Methyl transfer in glycine N-methyltransferase. A theoretical study

Density functional theory calculations using the hybrid functional B3LYP have been performed to study the methyl transfer step in glycine N-methyltransferase (GNMT). This enzyme catalyzes the S-adenosyl-L-methionine (SAM)-dependent methylation of glycine to form sarcosine. The starting point for the calculations is the recent X-ray crystal structure of GNMT complexed with SAM and acetate. Several quantum chemical models with different sizes, employing up to 98 atoms, were used. The calculations demonstrate that the suggested mechanism, where the methyl group is transferred in a single S(N)2 step, is thermodynamically plausible. By adding or eliminating various groups at the active site, it was furthermore demonstrated that hydrogen bonds to the amino group of the glycine substrate lower the reaction barrier, while hydrogen bonds to the carboxylate group raise the barrier.     

Differential SERS activity of gold and silver nanostructures enabled by adsorbed poly(vinylpyrrolidone)

We report that poly(vinylpyrrolidone) (PVP), a common stabilizer of colloidal dispersions of noble metal nanostructures, has a dramatic effect on their surface-enhanced Raman scattering (SERS) activity and enables highly selective SERS detection of analytes of various type and charge. Nanostructures studied include PVP-stabilized Au-Ag nanoshells synthesized by galvanic exchange reaction of citrate-reduced Ag nanoparticles (NPs), as well as solid citrate-reduced Ag and Au NPs, both before and after stabilization with PVP. All nanostructures were characterized in terms of their size, surface plasmon resonance wavelength, surface charge, and chemical composition. While the SERS activities of the parent citrate-reduced Ag and Au NPs are similar for rhodamine 6G (R6G) and 1,2-bis(4-pyridyl)ethylene (BPE) at various pH values, PVP-stabilized nanostructures demonstrate large differences in SERS enhancement factors (EFs) between these analytes depending on their chemical nature and protonation state. At pH values higher than BPE's pK(a2) of 5.65, where the analyte is largely unprotonated, the PVP-coated Au-Ag nanoshells showed a high SERS EF of >10(8). In contrast, SERS EFs were 10(3)- to 10(5)-fold lower for the protonated form of BPE at lower pH values, or for the usually highly SERS-active cationic R6G. The differential SERS activity of PVP-stabilized nanostructures is a result of discriminatory binding of analytes within-adsorbed PVP monolayer and a subsequent increase of analyte concentration at the nanostructure surface. Our experimental and theoretical quantum chemical calculations show that BPE binding with PVP-stabilized Au-Ag nanoshells is stronger when the analyte is in its unprotonated form as compared to its cationic, protonated form at a lower pH.     

A convenient synthesis of long-chain 4-substituted benzyloxycarbonyl alkanethiols for the formation of self assembled monolayers on metal substrates

New 4-substituted benzyl esters of 16-mercaptohexadecanoic acid were prepared by developing a practicable synthetic procedure and using readily available staring materials. The compounds synthesized have been characterized by NMR, MS, IR spectra and elemental analysis. The mercapto derivatives are precursors for the formation of self-assembled monolayers on metal substrates. Dedicated to Professor Dr. Manfred Hesse on the occasion of his 70 ^th birthday     

An efficient approach towards the convergent synthesis of "fully-carbohydrate" mannodendrimers

