Jan K. Maurin

On the Primary Ionization Mechanism(s) in Matrix-Assisted Laser Desorption Ionization

A mechanism is proposed for the first step of ionization occurring in matrix-assisted laser desorption ionization, leading to protonated and deprotonated matrix (Ma) molecules ([Ma + H]+ and [Ma − H]− ions). It is based on observation that in solid state, for carboxyl-containing MALDI matrices, the molecules form strong hydrogen bonds and their carboxylic groups can act as both donors and acceptors. This behavior leads to stable dimeric structures. The laser irradiation leads to the cleavage of these hydrogen bonds, and theoretical calculations show that both [Ma + H]+ and [Ma − H]− ions can be formed through a two-photon absorption process. Alternatively, by the absorption of one photon only, a heterodissociation of one of the O–H bonds can lead to a stable structure containing both cationic and anionic sites. This structure could be considered an intermediate that, through the absorption of a further photon, leads to the formation of matrix ions. Some experiments have been performed to evaluate the role of thermal ionization and indicate that its effect is negligible. Some differences have been observed for different matrices in the formation of analyte molecule (M) ion [M + H]+, [M − H]−, M+•, and [M − 2H]-•, and they have been explained in terms of ionization energies, pKa values, and thermodynamic stability.

Diastereoselective synthesis of lortalamine analogs

Abstract ( R )-(+)-1-(1-Naphthyl)ethylamine was found to be a more effective chiral auxiliary in the asymmetric synthesis of lortalamine analogs than ( S )-(−)-α-methylbenzylamine. Chiral, non-racemic ketones 6 and 7 were prepared and transformed into pairs of diastereomeric intermediates of final lortalamine analogs. For one of them, the stereochemistry on the heterocyclic part was established on the basis of X-ray crystallography. Enantiomerically pure (+)- des -chloro-lortalamine 2 was finally obtained.

Genistein complexes with amines: structure and properties

New amine complexes of genistein in the crystal and the solution state have been studied by X-ray crystallography and by 1H and 13C NMR spectroscopy. The gas-phase structures have been modelled with ab initio quantum chemical calculations. The morpholine–genistein hydrogen bonded complex has been investigated by all the above methods whereas the triethylamine, morpholine and piperazine complexes have been investigated with 1H NOE and 13C NMR spectroscopy. The X-ray results show the genistein–morpholine complex to be formed as a result of proton transfer from the genistein OH group at position C7 to the morpholine nitrogen atom. This complex also has the lowest total energy when compared to other possible complexes. The NMR measurements in solution indicate that the protonated amine is in fast exchange between various interaction sites, the most stable complex being formed at position C7 as in the crystal. The ab initio quantum mechanical calculations show that this position is also the best for interactions. The 13C NMR chemical shifts calculated theoretically are in agreement with experimental values.

Crystallographic studies and physicochemical properties of π-electron systems: Part VIII. Crystal and molecular structure of N,N-dimethyl-4-nitro-2, 6-xylidine. Application of the Walsh Rule to p-substituted nitrobenzenes

Abstract The crystal and molecular structure of N,N -dimethyl-4-nitro-2, 6-xylidine has been determined by X-ray diffraction. The compound crystallizes as tetragonal bipyramids, space group P 4 1 2 1 2 with cell dimensions a =764.1(1), c =1807.9(2) pm. The absolute configuration was not determined. The structure was solved by direct methods and refined to R =0.042 for 578 observed reflections. The e.s.d. values of bond lengths between the heavy atoms are 0.3–0.4 pm and of the valence angles are 0.1–0.3°. The NO 2 -plane is coplanar with e.s.d. 0.21° with the plane of the benzene ring, whereas the NMe 2 -plane is twisted by 60.40(30)° outwards. The through resonance is hence hindered and in turn, the geometry of the ring exhibits less quinoidal character. These deformations are analyzed in terms of the Walsh rule applying parameters determined directly from bond lengths, as well as applying the HOSE-model. Comparison with 14 well solved structures for which the e.s.d. values for bond lengths are not higher than 0.5 pm of p -substituted nitrobenzenes, leads to the conclusion that consideration of the difference in length for bonds C2C3 and C1C2 describes the substituent effect on the geometry of the ring almost as well as the valence angle α at C1.

