Discrimination of constructed air samples using multivariate analysis of full scan membrane introduction mass spectrometry (MIMS) data.

RATIONALE: Volatile and semi-volatile organic compounds (S/VOCs) are important atmospheric pollutants affecting both human and environmental health. They are directly measured as an unresolved mixture using membrane introduction mass spectrometry (MIMS). We apply chemometric techniques to discriminate, classify, and apportion air samples from a variety of sources. METHODS: Full scan mass spectra of lab-constructed air samples were obtained using a polydimethylsiloxane membrane interface and an electron ionization ion trap mass spectrometer. Normalized full scan spectra were analyzed using principal component analysis (PCA), cluster analysis, and k-nearest neighbours (kNN) for sample discrimination and classification. Multivariate curve resolution (MCR) was used to extract pure component contributions. Similar techniques were applied to VOC mixtures sampled from different woodsmoke emissions and from the headspace above aqueous hydrocarbon solutions. RESULTS: PCA successfully discriminated 32 constructed VOC mixtures from nearly 300 air samples, with cluster analysis showing similar results. Further, kNN classification (k = 1) correctly classified all but one test set sample, and MCR successfully identified the pure compounds used to construct the VOC mixtures. Real-world samples resulting from the combustion of different wood species and those associated with water contaminated with different commercial hydrocarbon products were similarly discriminated by PCA. CONCLUSIONS: Chemometric techniques have been evaluated using full scan MIMS spectra with a series of VOC mixtures of known composition containing known compounds, and successfully applied to samples with known sources, but unknown molecular composition. These techniques have application to source identification and apportionment in real-world environmental samples impacted by atmospheric pollutants.

Structure-activity correlation in transfection promoted by pyridinium cationic lipids.

The efficiency of the transfection of a plasmid DNA encoding a galactosidase promoted by a series of pyridinium lipids in mixtures with other cationic lipids and neutral lipids was assessed in CHO-K1 cells. We identify key molecular parameters of the lipids in the mixture - clog P, lipid length, partial molar volume - to predict the morphology of the lipid-DNA lipoplex and then correlate these same parameters with transfection efficiency in an in vitro assay. We define a Transfection Index that provides a linear correlation with normalized transfection efficiency over a series of 90 different lipoplex compositions. We also explore the influence of the same set of molecular parameters on the cytotoxicity of the formulations.

Cyclodextrin ion channels.

Seventeen derivatives of α- and ß-cyclodextrins were prepared from the cyclodextrin per-6-azide by "click" cyclization with terminal alkynes. Sixteen of these "half-channel" compounds showed significant activity as ion channels in planar bilayer members as assessed by the voltage-clamp technique. Activity ranged from persistent square-top openings to highly erratic conductance; mixed behaviours were evident in virtually all data recorded. Some of the erratic behaviours were shown to follow an apparent power-law distribution of open duration times. The activities observed for the suite were summarized using a model-free activity grid method which displays conductance, duration, and opening behaviour. The overall activity shows the clustering of conductance-duration indicating that activity arises from system properties rather that solely as a property of the compound. The activity grids also support an analysis of structure-activity relationships as they apply to the global behaviour of the compounds and reveal the complexity of a single structure change in controlling the distribution of concurrent conductance behaviours. Transient blockage of channel activity by the hydrophobic guest of the cyclodextrin (1-adamantyl carboxylate) is consistent with the formation of an end-to-end dimer channel among several other competing and interconverting structures.

Electrochemical release from gold-thiolate electrodes for controlled insertion of ion channels into bilayer membranes.

A proof-of-principle experiment to inject a sub-attomole amount of a channel compound into a bilayer membrane is described. The system is based on reductive cleavage of a self-assembled gold-thiol monolayer. In 'macroscopic' experiments, 11-biphenyloxyundecane thiol formed well-ordered monolayers by open-circuit or controlled potential deposition. The products of reductive release were determined by chromatographic analysis. In DMF, the sole reduction product is the corresponding disulfide. In acetonitrile and water, only the thiol is detected. The current efficiency is low due to competing electrolysis of water, and to the low solubility of the released thiol or disulfide layer. On a 'microscopic' scale, the half ester of dithiodibutyric acid with gramicidin was deposited on a gold microelectrode under open circuit conditions. The thoroughly washed microelectrode, placed in proximity to a bilayer, released gramicidin only following a 100 ms pulse of reducing potential. The transfer efficiency of this method for controlled positioning of ion channels is estimated to be better than 1 part in 10(5).

Ion channels from linear and branched bola-amphiphiles.

