T.-W. Dominic Chan

Synthesis, spectroscopic and electrochemical properties of substituted bis(phthalocyaninato)lanthanide(III) complexes

Abstract The sandwich-like bis(phthalocyaninato)lanthanide(III) complexes Ln[Pc′(R) 8 ] 2 (Ln = Eu, Gd; Pc′ = 2,3,9,10,16,17,23,24-octasubstituted phthalocyaninate; R = H, n-C 7 H 15 , OC 5 H 11 ) have been prepared by treating Ln(acac) 3 ·nH 2 O (Ln = Eu, Gd; acac = acetylacetonate) with dilithium phthalocyaninate or with the corresponding disubstituted phthalonitriles in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The spectroscopic and electrochemical properties of the substituted sandwich complexes have been investigated and compared with those of the unsubstituted analogues. The matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS) and electrospray ionisation mass spectrometry (ESI-MS) have also been employed to unambiguously characterise this novel class of compounds for the first time.

Fe3O4@MOF core–shell magnetic microspheres for magnetic solid-phase extraction of polychlorinated biphenyls from environmental water samples

Fe3O4@MIL-100 core-shell magnetic microspheres were, for the first time, used as the sorbent for the magnetic solid-phase extraction (MSPE) of polychlorinated biphenyls at trace levels in environmental water samples. GC coupled with tandem MS was used for sample quantification and detection. The Box-Behnken design was used to determine the optimum extraction parameters influencing extraction efficiency through response surface methodology. Under the optimized conditions, the developed method showed good linearity within the range of 5-4000ngL(-1), low limits of detection (1.07-1.57ngL(-1); signal-to-noise ratio=3:1), and good extraction repeatability (relative standard deviation<12%; n=5). Environmental samples collected from the Yellow River, local lake, wastewater, and snow water were processed using the developed method. The results demonstrated that the Fe3O4@MOF core-shell magnetic microspheres are promising sorbents in the MSPE of aromatic pollutants from environmental water samples.

Formulation of matrix solutions for use in matrix-assisted laser desorption/ionization of biomolecules

We report a simple method for converting solid matrices into useful matrix solutions for matrix-assisted laser desorption/ionization (MALDI). This method is based on the dissolution of the solid matrix in a liquid support of low volatility such as glycerol. An appropriate solubilizing reagent was added to promote the dissolution of the matrix materials into the liquid support. Selection of the solubilizing reagent is empirically related to an acid-base relationship, i.e., an acidic solid matrix requires a basic organic compound to form a stable matrix solution in the liquid support and vice versa. A tenfold increase in the solubility can be obtained for many solid matrices when appropriate solubilizing reagents are added into the glycerol support. This solubility enhancement is tentatively attributed to the ion-pair formation in a polar nonvolatile liquid support. In addition, the hydrophobicity of the solid matrix seems to play an important role in the efficiency of the resulting matrix solution. By using glycerol as liquid support, a hydrophilic matrix, such as 2,5-dihydroxybenzoic acid (DHB), showed a substantial “peripheral effect,” in which good analyte ion signals could only be recorded at the peripheral region of the sample droplet. More hydrophobic matrices, such as α-cyano-4-hydroxycinnamic acid (α-CCA), exhibit better and more homogeneous responses at different regions of the droplets. The performance of these matrix solutions was evaluated in terms of the durability, reproducibility, sensitivity, high mass capability, and generality. A typical sample droplet can afford more than an hour of repeated sampling with excellent shot-to-shot reproducibility. A low picomole sensitivity was demonstrated using a luteinizing hormone releasing hormone (LHRH) in a Fourier transform ion cyclotron resonance mass spectrometer with a homemade external MALDI ion source. By using a commercial MALDI time-of-flight mass spectrometer, proteins with masses as high as 66,000 Da were successfully analyzed by using these matrix solutions.

CATIONIZATION PROCESSES IN MATRIX-ASSISTED LASER DESORPTION/IONIZATION MASS SPECTROMETRY : ATTACHMENT OF DIVALENT AND TRIVALENT METAL IONS

In this paper, divalent and trivalent metal ions have been used as cationization reagents in matrix-assisted laser desorption/ionization (MALDI) experiments. Analytes studied include polyethylene glycol, polyethylene oxide dimethyl ether, cyclic polyethylene oxide and α-cyclodextrin. Independent of the valence of the metal ions and the nature of the analyte molecules studied, singly charged quasimolecular ions were generated in all cases. These quasimolecular ions were shown to be generated in different forms depending on the type of the di- or trivalent metal salts and the chemical properties of the analyte molecules. For Cu2+ ions, the quasimolecular ions were formed by the adduction of the reduced metal ions, i.e. Cu+. For Pb2+, Cd2+, Ba2+, Sr2+ and La3+ ions, singly charged quasimolecular ions were formed by adduction of the metal ions with either simultaneous deprotonation of the analyte molecule or adduction of an additional deprotonated matrix moiety onto the analyte molecule. Of these two choices, deprotonation of the analyte molecule is always preferred if labile protons, such as those in hydroxyl groups, are available in the analyte molecule. A model of ion formation from MALDI based on solid-state molecular pre-arrangement is used to account for the formation of such complex singly charged quasimolecular ions.

