Influence of Decreasing Solvent Polarity (1,4‐Dioxane/Water Mixtures) on the Acid–Base and Copper(II)‐Binding Properties of Guanosine 5′‐Diphosphate

The acidity constants of twofold protonated guanosine 5′‐diphosphate, H₂(GDP)^?, and the stability constants of the [Cu(H;GDP)] and [Cu(GDP)]^? complexes were determined in H₂O as well as in 30 or 50% (v/v) 1,4‐dioxane/H₂O by potentiometric pH titrations (25°; I=0.1M , NaNO₃). The results showed that in H₂O one of the two protons of H₂(GDP)^? is located mainly at the N(7) site and the other one at the terminal β‐phosphate group. In contrast, for 50% 1,4‐dioxane/H₂O solutions, a micro acidity‐constant evaluation evidenced that ca. 75% of the H₂(GDP)^? species have both protons phosphate‐bound, because the basicity of pyridine‐type N sites decreases with decreasing solvent polarity whereas the one of phosphate groups increases. In the [Cu(H;GDP)] complex, the proton and the metal ion are in all three solvents overwhelmingly phosphate‐bound, and the release of this proton is inhibited by decreasing polarity of the solvent. Based on previously determined straight‐line plots of log K vs. pK (where R represents a non‐interacting residue in simple diphosphate monoesters ROP(O^?)(?O)? O? P(?O)(O^?)₂, R? DP^3?), which were now extended to mixed solvents (based on analogies), it is concluded that, in all three solvents, the [Cu(GDP)]^? complex is more stable than expected based on the basicity of the diphosphate residue. This increased stability is attributed to macrochelate formation of the phosphate‐coordinated Cu²⁺ with N(7) of the guanine residue. The formation degree of this macrochelate amounts in aqueous solution to ca. 75% (being thus higher than that of the Cu²⁺ complex of adenosine 5′‐diphosphate) and in 50% (v/v) 1,4‐dioxane/H₂O to ca. 60%, i.e., the formation degree of the macrochelate is only relatively little affected by the change in solvent, though it needs to be emphasized that the overall stability of the [Cu(GDP)]^? complex increases with decreasing solvent polarity. By including previously studied systems in the considerations, the biological implications are shortly discussed, and it is concluded that Nature has here a tool to alter the structure of complexes by shifting them on a protein surface from a polar to an apolar region and vice versa.     

The formation of neutral CCCO2H and HCCCO2 molecules from anionic precursors in the gas phase: a joint experimental and theoretical study

Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory indicate that the anions -CCCO2H and HCCCO2(-) are stable species in their singlet states. Upon collision-induced, vertical one-electron oxidation under neutralisation-reionisation (-NR+) conditions, they produce the neutral molecules CCCO2H and HCCCO2, respectively. Some of the CCCO2H neutrals should be stable for the duration of the neutralisation-reionisation experiment (10(-6) s), while others will dissociate to CCCO and OH (requires 125 kJ mol(-1)). In contrast, neutral HCCCO2 is expected to be much less stable, and dissociate to HCC and CO2 (37 kJ mol(-1)). Neither CCCO2H nor HCCCO2 is expected to interconvert, or to rearrange to other isomers. The anions -CCCO2H and HCCCO2(-) have been formed in the ion source of the mass spectrometer by the reactions between (CH3)3Si-C[triple bond]C-CO2H and F- and HC[triple bond]C-CO2Si(CH3)3 and F-, respectively. The -NR+ spectrum of -CCCO2H shows a recovery signal and also indicates that the lowest energy dissociation pathway of neutral CCCO2H corresponds to the loss of OH. The -NR+ spectrum of HCCCO2 displays little or no recovery signal, and the spectrum is dominated by the [CO2]+ ion. The experimental observations are in agreement with the predictions of the extensive theoretical studies.     