The synthesis and characterization of the ion channel activity of three new bola-amphiphiles is described. These compounds are conceptually derived from a previously reported bis-cyclophane bola-amphiphile through opening of the cyclophanes to acyclic structures and were found to readily form ion channels in planar bilayer membranes as assessed by bilayer clamp single-channel analysis. All three compounds behaved very similarly: the dominant channels formed by all three are Ohmic with specific conductance of 10 +/- 1 pS (NaCl electrolyte) and 39 +/- 1 pS (CsCl electrolyte). Single-ion permeability ratios, determined from dissymmetric electrolyte experiments, showed the selectivity P(Cs(+)) > P(Na(+)) > P(Cl(-)). Less frequently, lower conductance channels were also observed to act independently of the dominant channels. The lifetimes of the dominant channels range from 70 to 280 ms for the three compounds with some very long-lived openings (20-40 s) observed for two of the three. The lower conductance states have shorter lifetimes. This study demonstrates that bis-macrocyclic compounds are not essential for channel formation by bola-amphiphiles, and opens a new class of channel-forming compounds for structure-activity optimization.

Transmembrane ion conductance by an acyclic bolaamphiphile.

[reaction in text] An acyclic bolaamphiphile forms ion channels in vesicles and planar bilayer membranes that are closely similar to the channels formed by related bismacrocyclic compounds. Thus the function of these ion channels does not depend on synthetically difficult macrocyclic subunits.

Synthesis and characterization of photoswitchable lipids containing hemithioindigo chromophores.

The synthesis of four lipids containing the hemithioindigo chromophore as part of the fatty acid is described. Heck reaction of bromophenyl thioacetate esters with acrylonitrile, followed by reduction, ester hydrolysis, and Friedel--Craft acylation--cyclization gave a substituted thioindoxyl that condensed with an alkoxy benzaldehyde to produce the hemithioindigo. "Solventless" nitrile hydrolysis followed by mixed anhydride coupling of the acid with glycerophosphocholine produced lipids bearing two hemithioindigo chromophores. The photochemistry of various hemithioindigo derivatives was studied to confirm the expected photoisomerization in both homogeneous organic solution, and in vesicle bilayer membranes. Characteristic changes in the UV--visible spectra are consistent with fully reversible Z--E photoisomerization. Chromatographic separation of the Z and E isomers of a compound containing a single hemithioindigo chromophore confirmed the spectroscopic analysis and provided a quantitative analysis of the compositions of Z--E isomer mixtures.

Bilayer membranes and transporter models.

Significant advances in the understanding of the mechanisms of artificial transporters appeared in the past year which provide insights into natural transporters and suggest directions for regulation of transport. Parallel developments in the control of bilayer formation and molecular recognition by synthetic lipids have led to simple mimics of 'whole cell' types of processes.

Pores Formed by Bis-macrocyclic Bola-amphiphiles in Vesicle and Planar Bilayer Membranes.

A new series of bis-macrocyclic bola-amphiphiles were prepared, and their transport activities in vesicle and planar bilayer membranes were evaluated. From vesicle experiments, the apparent kinetic order in transporter indicates that aggregates are the kinetically active species. Step-conductance changes observed in planar bilayer membranes indicate that the compounds act as channels. Multiple copies of the same channel form in which the conductance is controlled by the macrocyclic portions of the structures. The pores are ion-selective in the sequence Cs(+) > K(+) > Na(+) > Cl(-), controlled by the polar head-groups of the structures. The data are consistent with a model involving the formation of active dimers.

Coated-wire electrodes containing polymer immobilized ionophores blended with poly(vinyl chloride).

Polymers containing covalently attached 18-crown-6 or 2.2.2 cryptand units were incorporated into plasticized PVC membranes and the composite membranes were examined as potassium ion sensor elements. Ionophores were linked to carboxy-PVC and to poly(acrylic acid) via amide linkages to an alkyl spacer unit. Coated-wire electrodes (CWEs) from the immobilized ionophores gave acceptable responses, but conventional ion-selective membrane electrodes (ISEs) prepared by solvent casting were inactive. Dip-cast membranes did give active ISEs. Potassium electrode performance was independent of the loading of the ionophore within the acrylate support polymer, but depended upon the spacer length. Ion selectivity varied with the ionophore loading within the support polymer. Selectivity is a composite of the ionophore selectivity and ion-exchange interactions with the acrylate backbone, giving selectivities akin to carboxylate substituted crown ethers, notably enhanced monovalent/divalent ion discrimination relative to the ionophore in solution. Polymer immobilization extended the lifetime of active electrodes.