Tandem mass spectrometric analysis of cyclophosphamide, ifosfamide and their metabolites

A detailed multi-stage (MSn) fragmentation study of cyclophosphamide (CP), ifosfamide (IF) and their major metabolites, using an ion-trap mass spectrometer and a Q-TOF mass spectrometer, was performed with the aid of specifically deuterium-labeled analogs. The analytes showed good responses in positive-ion electrospray mass spectrometry as [MH]+ ions. Tandem mass spectra revealed a wealth of structurally specific ions, allowing characterization of the fragmentation pathways of these analytes. The major fragmentation pathways of the protonated CP and IF are elimination of ethylene from C5 and C6 of 1,3,2-oxazaphosphorine-2-oxide via a McLafferty rearrangement, and cleavage of the P-N bond. However, their activated 4-OOH and 4-OH metabolites primarily underwent hydrogen peroxide elimination and dehydration, respectively, followed by fragmentation pathways similar to those of CP and IF. These results should prove useful in structural elucidation of future analogs of CP and IF, and/or of their metabolites.     

Detection and sequencing of phosphopeptides

Consecutive enzymatic reactions of analytes which are affinity bound to immobilized metal ion beads with subsequent direct analysis of the products by matrix-assisted laser desorption/ionization mass spectrometry have been used for detecting phosphorylation sites. The usefulness of this method was demonstrated by analyzing two commercially available phosphoproteins, beta-casein and alpha-casein, as well as one phosphopeptide from a kinase reaction mixture. Agarose loaded with either Fe3+ or Ga3+ was used to isolate phosphopeptides from the protein digest. Results from using either metal ion were complementary. Less overall suppression effect was achieved when Ga3+-loaded agarose was used to isolate phosphopeptides. The selectivity for monophosphorylated peptides, however, was better with Fe3+-loaded agarose. This technique is easy to use and has the ability to analyze extremely complicated phosphopeptide mixtures. Moreover, it eliminates the need for prior high-performance liquid chromatography separation or radiolabeling, thus greatly simplifying the sample preparation.     

Solvent effects on chemical processes. 4. Complex formation between naphthalene and theophylline in binary aqueous-organic solvents

The standard free energy change for complex formation is written as a sum of effects arising from solvent-solvent interactions (the general medium effect), solvent-solute interactions (the solvation effect), and solute-solute interactions (the intersolute effect). The general medium effect is given by gA(–_o), where g is a curvature correction factor to the solvent surface tension , A is the change in surface area as the two solvent cavities containing the substrate (naphthalene) and ligand (theophylline) collapse into a single cavity containing the complex, and _o is the value of surface tcnsion at which there is no net solvophobic interaction; is defined to be the value appropriate to the equilibrium mean solvation shell composition. The solvation effect is modeled by equilibrium stoichiometric formation of solvated species. All data are related to the fully aqueous system to give _MG^o, the solvent effect on the free energy change, as an explicit function of solvent composition. Solvent effects on bimolecular association are related to solvent effects on the solubilities of the substrate, ligand, and complex. Approximation methods for interpreting such systems are described and are applied to the naphthalene-theophylline complex. It is shown that complex destabilization in mixed aqueous-organic solvents (relative to the fully aqueous system) may receive contributions from both the general medium and the solvation effects, and that these contributions can be quantitatively estimated.     

Comparison of atomic charges derived via different procedures

Atomic charges were obtained from ab initio molecular orbital calculations using a variety of procedures to compare them and assess their utility. Two procedures based on the molecular orbitals were examined, the Mulliken population analysis and the Weinhold–Reed Natural Population Analysis. Two procedures using the charge density distribution were included; the Hirshfeld procedure and Bader's Atoms in Molecules method. Charges also were derived by fitting the electrostatic potential (CHELPG) and making use of the atomic polar tensors (GAPT). The procedures were first examined for basis set independence, and then applied to a group of hydrocarbons. The dipole moments for these molecules were computed from the various atomic charges and compared to the total SCF dipole moments. This was followed by an examination of a series of substituted methanes, simple hydrides, and a group of typical organic compounds such as carbonyl derivatives, nitriles, and nitro compounds. In some cases, the ability of the charges to reproduce electrostatic potentials was examined. © John Wiley & Sons, Inc.     