Determination of the Configuration of Wine Lactone

The intense sweet and coconut-like smelling odorant 1 , named ‘wine lactone’, was isolated from different wine varieties. The chemical structure of this compound, which has not yet been detected in wine or a food, was identified by high-resolution mass spectrometry (HR-MS) as 3a,4,5,7a-tetrahydro-3,6-dimethylbenzofuran-2(3H)-one. For the evaluation of the configuration of wine lactone, stereochemically controlled syntheses were developed. All eight isomers were characterized by NMR, MS, IR, and CD measurements. The configuration of ‘wine lactone’ was in agreement with synthesized (3S,3aS,7aR)-enantiomer ( 1a ) on the basis of enantioselective GC. For this isomer, the lowest odor threshold (0.02 pg/1 air) was detected.     

3-(2,5-Dioxopyrrolidin-1-yl), 3-(2,6-dioxopiperidin-1-yl), and 3-(1,3-dioxoisoindolin-2-yl)rhodanines. a novel type of rhodanine derivatives

3-Aminorhodanine 2 was reacted with cyclic anhydrides yielding 3-(2,5-dioxopyrrolidin-1-yl), 3-(2,6-dioxopiperidin-1-yl), and 3-(1,3-dioxoisoindolin-2-yl)rhodanine 6-8 , which in turn were condensated in 5 position with several aldehydes yielding the new rhodanine derivatives 9-11 .     

Homo-metathesis of vinylsilanes catalysed by ruthenium carbene complexes

Effective homo-metathesis of a series of dichloro-substituted vinylsilanes H₂C = C(H)SiCl₂R (where R = Me, OSiMe₃, C₆H₅, C₆H₄–Me-4, C₆H₄–CF₃-4) in the presence of second generation Grubbs catalyst [Cl₂(PCy₃)(IMesH₂)Ru(=CHPh)] (I) and Hoveyda–Grubbs catalyst (II) leads to selective formation of E-1,2-bis(silyl)ethenes and ethene. On the basis of the results of experiments with deuterium-labelled reagents, a metallacarbene mechanism has been suggested for these reactions.     

The Reaction of Transient 1,1‐Bis(trimethylsilyl)silenes with Tris(trimethylsilyl)silyllithium – Synthesis and Structure of Sterically Congested 1‐Trimethylsilylalkylpolysilanes

Tris(trimethylsilyl)silyllithium ( 3 ) reacted with aldehydes and ketones (molar ratio 2 : 1) according to a modified Peterson mechanism under formation of transient silenes, which were immediately trapped by excess 3 to give the organolithium derivatives (Me₃Si)₃SiSi(SiMe₃)₂C(Li)R¹R² ( 7 ). Hydrolysis of 7 afforded the alkylpolysilanes (Me₃Si)₃SiSi(SiMe₃)₂CHR¹R² ( 8 ). Depending on the substituents R¹ and R², 7 proved to be rather unstable in THF solution and underwent a rapid rearrangement, involving a 1,3‐Si,C‐trimethylsilyl migration, resulting in the formation of the lithium silanides (Me₃Si)₂Si(Li)Si(SiMe₃)₂C(SiMe₃)R¹R² ( 9 ), which were hydrolized during the aqueous workup to give the H‐silanes (Me₃Si)₂Si(H)Si(SiMe₃)₂C(SiMe₃)R¹R² ( 10 ). Reaction of 9 with chlorotrimethylsilane produced the 1‐trimethylsilylalkylpolysilanes (Me₃Si)₃SiSi(SiMe₃)₂C(SiMe₃)R¹R² ( 11 ). The structures of the products described were elucidated by comprehensive spectral analyses. The results of X‐ray crystal structure analyses, performed for 8 l (R¹ = H, R² = 2,4,6‐(MeO)₃C₆H₂), 10 d (R¹ = H, R² = Mes) and 11 d (R¹ = H, R² = Mes) are discussed and confirm the expected extreme sterical congestion of the molecules.     

