Uranium-sulfilimine chemistry. The preparation of Cp2*UCl2(HNSPh2) and its hydrolysis with HNSPh2 · H2O

The reaction of Cp₂*UCl₂ with HNSPh₂ produces Cp₂*UCl₂(HNSPh₂), which is the first structurally characterized complex of a sulfilimine. The hydrolysis of Cp₂*UCl₂(HNSPh₂) with HNSPh₂ · H₂O yields a tetrauranium cluster whose heavy atom structure has been determined by x-ray diffraction and which is formulated as a U^IV/U^V complex: [Cp*(Cl)(HNSPh₂)U(μ₃-O)(μ₂-O)₂U(Cl)(HNSPh₂)₂]₂.     

Fragmentation of Homoallylic Alkoxides. Preparation of 1-(3′-cyclopentenyl)-2-alkanones from 2-substituted bicyclo[2.2.1]hept-5-en-2-ols

The potassium 2-substituted bicyclo[2.2.1]hept-5-en-2-alkoxides derived from alcohols 2–9 at 30° in hexamethylphosphoric triamide (HMPA) afford 1-(3′-cyclopentenyl)-2-alkanones 10–19 via heterolytic C(1), C(2)-allylic bond cleavage in the substrate alkoxide followed by intramolecular protonation of the resultant transient allylic anion.     

Synthesis of functionalized rab GTPases by a combination of solution- or solid-phase lipopeptide synthesis with expressed protein ligation

Prenylated proteins with non-native functionalities are generally very difficult to obtain by recombinant or enzymatic means. The semisynthesis of preparative amounts of prenylated Rab guanosine triphosphatases (GTPases) from recombinant proteins and synthetic prenylated peptides depends largely on the availability of functionalised prenylated peptides corresponding to the proteins' native structure or modifications thereof. Here, we describe and compare solution-phase and solid-phase strategies for the generation of peptides corresponding to the prenylated C terminus of Rab7 GTPase. The solid-phase with utilisation of a hydrazide linker emerges as the more favourable approach. It allows a fast and practical synthesis of pure peptides and gives a high degree of flexibility in their modification. To facilitate the analysis of semisynthetic proteins, the synthesised peptides were equipped with a fluorescent group. Using the described approach, we introduced fluorophores at several different positions of the Rab7 C terminus. The position of the incorporated fluorescent groups in the peptides did not influence the protein-ligation reaction, as the generated peptides could be ligated onto thioester-tagged Rab7. However, it was found that the positioning of the fluorescent group had an influence on the functionality of the Rab7 proteins; analysis of the interaction of the semisynthetic Rab7 proteins with REP (Rab escort protein) and GDI (guanosine diphosphate dissociation inhibitor) molecules revealed that modification of the peptide side chains or of the C-terminal isoprenoid did not significantly interfere with complex formation. However, functionalisation of the C terminus was found to have an adverse effect on complex formation and stability, possibly reflecting low structural flexibility of the Rab GDI/REP molecules in the vicinity of the lipid-binding site.     

Luminol Oxidation by Hydrogen Peroxide Catalyzed by Tobacco Anionic Peroxidase: Steady-State Luminescent and Transient Kinetic Studies

The properties of a newly isolated anionic tobacco peroxidase from transgenic tobacco plants overexpressing the enzyme have been studied with respect to the chemiluminescent reaction of luminol oxidation. These were compared to the properties of horseradish peroxidase in the cooxidation of luminol and p -iodophenol, the enhanced chemiluminescence reaction. The pH, luminol and hydrogen peroxide concentrations were optimized for maximum sensitivity using the tobacco enzyme. The detection limit for the latter under the optimal conditions (2.5 m M luminol, 2 m M hydrogen peroxide, 100 m M Naborate buffer, pH 9.3) was about 0.1 p M , which is at least five times lower than that for horseradish peroxidase in enhanced chemiluminescence with p -iodophenol. The rate constants for the elementary steps of the enzyme-catalyzed reaction have been determined: k ₁= 4.9 × 10⁶ M ⁻¹ s¹, k ₂= 7.3 × 10⁶ M ⁻¹ s⁻¹, k ³= 2.1 × 10⁶ M ⁻¹ s⁻¹ (pH 9.3). The similarity of these rate constants is unusual for plant peroxidases. The high catalytic activity of tobacco peroxidase in the luminescent reaction is explained by the high reactivity of its Compound II toward luminol and the high stability of the holoenzyme with respect to heme dissociation. This seems to be a unique property of this particular enzyme among other plant peroxidases.     

A novel one-pot method for the preparation of pyrazoles by 1,3-dipolar cycloadditions of diazo compounds generated in situ

A convenient one-pot procedure for the preparation of pyrazoles by 1,3-dipolar cycloaddition of diazo compounds generated in situ has been developed. Diazo compounds derived from aldehydes were reacted with terminal alkynes to furnish regioselectively 3,5-disubstituted pyrazoles. Furthermore, the reaction of N-vinylimidazole and diazo compounds derived from aldehydes gave exclusively 3-substituted pyrazoles in a one-pot process.     