Glycosylation of sugar trityl ethers with sugar 1,2-O-(1-cyano)ethylidene derivatives (the trityl-cyanoethylidene condensation) has been applied to the synthesis of highly branched (dendritic) mannooligosaccharides incorporating a Manalpha1-->3(Manalpha1-->6)Man structural motif. The convergent synthetic strategy used to assemble these oligosaccharides was based on the use of glycosyl acceptors and/or a glycosyl donor already bearing this structural motif. The former were represented by mono- and ditrityl ethers of ManalphaOMe, Manalpha1-->3ManalphaOMe, and Manalpha1-->3(Manalpha1-->6)ManalphaX, where X=OMe or SEt. The pivotal glycosyl donor was the peracetylated 1,2-O-(1-cyano)ethylidene-3,6-di-O-(alpha-D-mannopyranosyl)-beta-D-mannopyranose (1), prepared by orthogonal Helferich glycosylation of the known 1,2-O-(1-cyano)ethylidene-beta-D-mannopyranose with tetra-O-acetyl-alpha-D-mannopyranosyl bromide followed by O-acetylation. Glycosylation of acetates of methyl 6-O-trityl-alpha-D-mannopyranoside and methyl 3,6-di-O-trityl-alpha-D-mannopyranoside with one equivalent of the donor 1 gave rise to the isomeric tetrasaccharide derivatives, Manalpha1-->3(Manalpha1-->6)Manalpha1-->6ManalphaOMe and Manalpha1-->3(Manalpha1-->6)Manalpha1-->3ManalphaOMe, respectively. The latter derivative was further mannosylated at the remaining 6-O-trityl acceptor site to give the protected pentasaccharide Manalpha1-->3(Manalpha1-->6)Manalpha1-->3(Manalpha1-->6)ManalphaOMe. The isomeric pentasaccharide, Manalpha1-->3(Manalpha1-->6)Manalpha1-->6(Manalpha1-->3)ManalphaOMe, was prepared by reaction of 1 with the 6-O-trityl derivative of (Manalpha1-->3)ManalphaOMe. In a similar fashion, 6'- and 6"-O-trityl derivatives of the branched trisaccharide Manalpha1-->3(Manalpha1-->6)ManalphaOMe served as precursors for two isomeric mannohexaosides. The 3,6-di-O-trityl ether of ManalphaOMe and the 6',6"-di-O-trityl ether of Manalpha1-->3(Manalpha1-->6)ManalphaX (X=OMe or SEt) were efficiently bis-glycosylated with the donor 1 to give the corresponding protected mannoheptaoside and mannononaoside. The yields of these glycosylations with the donor 1 ranged from 50 to 66 %. Final deprotection of all the oligosaccharides was straightforward and afforded the target products in high yields. Both the acylated and deprotected products were characterized, and the intersaccharide connectivities were elucidated by extensive one- and two-dimensional NMR spectroscopy. The described blockwise convergent approach allows assembly of a variety of 3,6-branched mannooligosaccharides.     

Structural Studies and Anticancer Activity of a Novel Class of β‐Peptides

Functionalized oligomeric organic compounds with well‐defined β‐proline scaffold have been synthesized by a cycloadditive oligomerization approach in racemic and enantiopure forms. The structure of the novel β‐peptides was investigated by NMR spectroscopic and X‐ray methods determining the conformational shapes of the β‐proline oligomers in solution and solid states. The main structural elements subject to conformational switches are β‐peptide bonds between 5‐arylpyrrolidine‐2‐carboxylic acid units existing in Z/E configurations. The whole library of short β‐peptides and intermediate acrylamides has been tested on antiproliferative activity towards the hormone‐refractory prostate cancer cell line PC‐3 revealing several oligomeric compounds with low micromolar and submicromolar activities. Bromine‐substituted dimeric and trimeric acrylamides induced caspase‐dependent apoptosis of PC‐3 cells through cell‐cycle arrest and mitochondrial damage.     

Theoretical study of the methyl transfer in guanidinoacetate methyltransferase

The reaction mechanism of the guanidinoacetate methyltransferase (GAMT) enzyme has been investigated by means of density functional theory using the B3LYP hybrid functional. GAMT catalyzes the S-adenosyl-L-methionine (SAM)-dependent methylation of guanidinoacetate (GAA) to form creatine. A quantum chemical model was built on the basis of the recent crystal structure of GAMT complexed with S-adenosylhomocysteine (SAH) and GAA. The methyl group transfer from SAM to N(E) of GAA is shown to occur concertedly with a proton transfer from NE to the neighboring OD1 of Asp134. Good agreement is found between the calculated barrier and the experimental rate.