The nominal butyl ester ion in the mass spectra of long-chain n-alkyl esters: A postscript

n-Octadecyl benzoate, taken as a model for long-chain n-alkyl carboxylates generally, loses C₁₄H₂₈ under electron impact to yield a product with the same elemental composition as the butyl benzoate molecular ion. This product retains quantitatively one hydrogen from C-6, and seems to be formed as an oxygen-protonated 4-benzoyloxybutyl radical. It reacts further to lose H₂O, in which deuterium labeling demostrates that the second hydrogen atom comes predominantly from C-4. The intermediate reorganization, for which the driving force is presumably furnished by the instability associated with a primary radical, is pictured in terms of cyclization via bonding between the C-4 radical site and the benzoyl carbon concerted with hydrogen migration via a 4-membered quasicyclic transition state.     

Synthesis,structure, and spectroscopic,photochemical, redox,and catalytic properties of ruthenium(II) isomeric complexes containing dimethyl sulfoxide,chloro, and the dinucleating bis(2-pyridyl)pyrazole ligands

Two isomeric Ru(II) complexes containing the dinucleating Hbpp (3,5-bis(2-pyridyl)pyrazole) ligand together with Cl and dmso ligands have been prepared and their structural, spectroscopic, electrochemical, photochemical, and catalytic properties studied. The crystal structures of trans,cis-[Ru(II)Cl(2)(Hbpp)(dmso)(2)], 2a, and cis(out),cis-[Ru(II)Cl(2)(Hbpp)(dmso)(2)], 2b, have been solved by means of single-crystal X-ray diffraction analysis showing a distorted octahedral geometry for the metal center where the dmso ligands coordinate through their S atom. 1D and 2D NMR spectroscopy corroborates a similar structure in solution for both isomers. Exposure of either 2a or 2b in acetonitrile solution under UV light produces a substitution of one dmso ligand by a solvent molecule generating the same product namely, cis(out)-[Ru(II)Cl(2)(Hbpp)(MeCN)(dmso)], 4. While the 1 e(-) oxidation of 2b or cis(out),cis-[Ru(II)Cl(2)(bpp)(dmso)(2)](+), 3b, generates a stable product, the same process for 2a or trans,cis-[Ru(II)Cl(2)(bpp)(dmso)(2)](+), 3a, produces the interesting linkage isomerization phenomenon where the dmso ligand switches its bond from Ru-S to Ru-O (K(III)(O)(-->)(S) = 0.25 +/- 0.025, k(III)(O)(-->)(S) = 0.017 s(-1), and k(III)(S)(-->)(O) = 0.065 s(-1); K(II)(O)(-->)(S) = 6.45 x 10(9), k(II)(O)(-->)(S) = 0.132 s(-1), k(II)(S)(-->)(O) = 2.1 x 10(-11) s(-1)). Finally complex 3a presents a relatively high activity as hydrogen transfer catalyst, with regard to its ability to transform acetophenone into 2-phenylethyl alcohol using 2-propanol as the source of hydrogen atoms.     

Gas-phase noncovalent interactions between vancomycin-group antibiotics and bacterial cell-wall precursor peptides probed by hydrogen/deuterium exchange

Gas-phase structures of noncovalent complexes between the glycopeptide antibiotics vancomycin, eremomycin, ristocetin, and pseudo aglyco-ristocetin and the cell-wall mimicking peptides N-acetyl-D-Alanyl-D-Alanine, N-acetyl-Glycyl-D-Alanine, and N,N′-di-acetyl L-Lysyl-D-Alanyl-D-Alanine have been probed by hydrogen/deuterium (H/D) exchange using ND₃ as reagent gas. The noncovalent complexes were transferred from solution to the vacuum using electrospray ionization. The H/D exchange of the solvent-free ions was studied in a Fourier transform ion cyclotron resonance mass spectrometer. The H/D exchange behavior of the free antibiotics and the free peptides were compared with the exchange observed for the antibiotic–peptide complexes. A general increase was found in the degree of deuterium incorporation upon complex formation with the ligand, which indicates that the peptide binding makes more sites on the antibiotic capable of taking part in the H/D exchange. Apart from H/D exchange, adduct formation with ND₃ was observed, but only for the singly protonated peptides and the doubly protonated [ristocetin+N-acetyl-D-Alanyl-D-Alanine]. This marked difference in chemical reactivity of closely related systems such as [ristocetin+N-acetyl-Glycyl-D-Alanine] and [ristocetin+N-acetyl-D-Alanyl-D-Alanine] indicates that the gas-phase structures of these noncovalent complexes are quite sensitive to small changes in the primary structures of the peptides. The gas-phase structures of the antibiotic–peptide complexes are probably different from the solution-phase structures, with the peptides no longer bound to the characteristic solution-phase binding pockets of the antibiotics.