The Raman active internal vibrational modes of potassium nitrate

The Raman active internal vibrational modes of single crystal orthorhombic potassium nitrate have been studied in various polarizations. The full multiplet structure predicted by factor group analysis for the v₂ and v₃ regions has been observed for the first time. The expected site group splitting of the v₄ mode was not observed and can be assumed to be less than 0.5 cm^?1.     

Multivalent ion/polyelectrolyte exchange processes in exponentially growing multilayers

We show, in this paper that multivalent ferrocyanide anions can penetrate into exponentially growing (PGA/PAH)n multilayer films whatever the nature of the last deposited layer. These ions are not able to diffuse out of the film when it is brought in contact with a pure buffer solution. However, the contact of this film with a poly(allylamine) (PAH) or a poly(L-glutamic acid) (PGA) solution leads to the release of ferrocyanide ions from the multilayer. It is shown that the release of ferrocyanide anions, when the film is in contact with a PGA solution, is due to the diffusion of the PGA chains into the film so that an exchange between ferrocyanide ions and PGA chains takes place inside the film. On the other hand, PAH chains do not diffuse into PGA/PAH multilayers. When the film is then brought in contact with a PAH solution, the PAH chains from the solution are expected to strongly interact with the ferrocyanide ions and thus induce a diffusion mechanism of the multivalent anions out of the film, the film/solution interface playing the role of a sink for these ions. This work thus shows that interactions between multivalent ions and exponentially growing films are much more complex than expected at first sight and that polyelectrolyte multilayers must be seen as dynamic entities in which diffusion and exchange processes can take place.     

Role of the spacer in the singlet-singlet energy transfer mechanism (Förster vs Dexter) in cofacial bisporphyrins

The cofacial bisporphyrins H4DPS (DPS = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzothiophene), H4DPO (DPO = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzofuran), H4DPX (DPX = 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene), H4DPA (DPA = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]anthracene), and H4DPB (DPB = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]biphenylene) have been monometalated by Zn(OAc)2.2H2O and by GaCl3 to explore the singlet-singlet energy transfer from the photoexcited metal porphyrin center to the linked free base porphyrin. The spectroscopic (UV-vis and fluorescence) and photophysical properties (fluorescence lifetimes, tauF, and quantum yields, phiF) have been investigated at 298 and 77 K in degassed 2-MeTHF for the donor-acceptor systems, (Zn)H2DPS, (Zn)H2DPO, (Zn)H2DPA, (Zn)H2DPX, and (Zn)H2DPB, as well as for the bis-zinc complexes, (Zn)2DPS, (Zn)2DPO, (Zn)2DPX, and (Zn)2DPB, respectively, and the monoporphyrin derivatives, H2P, (Zn)P, and (Ga-OMe)P (P2- = 5-phenyl-2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrin-dianion). The singlet-singlet energy transfer rate constants (KET) were obtained using KET = (1/tauF -1/tauFo), where tauFo is the fluorescence lifetime of the corresponding bis-zinc(II) systems (or (Zn)P and (Ga-OMe)P) where no energy transfer occurs. The tauF value for three bis-zinc(II) compounds varies from 1.69 to 2.01 ns and is 1.84 (at 298 K) and 3.20 ns (at 77 K) for (Ga-OMe)P. In the donor-acceptor bismacrocycles, depending on the spacer and the temperature, the fluorescence lifetimes decrease down to 50-240 ps. The KET values range from approximately 4 to approximately 21 (ns(-1)) and have been analyzed considering both the F?rster and the Dexter mechanisms. Using the C(meso)-C(meso) distance parameters in the calculations, the F?rster and Dexter mechanisms operate for DPS and DPO, and for DPA, DPX, and DPB spacer systems, respectively. The limit distance where one mechanism dominates over the other is estimated to be around 5-6 A.     

The influence of the purine 2-amino group on DNA conformation and stability-II: Synthesis and physical characterization of d[CGT(2-NH2)ACG], d[CGU(2-NH2)ACG], and d[CGT(2-NH2)AT(2-NH2)ACG]

Synthesis of an N-protected derivative of 2-amino-2'-deoxyadenosine (dA') from 2'-deoxyguanosine is reported. The syntheses of several oligodeoxynucleotides containing this modified nucleoside are described, together with physical characterization via melting studies and CD conformational analysis. As expected, the 2-amino group is seen to add to the duplex stability. Although the sequence d(TA')₃ was found to undergo a salt-induced conformational transition, mixed sequences such as d(CGTA'CG) did not display this behavior. All guanine residues present in these sequences were O⁶ protected, either with the cyanoethyl group or the 4-nitrophenylethyl group, to eliminate guanine degradation during phosphorylation and condensation reactions. Procedures for the introduction and removal of these O⁶ protecting groups are described.     

The reactivity of 1,4-dihydronicotinamides towards reduction of 1,1′,1″-trifluoro-acetophenone : influence of methyl substitution on the pyridine ring.

In a series of methyl substituted 1-(2′,6′-dichlorobenzyl)-1,4-dihydronicotinamides those compounds with the methyl α to the pyridine nitrogen are at least one order of magnitude more reactive than the analogues without methyl or methyl on the 5-position.