Deoxygenation of alcohols employing water as the hydrogen atom source

Trialkylboranes (BMe3, BEt3, and BBu3) have been shown to mediate reductive deoxygenation reactions of O-alkyl-S-methyl dithiocarbonates (methyl xanthates) in which water or deuterium oxide functions as the source of hydrogen or deuterium. This method has proven versatile with regard to substrate scope and is capable of providing protio- or deuterioalkane products in high yields with excellent levels of D-incorporation. Ab initio calculations suggest that the trialkylborane-water complex possesses an unusually low O-H bond dissociation energy (73 kcal/mol) and support a radical chain mechanism for this process. Taken together, this report provides evidence for fundamentally novel and previously overlooked modes of reactivity for water and trialkylboranes of wide ranging importance in both theoretical and applied investigations.     

Thermodynamic properties of simple alkenes

Employing low temperature thermal measurements, heat capacities (C_s) in the crystal and liquid states, and phase transition data, T_m and ΔH_m, the condensed phase thermodynamic properties, (G_s -H°₀)/T, H_s -H°₀, S_s and C_s, in the temperature range 0–360 K were evaluated for the following eleven alkenes: ethylene, propylene, 1-butene, cis-2-butene, trans-2-butene, 1-pentene, cis-2-pentene, trans-2-pentene, 2-methyl-1-butene, 3-methyl-1-butene and 2-methyl-2-butene. The sources of experimental data, methods of evaluation, and the calculated results are described in detail.     

Mid-infrared characterization of the NH4 +(H2O)n clusters in the neighborhood of the n=20 "magic" number

Vibrational predissociation spectra are reported for size-selected NH4+ (H2O)n clusters (n=5-22) in the 2500-3900 cm(-1) region. We concentrate on the sharp free OH stretching bands to deduce the local H-bonding configurations of water molecules on the cluster surface. As in the spectra of the protonated water clusters, the free OH bands in NH4+ (H2O)n evolve from a quartet at small sizes (n<7), to a doublet around n=9, and then to a single peak at the n=20 magic number cluster, before the doublet re-emerges at larger sizes. This spectral simplification at the magic number cluster mirrors that found earlier in the H+(H2O)n clusters. We characterize the likely structures at play for the n=19 and 20 clusters with electronic structure calculations. The most stable form of the n=20 cluster is predicted to have a surface-solvated NH4+ ion that lies considerably lower in energy than isomers with the NH4+ in the interior.     

Direct measurement of the energy thresholds to conformational isomerization in tryptamine: experiment and theory

The methods of stimulated emission pumping-hole filling spectroscopy (SEP-HFS) and stimulated emission pumping population transfer spectroscopy (SEP-PTS) were applied to the conformation-specific study of conformational isomerization in tryptamine [TRA, 3-(2-aminoethyl)indole]. These experimental methods employ stimulated emission pumping to selectively excite a fraction of the population of a single conformation of TRA to well-defined ground-state vibrational levels. This produces single conformations with well-defined internal energy, tunable over a range of energies from near the zero-point level to well above the lowest barriers to conformational isomerization. When the SEP step overcomes a barrier to isomerization, a fraction of the excited population isomerizes to form that product. By carrying out SEP excitation early in a supersonic expansion, these product molecules are subsequently cooled to their zero-point vibrational levels, where they can be detected downstream with a third tunable laser that probes the ground-state population of a particular product conformer via a unique ultraviolet transition using laser-induced fluorescence. The population transfer spectra (recorded by tuning the SEP dump laser while holding the pump and probe lasers fixed) exhibit sharp onsets that directly determine the energy thresholds for conformational isomerization in a given reactant-product conformer pair. In the absence of tunneling effects, the first observed transition in a given X-Y PTS constitutes an upper bound to the energy barrier to conformational isomerization, while the last transition not observed constitutes a lower bound. The bounds for isomerizing conformer A of tryptamine to B(688-748 cm(-1)), C(1)(860-1000 cm(-1)), C(2)(1219-1316 cm(-1)), D(1219-1282 cm(-1)), E(1219-1316 cm(-1)), and F(688-748 cm(-1)) are determined. In addition, thresholds for isomerizing from B to A(<1562 cm(-1)), B to F(562-688 cm(-1)), and out of C(2) to B(<747 cm(-1)) are also determined. The A-->B and B-->A transitions are used to place bounds on the relative energies of minima B relative to A, with B lying at least 126 cm(-1) above A. The corresponding barriers have been computed using both density functional and second-order many-body perturbation theory methods in order to establish the level of theory needed to reproduce experimental results. While most of the computed barriers match experiment well, the barriers for the A-F and B-F transitions are too high by almost a factor of 2. Possible reasons for this discrepancy are discussed.