Magnetic field gradients in solid state magic angle spinning NMR.

Magnetic field gradients have proven useful in NMR for coherence pathway selection, diffusion studies, and imaging. Recently they have been combined with magic angle spinning to permit high-resolution measurements of semi-solids, where magic angle spinning averages any residual dipolar couplings and local variations in the bulk magnetic susceptibility. Here we show the first examples of coherence pathway selection by gradients in dipolar coupled solids. When the gradient evolution competes with dipolar evolution the experiment design must take into account both the strength of the dipolar couplings and the means to refocus it. Examples of both homonuclear and heteronuclear experiments are shown in which gradients have been used to eliminate the need for phase cycling.

Rapid characterization of combinatorial libraries using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry.

The relatively new field of combinatorial chemistry has enabled researchers to create large mixtures of compounds that can be screened for leads in developing potential drug candidates. The new synthetic method has also created a need for better procedures to analyze the complex mixtures that are generated. The immediate goal in most cases is to verify the synthetic procedure and to determine the purity and completeness of the library sample before binding studies are initiated. We report here a method to rapidly characterize small-molecule combinatorial libraries in solution. All combinatorial library samples were synthesized by combining a core molecule bearing two acid chloride functionalities with various amino acids to generate libraries of 36, 78 and 120 components. Using electrospray ionization fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) we were able to identify 70-80% of the library components. All samples were analyzed as mixtures by direct infusion without chromatographic separation. Furthermore, nominally isobaric components could be resolved and identified through exact mass assignments without tandem mass spectrometery. ESI-FTICR-MS is a rapid and convenient tool for the characterization of small-molecule libraries. The method is especially useful for the analysis of larger libraries that contain many nominally isobaric components and impurities.

Electrophore mass tag dideoxy DNA sequencing.

Toward a goal of dideoxy sequencing DNA utilizing electrophore labels, we prepared four electrophore-labeled DNA oligonucleotide primers. Each primer has a different electrophore and DNA sequence but a common glycol keto (alpha,beta-dihydroxyketo) release group. Cleavage of this latter group by either periodate oxidation or a thermal retroaldol reaction releases the electrophores for detection by mass spectrometry. Successful sequencing data with these primers was obtained by capillary electrophoresis on an ABI Model 310 after fluorescence dideoxy terminator cycle sequencing reactions were conducted. In a separate experiment, it was demonstrated that a cocktail of the four electrophore DNA primers could be detected as a dried sample spot by CO2 laser desorption/capillary collection/gas chromatography electron capture mass spectrometry. These results establish some feasibility for our long-term goal of high-speed multiplex electrophore mass tag dideoxy DNA sequencing. Ultimately we plan to use a higher number of electrophore mass tags and to rely on direct detection of the desorbed electrophores by electron capture time-of-flight mass spectrometry.

Pulsed-gas glow discharge for ultrahigh mass resolution measurements with fourier transform ion cyclotron resonance mass spectrometry.

A new pulsed-gas glow discharge (GD) source has been developed for use with an external ion source Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. With pulsed argon gas introduction into the GD source, the gas load and pressure in the mass analyzer region were greatly reduced; this resulted in improved mass resolution. Mass resolution of greater than 1 450 000 (fwhm) has been achieved for Cu(+) ions from a brass sample, the highest reported for any type of GD mass spectrometer. The pulsed-gas GD source promises analytical usefulness for ultrahigh resolution measurements in GD mass spectrometry.

Deprotonation reactions of multiply protonated ubiquitin ions.

The gas-phase deprotonation reactions of multiply protonated ubiquitin ions have been studied in a Fourier-transform ion cyclotron resonance mass spectrometer. Electrospray ionization was used to generate ubiquitin ions with attachment of 7-13 protons. Rate constants were measured for the reactions of these protein ions with four amines: n-propylamine, di-n-propylamine, tri-n-propylamine, and N,N,N',N'-tetramethyl-1,4-diaminobutane. The gas-phase basicities of the amines ranged from 210.1 kcal/mol to 232.6 kcal/mol. The rate constants were found to increase as the charge state of the ion increased and as the basicity of the amine increased. Several reactions proceed at near the collision rate and have rate constants in excess of 10(-8) cm3 molecule-1 s-1. With the more basic reactants, multiple protons could be stripped sequentially from ubiquitin ions at roughly equivalent rates, suggesting that these protons are attached to sites with similar basicities. In general, deprotonation occurs if the gas-phase basicity of the amine is within 10 kcal/mol of the intrinsic gas-phase basicity of the amino acid residue being deprotonated. For [M+nH]n+, n = 4-6, nonlinear pseudo-first-order kinetic behavior indicated the presence of multiple ion structures. Kinetic, structural and thermodynamic aspects of these reactions are discussed.