Mass spectrometric investigations of α‐ and β‐cyclodextrin complexes with ortho‐, meta‐ and para‐coumaric acids by negative mode electrospray ionization

The mass spectrometric characterization of aqueous solutions of α‐ and β‐cyclodextrins (CDs) and o‐, m‐ and p‐coumaric acids (CAs) by negative ion electrospray ionization (ESI) indicates that the [CD+CA]^? ions were sourced from the inclusion complex present in solution and from the anion attached to CD molecules formed in the spray processes. The anion adducts formed in the spray process contribute significantly to the signal intensity of an ionized inclusion complex thus overestimating the calculated stability constant (K) of solution‐phase complexes by one to two orders of magnitude. The relative intensities of anion adducts in mass spectra depend on the concentration ratio of the anion and the CD in spray droplets, while the relative intensity of the ionized inclusion complex depends on CD and CA concentrations in solutions and the value of K. Ion Mobility Spectrometry Mass Spectrometry [IMS‐MS] measurements show that the collision cross‐section (Ω) values of the [CD+CA]^? or [(CD)₂₊CA]^2? and [CD+CA] complex ions are 5–6% larger than or equal to CD^? or [CD], respectively. Therefore, in the gas phase the anion adducts [CD+CA^?] on cyclodextrin molecules possess the same conformations as the ionized inclusion complexes [CD+CA]^?. Copyright © 2008 John Wiley & Sons, Ltd.     

Dimeric calixarenes: a new family of major-groove binders

A new class of potent DNA binding agents is presented. Dimeric calix[4]arenes with cationic groups at their upper rims and flexible alkyl bridges can be synthesized from triply acyl-protected calix[4]arene tetramines in relatively short synthetic sequences (3-5 steps). The compounds attach themselves to double-stranded nucleic acids in a noncovalent fashion, with micro- to nanomolar affinities. Guanidinium headgroups with their extended hydrogen-bonding "fingers" are more powerful than ammonium groups, and the benzylamine series is superior to the anilinium series (see below). The new ligands easily distinguish between RNA and various DNA types, and produce characteristic changes in UV/Vis, fluorescence, CD, as well as NMR spectra. Especially extended oligonucleotides of more than 100 base pairs are bound with affinities increasing from RNA (10?μM K(d))相似文献   

A novel bis-phenanthridine triamine with pH controlled binding to nucleotides and nucleic acids

The new bis-phenanthridine triamine is characterised by three pK(a) values: 3.65; 6.0 and >7.5. A significant difference in the protonation state of at pH = 5 (four positive charges) and at pH = 7 (less than two positive charges) accounts for the strong dependence of -nucleotide binding constants on nucleotide charge under acidic conditions, whereas at neutral pH all -nucleotide complexes are of comparable stability. All experimental data point at intercalation as the dominant binding mode of to polynucleotides. However, there is no indication of bis-intercalation of the two phenanthridine subunits in binding to double stranded polynucleotides, the respective complexes being most likely mono-intercalative. Thermal stabilisation of calf thymus DNA (ct-DNA) and poly A-poly U duplexes upon addition of is significantly higher at pH = 5 than at neutral conditions. This is not the case with poly dA-poly dT, indicating that the specific secondary structure of the latter, most likely the shape of the minor groove, plays a key role in complex stability. At pH = 5 acts as a fluorimetric probe for poly G (emission quenching) as opposed to other ss-polynucleotides (emission increase), while at neutral conditions this specificity is lost. One order of magnitude higher cytotoxicity of compared to its "monomer" can be accounted for by cooperative action of two phenanthridinium units and the charged triamine linker. The results presented here are of interest to the development of e.g. sequence-selective cytostatic drugs, and in particular for the possibility to control the drug activity properties over binding to DNA and/or RNA by variation of the pH of its surrounding.     

