Analysis of Glycosaminoglycan Oligosaccharides by Combined HPTLC/MALDI-TOF MS: Reduced Silica Gel Thickness Leads to Improved Spectral Qualities and Reduced Side Reactions

Thin-layer chromatography (TLC) is a simple, fast and inexpensive separation method which can be applied to virtually all natural products including oligosaccharides. Unfortunately, however, the unequivocal identification of a TLC spot is normally difficult. Fortunately, this problem can be minimized when mass spectrometry (MS) such as matrix-assisted laser desorption and ionization time-of-flight is used to identify the TLC spots. This work is dedicated to the TLC/MS analysis of oligosaccharides derived from native chondroitin sulfate and hyaluronan. We will show that the thickness of the silica gel layer (200 versus 100 µm) has a tremendous influence on the quality of the mass spectra: a reduced silica gel thickness enhances the spectral quality and, in particular, improves the achievable signal-to-noise ratio. Additionally, unwanted formylation of the GAG oligosaccharides (which occurs due to the high moiety of formic acid in the mobile phase) can also be minimized if MS-grade HPTLC plates are used.

     

Sandmeyer‐Type Trifluoromethylthiolation and Trifluoromethylselenolation of (Hetero)Aromatic Amines Catalyzed by Copper

Aromatic and heteroaromatic diazonium salts were efficiently converted into the corresponding trifluoromethylthio‐ or selenoethers by reaction with Me₄NSCF₃ or Me₄NSeCF₃, respectively, in the presence of catalytic amounts of copper thiocyanate. These Sandmeyer‐type reactions proceed within one hour at room temperature, are applicable to a wide range of functionalized molecules, and can optionally be combined with the diazotizations into one‐pot protocols.     

High resolution and low-temperature photoelectron spectroscopy of an oxygen-linked fullerene dimer dianion: C(120)O(2-)

C(120)O comprises two C(60) cages linked by a furan ring and is formed by reactions of C(60)O and C(60). We have produced doubly charged anions of this fullerene dimer (C(120)O(2-)) and studied its electronic structure and stability using photoelectron spectroscopy and theoretical calculations. High resolution and vibrationally resolved photoelectron spectra were obtained at 70 K and at several photon energies. The second electron affinity of C(120)O was measured to be 1.02+/-0.03 eV and the intramolecular Coulomb repulsion was estimated to be about 0.8 eV in C(120)O(2-) on the basis of the observed repulsive Coulomb barrier. A low-lying excited state ((2)B(1)) was also observed for C(120)O(-) at 0.09 eV above the ground state ((2)A(1)). The C(120)O(2-) dianion can be viewed as a single electron on each C(60) ball very weakly coupled. Theoretical calculations showed that the singlet and triplet states of C(120)O(2-) are nearly degenerate and can both be present in the experiment. The computed electron binding energies and excitation energies, as well as Franck-Condon factors, are used to help interpret the photoelectron spectra. A C-C bond-cleaved isomer, C(60)-O-C(60) (2-), was also observed with a higher electron binding energy of 1.54 eV.     

Homo- und Hetero-Zweikernkomplexe des D2h-symmetrischen Bis(chelat)-Liganden 2,2′-Bipyrimidin mit den elektronenreichen Metallfragmenten Mo(CO)4, Re(CO)3Cl, [Cu(PPh3)2]+ und [Ru(bpy)2]2+

Homo- and Heterodinuclear Complexes of the D_2h-symmetric Bis(chelate) Ligand 2,2′-Bipyrimidine with Electron-Rich Metal Fragments Mo(CO)₄, Re(CO)₃Cl, [Cu(PPh₃)₂]⁺, and [Ru(bpy)₂]²⁺ All homo- and heterodinuclear complexes (L_nM)(μ-bpym)(ML_n)′, bpym = 2,2′-bipyrimidine, ML_n (ML_n)′ = Mo(CO)₄, Re(CO)₃Cl, [Cu(PPh₃)₂]⁺, [Ru(bpy)₂]²⁺, have been synthesized and studied by cyclic voltammetry, absorption spectroscopy, and by electron spin resonance of singly reduced forms. The individual capabilities of the low-valent metal fragments to undergo oxidation and to shift the reduction potential of the bpym π acceptor ligand on coordination combine to result in variable electrochemical potential differences. After consideration of different Franck-Condon factors, absorption intensities, additional low-lying unoccupied orbitals of the bridging acceptor ligand and solvatochromic effects, we have assigned the considerably varying metal-to-ligand charge transfer transitions in the visible.     

Isomer-selected photoelectron spectroscopy of isolated DNA oligonucleotides: phosphate and nucleobase deprotonation at high negative charge states

Fractionation according to ion mobility and mass-to-charge ratio has been used to select individual isomers of deprotonated DNA oligonucleotide multianions for subsequent isomer-resolved photoelectron spectroscopy (PES) in the gas phase. Isomer-resolved PE spectra have been recorded for tetranucleotides, pentanucleotides, and hexanucleotides. These were studied primarily in their highest accessible negative charge states (3-, 4-, and 5-, respectively), as provided by electrospraying from room temperature solutions. In particular, the PE spectra obtained for pentanucleotide tetraanions show evidence for two coexisting classes of gas-phase isomeric structures. We suggest that these two classes comprise: (i) species with excess electrons localized exclusively at deprotonated phosphate backbone sites and (ii) species with at least one deprotonated base (in addition to several deprotonated phosphates). By permuting the sequence of bases in various [A(5-x)T(x)](4-) and [GT(4)](4-) pentanucleotides, we have established that the second type of isomer is most likely to occur if the deprotonated base is located at the first or last position in the sequence. We have used a combination of molecular mechanics and semiempirical calculations together with a simple electrostatic model to explore the photodetachment mechanism underlying our photoelectron spectra. Comparison of predicted to measured photoelectron spectra suggests that a significant fraction of the detected electrons originates from the DNA bases (both deprotonated and neutral).     

Synthesis of Aryl Tri‐ and Difluoromethyl Thioethers via a CH‐Thiocyanation/Fluoroalkylation Cascade

An AlCl₃‐catalyzed C?H thiocyanation was discovered and combined with a Langlois‐type trifluoromethylation to afford aryl trifluoromethyl thioethers directly from arenes, N‐thiocyanatosuccinimide (NTS) and Ruppert–Prakash reagent. An analogous combination with a copper‐mediated difluoromethylation gives access to aryl difluoromethyl thioethers. Both processes proceed with exceptional regioselectivity for the most electron‐rich, sterically least hindered position of the arene. The sulfur and fluoroalkyl groups originate from different sources, so that the use of expensive, preformed fluoroalkylthiolation reagents is avoided.     

Probing electrostatic interactions and structural changes in highly charged protein polyanions by conformer-selective photoelectron spectroscopy

We have recorded the first conformer-selective photoelectron spectra of a protein polyanion in the gas-phase. Bovine cytochrome c protein was studied in 8 different negative charge states ranging from 5- to 12-. Electron binding energies were extracted for all charge states and used as a direct probe of intramolecular Coulomb repulsion. Comparison of experimental results with simulations shows that the experimental outcome can be reproduced with a simple electrostatic model. Energetics are consistent with a structural transition from a folded to an unfolded conformational state of the protein as the number of charges increases. Furthermore, the additional ion-mobility data show that the onset of unfolding can be assigned to charge state 6- where three conformers can be distinguished.