An efficient approach for the characterization of mucin-type glycopeptides: the effect of O-glycosylation on the conformation of synthetic mucin peptides

Despite the growing importance of mucin core O-glycosylation in many biological processes including the protection of epithelial cell surfaces, the immune response, cell adhesion, inflammation, and tumorigenesis/metastasis, the regulation mechanism and conformational significance of the multiple introduction of α-GalNAc residues by UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (ppGalNAcTs) remains unclear. Here we report an efficient approach by combining MS and NMR spectroscopy that allows for the identification of O-glycosylation site(s) and the effect of O-glycosylation on the peptide backbone structures during enzymatic mucin domain assembly by using an isoform UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase-T2 (ppGalNAcT2) in vitro. An electron-capture dissociation device in a linear radio-frequency quadrupole ion trap (RFQ-ECD) combined with a time-of-flight (TOF) mass spectrometer was employed for the identification of Thr/Ser residues occupied by α-GalNAc branching among multiple and potential O-glycosylation sites in the tandem repeats of human mucin glycoproteins MUC4 (Thr-Ser-Ser-Ala-Ser-Thr-Gly-His-Ala-Thr-Pro-Leu-Pro-Val-Thr-Asp) and MUC5AC (Pro-Thr-Thr-Val-Gly-Ser-Thr-Thr-Val-Gly). In the present study, O-glycosylation was initiated specifically at Thr10 in naked MUC4 peptide and additional introduction of α-GalNAc proceeded preferentially but randomly at three other Thr residues to afford densely glycosylated MUC4 containing six α-GalNAc residues at Thr1, Ser2, Ser5, Thr6, Thr10, and Thr15. On the contrary, O-glycosylation of naked MUC5AC peptide occurred predominantly at consecutive Thr residues and led to MUC5AC with four α-GalNAc residues at Thr2, Thr3, Thr7, and Thr8. The solution structures determined by NMR spectroscopic studies elicited that the preferential introduction of α-GalNAc at Thr10 of MUC4 stabilizes specifically a β-like extended backbone structure at this area, whereas other synthetic models with a single α-GalNAc residue at Thr1, Thr6, or Thr15 did not exhibit any converged three-dimensional structure at the proximal peptide moiety. Such conformational impact on the underlying peptides was proved to be remarkable in the glycosylation at the consecutive Thr residues of MUC5AC.     

Direct enantioseparation of lipoic acid in dietary supplements by capillary electrophoresis using trimethyl-β-cyclodextrin as a chiral selector

Lipoic acid, an antioxidant, naturally occurs as the (R)-enantiomer, while synthetic lipoic acid is racemic. It is thus of interest to know the (R)-enantiomer content of lipoic acid supplements. Here, we used capillary electrophoresis to directly enantioseparate lipoic acid in dietary supplements by using a sulfonated capillary with an effective voltage of +18 kV and direct detection at 200 nm. Factors affecting migration time and resolution of lipoic acid were investigated. The optimum background electrolyte was found to be 100 mM phosphate buffer (pH 7.0) containing 8 mM trimethyl-β-cyclodextrin as a chiral selector at 20°C. Under the proposed conditions, direct chiral resolution of lipoic acid in dietary supplements was conducted successfully.     

A highly negatively charged γ-Keggin germanodecatungstate efficient for Knoevenagel condensation

A tetra-n-butylammonium (TBA) salt of a γ-Keggin -6-charged germanodecatungstate, [γ-H(2)GeW(10)O(36)](6-) (I), could act as an efficient homogeneous catalyst for Knoevenagel condensation of active methylene compounds with carbonyl compounds.     

A catalytic asymmetric Ugi-type reaction with acyclic azomethine imines

An unconventional Ugi: A catalytic asymmetric Ugi-type reaction can be realized by use of N'-alkylbenzohydrazide, instead of a conventional amine, in the presence of an axially chiral dicarboxylic acid. The reaction proceeds through a key acyclic azomethine imine intermediate, which has recently emerged as a promising electrophile in asymmetric catalysis.     

Preferential extraction of left- or right-handed single-walled carbon nanotubes by use of chiral diporphyrin nanotweezers

Handedness- and diameter-based separation of single-walled carbon nanotubes was achieved through preferential extraction of 76-CoMoCAT with 2,6-pyridylene-bridged chiral diporphyrin nanotweezers.     

Synthesis of novel fatty-acyl gratisin derivatives

To find candidates with high antimicrobial and low hemolytic activities, many gratisin (GR) analogues have been designed and synthesized. In the present account, we synthesized novel derivatives of GR having both the polycationic and fatty acyl groups, cyclo{-Val(1)-Orn(2)-Leu(3)-D-Phe(4)-Pro(5)-D-Lys(6)(X)-Val(7)-Orn(8)-Leu(9)-D-Phe(10)-Pro(11)-D-Lys(12)-} {X=-CO(CH(2))(6)CH(3) (1), -Lys-CO(CH(2))(6)CH(3) (2), -(Lys)(2)-CO(CH(2))(6)CH(3) (3), and -(Lys)(3)-CO(CH(2))(6)CH(3) (4)}, and examined the biological activities. Among them, we found that 2-4 have differential ionic interaction against the prokaryotic membrane and eukaryotic membrane. In other words, the dissociation with high antimicrobial activity and low hemolytic activity is caused by the addition of D-Lys(6)-{(Lys)(n)-CO(CH(2))(6)CH(3)} residues at position 6 of [D-Lys(6,12)]-GR. Our findings should be helpful in finding drug candidates with high antimicrobial activity and low hemolytic activity that are capable of combating microbial resistance.     

Configurational Stability of Optically Active Dichloromethyl p‐Tolyl Sulfoxide and Its Anionic Species: Experimental and Theoretical Study

Racemization of optically active dichloromethyl p‐tolyl sulfoxide took place at −78°C in the presence of potassium bis(trimethylsilyl)amide (KHMDS), while the same racemization did not occur under reflux in toluene in the absence of KHMDS. Density functional theory calculations suggested that the pyramidal inversion at the sulfur center was unlikely to be involved in the racemization mechanism. An anionic species of the sulfoxide was found to be gradually converted into chlorobis(p‐tolylsulfinyl)methane and dichlorocarbene. We propose a racemization mechanism mediated by achiral potassium p‐toluenesulfenate and chloro(p‐tolylsulfinyl)methylene. © 2013 Wiley Periodicals, Inc. Heteroatom Chem 24:131–137, 2013; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21074     

Presentations of Grothendieck Constructions

We will give quiver presentations of the Grothendieck constructions of functors from a small category to the 2-category of 𝕜-categories for a commutative ring 𝕜.