol as the acyl acceptor and oleoyl-CoA as the acyl donor, 97-98% of the diacylglycerol product was 1,2(2,3)-dioleoyl-sn-glycerol, 90% of which was the sn-1,2- and 10% the sn-2,3-enantiomer. The remaining diacylglycerol (less than 3%) was the sn-1,3-isomer. The overall yield of acylation products was 70%, of which 60% were diacylglycerols and 40% triacylglycerols. With 2-oleylglycerol ether as the acyl acceptor and [1-14C]oleoyl-CoA as the acyl donor, 90% of the diradylglycerol was 1-oleoyl-2-oleyl-sn-glycerol and 10% was the 2-oleyl-3-oleoyl-sn-glycerol. The diradylglycerols made up 96% and the triradylglycerols 4% of the radioactive product. With 1-palmitoyl-sn-glycerol as the acyl acceptor and [1-14C]oleoyl-CoA as the acyl donor, the predominant reaction product was 1-palmitoyl-3-oleoyl-sn-glycerol. The 3-palmitoyl-sn-glycerol was not a suitable acyl acceptor. Both 1,2- and 2,3-diacyl-sn-glycerols were substrates for diacylglycerol acyltransferase as neither isomer was favored when 1,2-dioleoyl-rac-[2-3H]glycerol was used as the acyl acceptor. There was a marked decrease in the acylation of the 1(3)-oleoyl-2-oleyl-sn-glycerol to the 1,3-dioleoyl-2-oleyl-sn-glycerol. It is concluded that neither monoacylglycerol nor diacylglycerol acyltransferase exhibit absolute stereospecificity for acylglycerols as fatty acid acceptors.