The mechanism of the loss of H2 and H2O from 1,4-cyclohexanediol anion and related compounds

First Direct Evidence of Interpartner Hydride/Deuteride Exchanges for Stored Sodiated Arginine/Fructose-6-phosphate Complex Anions within Salt-Solvated Structures

Mass spectrometric investigations of noncovalent binding between low molecular weight compounds revealed the existence of gas-phase (GP) noncovalent complex (NCC) ions involving zwitterionic structures. ESI MS is used to prove the formation of stable sodiated NCC anions between fructose (F6P) and arginine (R) moieties. Theoretical calculations indicate a folded solvated salt (i.e., sodiated carboxylate interacting with phosphate) rather than a charge-solvated form. Under standard CID conditions, [(F6P+R-H+Na)-H]^- competitively forms two major product ions (PIs) through partner splitting [(R-H+Na) loss] and charge-induced cross-ring cleavage while preserving the noncovalent interactions (noncovalent product ions (NCPIs)). MS/MS experiments combined with in-solution proton/deuteron exchanges (HDXs) demonstrated an unexpected labeling of PIs, i.e., a correlated D-enrichment/D-depletion. An increase in activation time up to 3000 ms favors such processes when limited to two H/D exchanges. These results are rationalized by interpartner hydride/deuteride exchanges (⟨HDX⟩) through stepwise isomerization/dissociation of sodiated NCC-d11 anions. In addition, the D-enrichment/D-depletion discrepancy is further explained by back HDX with residual water in LTQ (selective for the isotopologue NCPIs as shown by PI relaxation experiments). Each isotopologue leads to only one back HDX unlike multiple HDXs generally observed in GP. This behavior shows that NCPIs are zwitterions with charges solvated by a single water molecule, thus generating a back HDX through a relay mechanism, which quenches the charges and prevents further back HDX. By estimating back HDX impact on D-depletion, the interpartner ⟨HDX⟩ complementarity was thus illustrated. This is the first description of interpartner ⟨HDX⟩ and selective back HDX validating salt-solvated structures.

Stereochemical effects during [M-H]- dissociations of epimeric 11-OH-17beta-estradiols and distant electronic effects of substituents at C(11) position on gas phase acidity

The affinity of estradiol derivatives for the estrogen receptor (ER) depends strongly on nature and stereochemistry of substituents in C(11) position of the 17beta-estradiol (I). In this work, the stereochemistry effects of the 11alpha-OH-17beta-estradiol (III(alpha)) and 11beta-OH-17beta-estradiol (III(beta)) were investigated using CID experiments and gas-phase acidity (DeltaH degrees (acid)) determination. The CID experiments showed that the steroids decompose via different pathways involving competitive dissociations with rate constants depending upon the alpha/beta C(11) stereochemistry. It was shown that the fragmentations of both deprotonated [III(alpha)-H]- and [III(beta)-H]- epimers were initiated by the deprotonation of the most acidic site, i.e. the phenolic hydroxyl at C(3). This view was confirmed by H/D exchange and double resonance experiments. Furthermore, the DeltaH degrees (acid) of both epimers (III(alpha) and III(beta)), 17beta-estradiol (I), and 17-desoxyestradiol (II) was determined using the extended Cooks' kinetic method. The resulting values allowed us to classify steroids as a function of their gas-phase acidity as follows: (III(beta)) >> (II) > (I) > (III(alpha)). Interestingly, the alpha/beta C(11) stereochemistry appeared to influence strongly the gas-phase acidity. This phenomenon could be explained through stereospecific proton interaction with pi-orbital cloud of A ring, which was confirmed by theoretical calculation.

Effects of functional group interactions on the bimolecular and dissociation reactions of diols

The influence of functional group interactions on the bimolecular and dissociation reactions of diols were examined in a quadrupole ion trap mass spectrometer. Reactions of dimethyl ether ions with diols resulted in formation of (M + H)(+) ions and (M + 13)(+) ions (by net methyne addition). The product distribution depended on the relative separation of the hydroxyl groups within each diol, with the more proximate diols producing the greatest abundance of (M + 13)(+) ions compared to (M + H)(+) ions. The enhancement of the formation of (M + 13)(+) ions is attributed to the capability for electrostatic interactions between the hydroxyl groups and the electropositive methylene group of the methoxymethylene reagent ion. The enhancement is most significant for diols that can adopt five- or to a lesser extent six-membered ring transition states (i.e, any 1,2 or 1,3 diol). Collision-activated dissociation (CAD) techniques, including both sequential activation experiments (MS (n) ) and comparison of CAD spectra for model compounds, suggest that the (M + 13)(+) ions are protonated cyclic diethers.