Crystallographic studies of intra- and inter-molecular interactions. Crystal and molecular structure of N,N-dimethyl-4-nitro-3,5-xylidine. Structural evidence against the classical through-resonance concept in p-nitroaniline and derivatives

An analysis of the geometry of N,N-dimethyl-4-nitroaniline and its 3,5- and 2,6-dimethyl derivatives presents structural evidence against the classical concept of through-resonance(throughconjugation), according to which the most important contribution is the quinoid structure with full charge transfer from the amine to the nitro group. Application of the HOSE model gives results which are in line with calculations performed by Hiberty and Ohanessian on p-nitroaniline.

Argon ionization detector sensitive to hydrogen: Construction and mechanism of operation

Abstract The construction and optimal operating parameters of a radioionization detector having a sensitivity towards hydrogen as high as 10−11 g/sec are presented. The device also detects carbon monoxide, methane and ethane. Technical-grade argon is used as a carrier gas. The mechanism of operation involves the occurrence of either a Hornbeck-Molnar type-reaction (Ar* + H → ArH+ + e−) or a Penning ionization of the products of the decomposition of carbon monoxide, methane and ethane.

Crystal structure of lead(II) acetylacetonate and the structure of the acetylacetone solvated lead(II) ion in solution studied by large-angle X-ray scattering

The crystal structure of bis(acetylacetonato)lead(II) and the structure of the acetylacetone solvated lead(II) ion in solution have been determined by single-crystal X-ray diffraction and large-angle X-ray scattering (LAXS), respectively. The acetylacetone is deprotonated and acts as a bidentate anionic ligand (acac-) in the solid Pb(acac)2 compound. The lead(II) ion binds four oxygen atoms strongly in a nearly flat pyramidal configuration with Pb-O bond lengths in the range 2.32-2.37 A, and additionally three oxygens from neighboring complexes at 3.01-3.26 A. Acetylacetone acts as a solvent (Hacac) at dissolution of lead(II) trifluoromethanesulfonate forming a pentasolvate with a mean Pb-O bond distance of 2.724(5) A. The 6s2 lone electron pair on the lead(II) ion becomes stereochemically active in the crystalline acetylacetonate complex, while it is inactive in the solvate in solution. The solution was also analysed using IR and 1H NMR spectroscopy.

Diastereoselective synthesis of some β-carboline derivatives from l-amino acids

Abstract l -Ala and l -Val were used as chiral inductors in a series of reactions in which a Pictet–Spengler cyclization, completed under mild conditions was the key step. Diketopiperazines 7a and 8a having R configuration at the newly created stereogenic center were obtained with good diastereoselectivity due to 1,4-chirality transfer. Surprisingly, l -Pro promoted the formation of the product with S configuration in the predominant diastereomers 9a and 10a.

Chemical aspects of the primary ionization mechanisms in matrix-assisted laser desorption ionization.

It has been proposed that the primary ionization mechanism occurring in matrix-assisted laser desorption ionization (MALDI) experiments originates from the presence, in the solid-state matrix-analytes sample, of matrix dimers. These species are formed by the interaction of carboxylic groups present in the matrix molecules with the formation of strong hydrogen bonds. Theoretical calculations proved that the laser irradiation of these structures leads to one or two H-bridge cleavages, giving rise to an “open” dimer structure or to disproportionation with the formation of MH+ and [M – H]− species. The ions so formed can be considered highly effective in their reaction with analyte ions, leading to their protonation (or deprotonation). To achieve further evidence for these proposals, in the present study the energetics of the reactions of ions from different aromatic carboxylic acids with two amino acids (glycine and lysine) and three multipeptides (gly-gly, gly-gly-gly and gly-gly-gly-gly) was investigated. The lowest ΔG values were obtained for 2,5-dihydroxybenzoic acid, widely employed as the MALDI matrix. Also, for p-nitrobenzoic acid the reaction is slightly exothermic, while for the other aromatic carboxylic acids derivatives positives values of ΔG are present.

Pseudosymmetry in ammonium [(carboxymethyl)sulfanyl]acetate.

The structure of the title compound, NH(4)(+).C(4)H(5)O(4)S(-), is composed of monocarboxylate anions of [(carboxymethyl)sulfanyl]acetic acid linked into infinite chains via strong O-H...O(-) hydrogen bonds. The three-dimensional structure is completed by the ammonium cations, which interlink neighbouring chains via N-H...O hydrogen bonds. Solution and refinement in the true space group Pn led to an unambiguous position for the single carboxyl H atom. In the higher symmetry space group P2/n, the carboxylate anion would be located on a twofold axis.