Host-Guest Complexation of Amphiphilic Molecules at the Air-Water Interface Prevents Oxidation by Hydroxyl Radicals and Singlet Oxygen.

The oxidation of antioxidants by oxidizers imposes great challenges to both living organisms and the food industry. Here we show that the host-guest complexation of the carefully designed, positively charged, amphiphilic guanidinocalix[5]arene pentadodecyl ether (GC5A-12C) and negatively charged oleic acid (OA), a well-known cell membrane antioxidant, prevents the oxidation of the complex monolayers at the air-water interface from two potent oxidizers hydroxyl radicals (OH) and singlet delta oxygen (SDO). OH is generated from the gas phase and attacks from the top of the monolayer, while SDO is generated inside the monolayer and attacks amphiphiles from a lateral direction. Field-induced droplet ionization mass spectrometry results have demonstrated that the host-guest complexation achieves steric shielding and prevents both types of oxidation as a result of the tight and "sleeved in" physical arrangement, rather than the chemical reactivity, of the complexes.

Ion mobility-mass spectrometry with a radial opposed migration ion and aerosol classifier (ROMIAC).

The first application of a novel differential mobility analyzer, the radial opposed migration ion and aerosol classifier (ROMIAC), is demonstrated. The ROMIAC uses antiparallel forces from an electric field and a cross-flow gas to both scan ion mobilities and continuously transmit target mobility ions with 100% duty cycle. In the ROMIAC, diffusive losses are minimized, and resolution of ions, with collisional cross-sections of 200-2000 Å(2), is achieved near the nondispersive resolution of ~20. Higher resolution is theoretically possible with greater cross-flow rates. The ROMIAC was coupled to a linear trap quadrupole mass spectrometer and used to classify electrosprayed C2-C12 tetra-alkyl ammonium ions, bradykinin, angiotensin I, angiotensin II, bovine ubiquitin, and two pairs of model peptide isomers. Instrument and mobility calibrations of the ROMIAC show that it exhibits linear responses to changes in electrode potential, making the ROMIAC suitable for mobility and cross-section measurements. The high resolution of the ROMIAC facilitates separation of isobaric isomeric peptides. Monitoring distinct dissociation pathways associated with peptide isomers fully resolves overlapping peaks in the ion mobility data. The ability of the ROMIAC to operate at atmospheric pressure and serve as a front-end analyzer to continuously transmit ions with a particular mobility facilitates extensive studies of target molecules using a variety of mass spectrometric methods.

The unusually high proton affinity of aza-18-crown-6 ether: implications for the molecular recognition of lysine in peptides by lariat crown ethers.

Recent studies have shown that 18-crown-6 ether (18C6) will selectively form adducts in the gas phase with small, lysine containing peptides. The present study extends this work by investigating the ability of aza-18-crown-6 ether (A18C6) and L1 (a simple lariat crown ether derivative of A18C6) to form similar noncovalent adducts with the side chain of lysine in model peptides in the gas phase. The substitution of nitrogen for oxygen greatly increases the proton affinity of A18C6 relative to 18C6 and inhibits the formation of noncovalent adducts with small lysine containing peptides. The proton affinity of A18C6 is determined by the kinetic method to be 250 +/- 1 kcal/mol. This value is much higher than that for diethanolamine (228 kcal/mol) or for 18C6 (231 kcal/mol). This unusually high basicity is rationalized by semi-empirical calculations that suggest a highly symmetrical structure for protonated A18C6 in which the three most distant oxygens are able to fold back and hydrogen bond with the protonated nitrogen. In the case of L1, the lariat side chain is attached by an amide bond, lowering the proton affinity of LI relative to that of A18C6. This allows L1 to form noncovalent adducts with lysine despite the fact that steric repulsion within the cavity of the crown is increased to some extent. The relative ammonium ion affinities of these various crown ethers are shown to serve as qualitative predictors for the molecular recognition of lysine. The order of the relative ammonium ion affinities is 18C6>>L1>A18C6 as determined by the kinetic method. These results suggest that the substitution of nitrogen for oxygen in the crown ether is not beneficial for the molecular recognition of lysine.