Rate constants of the thermal cis‐trans isomerization of azobenzene dyes in solvents,acetone/water mixtures,and in microheterogeneous surfactant solutions

Rate constants k_iso of the thermal cis‐trans isomerization of four 4,4’‐nitro‐aminoazobenzenes with different amino groups have been determined in homogeneous aprotic solvents and polyglykol oligomers, primarily by means of conventional flash photolysis. The rate constants have been correlated with polarity (according to λ_max from UV/Vis absorption spectra of the trans isomers) and bulk viscosity of the solvents. Qualitative conclusions about the influence of varying concentrations of water with respect to polarity and hydrogen bonding on k_iso‐ and λ_max‐values in acetone/water mixtures were derived. Based on these results the data from microheterogeneous solutions have been interpreted. In microheterogeneous water/surfactant solutions k_iso‐values of selected azo dyes were strongly dependent on the concentrations of SDS, Triton^®X‐100, C₁₂EO₈ in water, and varied with the composition of bicontinuous microemulsions of Igepal^® CA‐520/ heptane/water. The large spread of isomerization rate constants is in part due to varying microviscosity. Replacement of H₂O by D₂O in aqueous surfactant solutions produced surprisingly large kinetic solvent isotope effects. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 337–350, 1999     

The influence of additives on the X-ray induced aggregation of malate synthase monitoring of the aggregation processin situ by time-resolved small-angle X-ray scattering

The X-ray induced aggregation of the sulfhydryl enzyme malate synthase in aqueous solution was monitoredin situ by time-resolved small-angle X-ray scattering. Experiments were performed in the absence/presence of various additives: formate, superoxide dismutase, catalase, NaCl, acetyl-CoA, glyoxylate, malate, pyruvate, -ketobutyrate, oxaloacetate, glycollate, lactate. The scattering curves were measured as a function of the time of irradiation and were analysed in terms of radii of gyration, degrees of aggregation, distance distribution functions, and parameters derived therefrom. Irradiation in the absence of additives resulted in a strong aggregation of enzyme particles. Each of the additives impeded aggregation, however to a different extent. The OH scavenger formate reduced aggregation efficiently; less pronounced effects were registered for superoxide dismutase and/or catalase (the scavengers for and H₂O₂), and for NaCl. Very pronounced diminutions of the aggregation phenomena were provided by substrates or analogues; the efficiency of these substances as radioprotectors may be explained by their action as both scavengers and specific ligands. Based on these results some implications for the performance of conventional small-angle X-ray scattering experiments on biopolymers are derived.

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Der Einfluß von Zusätzen auf die röntgeninduzierte Aggregation der Malatsynthase. Registrierung des Aggregationsvorganges in situ mittels zeitaufgelöster Röntgenkleinwinkelstreuung

Zusammenfassung Die röntgeninduzierte Aggregation des Sulfhydrylenzyms Malatsynthase in wäßriger Lösung wurdein situ mittels zeitaufgelöster Röntgenkleinwinkelstreuung messend verfolgt. Streuexperimente wurden in An- bzw. Abwesenheit verschiedener Zusätze durchgeführt: Formiat, Superoxiddismutase, Catalase, NaCl, Acetyl-CoA, Glyoxylat, Malat, Pyruvat, -Ketobutyrat, Oxalacetat, Glycolat, Lactat. Die Streukurven wurden als Funktion der Bestrahlungsdauer registriert. Die Auswertung lieferte Streumassenradien, Aggregationsgrade, Abstandsverteilungsfunktionen, und daraus abgeleitete Parameter. Die Bestrahlung in Abwesenheit von Zusätzen verursachte eine starke Aggregation der Enzymteilchen. Jeder der Zusätze verminderte die Aggregation, wenn auch in unterschiedlichem Ausmaß. Der OH Fänger Formiat verringerte die Aggregation wirksam; weniger stark ausgeprägte Effekte ergaben sich für die bzw. H₂O₂ Fänger Superoxiddismutase bzw. Catalase und für NaCl. Substrate und Substratanaloge reduzierten das Ausmaß der Aggregation besonders wirkungsvoll; der Schutzeffekt dieser Substanzen kann durch ihre zweifache Wirkung als Fänger bzw. spezifische Liganden erklärt werden. Ausgehend von diesen Ergebnissen werden einige Schlußfolgerungen für die Durchführung üblicher Röntgenkleinwinkelexperimente an Biopolymeren abgeleitet.