Fourier transform ion cyclotron resonance studies of lanthanide(III) porphyrin-phthalocyanine heteroleptic sandwich complexes by using electrospray ionization

A number of neutral Ianthanide(III) porphyrin-phthalocyanine heteroleptic sandwich complexes have been studied by using positive-ion electrospray ionization-Fourier transform ion cyclotron resonance mass spectrometry. The investigated compounds are represented as LnIII(TPP)(Pc), Ln2III(TPP)(Pc)2, and Ln2III(TPP)2(Pc), where Ln = Sm, Eu, or Gd, TPP is 5,10,15,20-tetraphenylporphyrinate, and Pc is phthalocyaninate. In all cases, intense signals corresponding to the singly charged molecular radical cations are observed. The formation of these molecular radical cations in electrospray ionization is attributed to electrochemical oxidation at the electrospray needle. Multiply charged molecular ions up to +5 are also observed. They are tentatively assigned to be formed from successive oxidation of the ligand(s). Apart from the molecular weight information, tandem mass spectrometry offers additional structural information on these complexes. From the fragmentation pattern of the europium complexes under collision-induced dissociation conditions, the configurations of the triple-decker complexes are assigned mass spectrometrically to be (TPP)Eu(Pc)Eu(Pc) and (TPP)Eu(Pc)Eu(TPP). In comparison with the previous spectroscopic findings that the positive “hole” is localized in the ligands of the complexes, there is evidence to suggest that intramolecular charge transfer or hole delocalization does occur within the macrocycles or between the metal centers and the macrocycles before fragmentation. The occurrence of this charge-transfer process is tentatively attributed to the result of collisional activation.

Synthesis and spectroscopic properties of heteroleptic sandwich-type (phthalocyaninato)(porphyrinato)lanthanide(III) complexes

Abstract Treatment of dilithium phthalocyaninate [Li 2 (Pc)] with Ln III (acac)(TPP) (Ln=Sm, Eu, Gd; acac=acetylacetonate; TPP=5,10,15,20-tetraphenylporphyrinate) prepared in situ from Ln(acac) 3 · n H 2 O and (TPP)H 2 in 1,2,4-trichlorobenzene led to the formation of heteroleptic triple-decker complexes (TPP)Ln III (Pc)Ln III (TPP) and (Pc)Ln III (Pc)Ln III (TPP) along with a small amount of the double-decker complexes Ln III (Pc)(TPP). The new compounds were characterised with 1 H NMR, UV–Vis, near-IR and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry.

Use of Ammonium Halides as Co‐matrices for Matrix‐assisted Laser Desorption/Ionization Studies of Oligonucleotides

Four ammonium salts, NH4F, NH4Cl, NH4Br and NH4I were tested as co-matrices for the matrix-assisted laser desorption/ionization (MALDI) analysis of a number of DNA homopolymers, including phosphorylated d(T)8, d(A)4, d(C)4 and non-phosphorylated d(G)4, using 2-amino-5-nitropyridine (ANP) matrix. In the present study, all the ammonium halides displayed significant enhancement effects on the signal intensities of the intact molecular ions of the DNA homopolymers. Among the halides used, NH4F was found to exhibit the greatest enhancement effects. By comparison with results obtained using the corresponding isomorphous potassium halides as co-matrices, it is postulated that both the cationic and anionic portions of the co-matrix molecule play important roles in the desorption/ionization of the oligonucleotides under typical MALDI conditions. It was also demonstrated that the ANP matrix exhibits a strong inhibitory effect on the formation of alkali-metal adducts in oligonucleotide analysis.

Sesquiterpene lactones from Elephantopus scaber

Abstract The whole plant of Elephantopus scaber afforded the known deoxyelephantopin and isodeoxy-elephantopin, and a new germacranolide sesquiterpene lactone named scabertopin, whose structure and stereo-chemistry were determined by spectroscopic methods and single-crystal X-ray analysis.

Electron transfer dissociation with supplemental activation to differentiate aspartic and isoaspartic residues in doubly charged peptide cations

Electron-transfer dissociation (ETD) with supplemental activation of the doubly charged deamidated tryptic digested peptide ions allows differentiation of isoaspartic acid and aspartic acid residues using the c + 57 or z • − 57 peaks. The diagnostic peak clearly localizes and characterizes the isoaspartic acid residue. Supplemental activation in ETD of the doubly charged peptide ions involves resonant excitation of the charge reduced precursor radical cations and leads to further dissociation, including extra backbone cleavages and secondary fragmentation. Supplemental activation is essential to obtain a high quality ETD spectrum (especially for doubly charged peptide ions) with sequence information. Unfortunately, the low-resolution of the ion trap mass spectrometer makes detection of the diagnostic peak, [M-60], for the aspartic acid residue difficult due to interference with side-chain loss from arginine and glutamic acid residues.

Differentiation of enantiomers using matrix-assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has successfully been used to differentiate pseudo-enantiomeric (isotopically labelled) amino acids by using cyclodextrin as complexing host. By using different pseudo-enantiomeric mixtures (i.e. R(Dn) + S; and R + S(Dn)), it has been demonstrated that the preference of cyclodextrin for S-enantiomers is not due to the size differences caused by the hydrogen/deuterium substitution. It is postulated that this method can be extended to differentiate enantiomers (and determine enantiomeric excess) by using a pair of enantiomeric hosts, as demonstrated previously using other ionization techniques, but with much higher sensitivity.