The niobium and tantalum riddle: gas-phase monocation reactions with pyrene and pyrene-D10

Metal monocation gas-phase reactions with pyrene (Py) have been investigated. Of those cations which bind Py by abstraction of two hydrogen atoms, there are two subgroups of which Nb⁺ and Ta⁺ are the most striking and puzzling representatives. The Ta⁺-like group binds three Py units consecutively, each by expulsion of two hydrogen atoms, whereas the Nb⁺ group binds four Py units; however, in this latter case, the second Py attachment occurs without the expulsion of any hydrogen. When perdeuterated Py-D₁₀ is used, the Ta⁺ group behavior is unaffected, but the Nb⁺ group now exhibits a considerable degree of Py attachment in which no deuterium atoms are lost in the first reaction step. We will attempt to explain this behavior by using the results of reactions of Nb⁺ and Ta⁺ with Py in a Fourier transform mass spectrometer (FTMS).     

A mass spectral study of the acid-catalysed oxygen exchange in laevulinic acid

The sequence, i.e. site selectivity of the acid-catalysed ¹⁶O/¹⁸O exchange in laevulinic acid (1) is studied by mass spectrometry, ion kinetic energy spectroscopy and accurate mass measurements on 1 and its ¹³C(1)-labelled congener. In the fragmentation pathways all ions of interest in the spectrum of 1 originate from the molecular ion, which may exist in six tautomeric or isomeric forms (1a–1d). The mechanism of gas-phase cyclization of 1a is rationalized in terms of three different pathways. Formation of the most prominent fragment ions and the sequence of ¹⁶O/¹⁸O exchange at the carbonyl and carboxy group are discussed. The results indicate the ions of m/z 56, m/z 61 and m/z 98 to be the only three which have incorporated oxygen atoms originating from the carboxy group in 1 and could thus be used in further elucidadon of the rearrangement mechanism of 5-hydroxymethylfuran (2) or 5-hydroxymethylfuran-2-carbaldehyde (3) into laevulinic acid (1).     

Studies of consecutive reactions of quinones in a reversed geometry mass spectrometer

Consecutive reactions involving the ejection of molecules of carbon monoxide from quinone systems have been studied. Using a reversed geometry double focusing mass spectrometer, the individual steps of a consecutive reaction may be separated into different field free regions. The structure of ions formed in the ion source may be compared with those formed from the fragmentation of metastable ions in a field free region by studying differences in the pattern of translational energies released when they break. In the case of anthraquinone it is found that two structures exist for the [M? CO]^+˙ ion. Critical energy measurements strengthen this argument. Furthermore, ¹⁸O labelling has been used to distinguish between the loss of carbon monoxide from different positions on the molecular ion of 1-hydroxyanthraquinone.     

Autoxidation of lipids in parchment

Historic parchment is a macromolecular material, which is complex due to its natural origin, inhomogeneity of the skin structure, unknown environmental history and potential localised degradation. Most research into its stability has so far focussed on thermal and structural methods of analyses.Using gas chromatographic analysis of the atmosphere surrounding parchment during oxidation, we provide the experimental evidence on the production of volatile aldehydes, which can be the products of lipid autoxidation. Oxidation of parchment with different aldehyde emissions was additionally followed in situ using chemiluminometry and the same techniques were used to evaluate the oxidation of differently delipidised parchment. It was shown that the production of peroxides and the emission of aldehydes from the material decrease with lower lipid content. Building on this evidence, we can conclude that the presence of lipids (either initially present in the skin or resulting from conservation intervention) leads to oxidative degradation of collagen and that the non-destructive analysis of the emission of volatiles could be used as a quick tool for evaluation of parchment stability.     

Nucleation of ordered Fe islands on Al2O3/Ni3Al(1 1 1)

Scanning tunneling microscopy (STM) has been used to investigate the nucleation and stability of iron clusters on the Al₂O₃/Ni₃Al(1 1 1) surface as a function of coverage and annealing temperature. We show that atomic beam deposition of iron leads to hexagonally ordered cluster arrangements with a distance of 24 Å between the clusters evidencing the template effect of the alumina film. The shape of the iron clusters is two-dimensional (2D) at deposition temperatures from 130 K to 160 K and three-dimensional (3D) at 300 K. However, the 2D iron clusters grown between 130 K and 160 K are stable up to 350 K.