Infinite-order phase transition in a classical spin system

For an exactly soluble classical spin model with long-range inhomogeneous coupling it is proved that in the absence of external magnetic field the free energy is aC function of the temperature at the critical point.     

Binding of single nucleotides to H+-ATP synthases observed by fluorescence resonance energy transfer

F(0)F(1)-ATP synthases couple proton translocation with the synthesis of ATP from ADP and phosphate. The enzyme has three catalytic nucleotide binding sites, one on each beta-subunit; three non-catalytic binding sites are located mainly on each alpha-subunit. In order to observe substrate binding to the enzyme, the H(+)-ATP synthase from Escherichia coli was labelled selectively with the fluorescence donor tetramethylrhodamine (TMR) at position T106C of the gamma-subunit. The labelled enzymes were incorporated into liposomes and catalysed proton-driven ATP synthesis. The substrate ATP-Alexa Fluor 647 was used as the fluorescence acceptor to perform intermolecular fluorescence resonance energy transfer (FRET). Single molecules are detected with a confocal set-up. When one ATP-Alexa Fluor 647 binds to the enzyme, FRET can be observed. Five stable states with different intermolecular FRET efficiencies were distinguished for enzyme-bound ATP-Alexa Fluor 647 indicating binding to different binding sites. Consecutive hydrolysis of excess ATP resulted in stepwise changes of the FRET efficiency. Thereby, gamma-subunit movement during catalysis was directly monitored with respect to the binding site with bound ATP-Alexa Fluor 647.     

Biosynthesis of isoprenoids. purification and properties of IspG protein from Escherichia coli

[Structure: see text]. The IspG protein is known to catalyze the transformation of 2-C-methyl-d-erythritol 2,4-cyclodiphosphate into 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate in the nonmevalonate pathway of isoprenoid biosynthesis. We have found that the apparent IspG activity in the cell extracts of recombinant Escherichia coli cells as observed by a radiochemical assay can be enhanced severalfold by coexpression of the isc operon which is involved in the assembly of iron-sulfur clusters. The recombinant protein was isolated by affinity chromatography under anaerobic conditions. With a mixture of flavodoxin, flavodoxin reductase, and NADPH as the reducing agent, stringent assay methods based on photometry or on 13C NMR detection of multiply 13C-labeled substrate/product ratios afforded catalytic activities greater than 60 nmol mg(-1) min(-1) for the protein "as isolated" (i.e., without reconstitution of any kind). Lower apparent activities were found using photoreduced deazaflavin as an artifactual electron donor, whereas dithionite was unable to serve as an artificial electron donor. The apparent Michaelis constant for 2-C-methyl-D-erythritol 2,4-cyclodiphosphate was 700 microM. The enzyme was inactivated by EDTA and could be reactivated by Mn2+. The pH optimum was at 9.0. The protein contained 2.4 iron ions and 4.4 sulfide ions per subunit. The replacement of any of the three conserved cysteine residues afforded mutant proteins which were devoid of catalytic activity and contained less than 6% of Fe2+ and less than 23% of S2- as compared to the wild-type protein. Sequence comparison indicates that putative IspG proteins of plants, the apicomplexan protozoan Plasmodium falciparum, and bacteria from the Bacteroidetes/Chlorobi group contain an insert of about 170-320 amino acid residues as compared with eubacterial enzymes.     

Fourier transform ion cyclotron resonance study of ion-molecule reactions of [M – OCH3]+ ions of methyl 2,3,4,6-tetra-O-methyl-d-hexopyranosides with ammonia

Glycosidic oxocarbenium ions A₁⁺ were formed by isobutane chemical ionization from methyl 2,3,4,6-tetra-O-methyl-β-D -mannopyranoside, methyl 2,3,4,6-tetra-0-methyl-β-D -galactopyranoside and methyl 2,3,4,6-tetra-O-methyl-β-D -glucopyranoside (the ring - O-being converted into ? O ? ), and then- reaction with ammonia was studied by Fourier transform ion cyclotron resonance Spectrometry. Very slow formation (reaction efficiency 0.6-1.4%) of the adduct ion [A₁ + NH₃]⁴ was observed as the main process for carefully thermalized ions A₁⁺. Interestingly, the efficiency of the adduct ion formation depends on the sterochemistry of ions A₁⁺.