DISTANCE-DEPENDENT FLUORESCENCE QUENCHING OF TRYPTOPHAN BY ACRYLAMIDE

Abstract We used GHz frequency-domain fluorometry to investigate the time-dependent intensity decays of N -acetyl -L-trytophanamide (NATA) when collisionally quenched by acrylamide in propylene glycol at 20°C. The intensity decays of NATA became increasingly heterogeneous in the presence of acrylamide. The NATA intensity decays were not consistent with the Collins-Kimball radiation boundary condition (RBC) model for quenching. The steady-state Stern-Volmer plots show significant upward curvature. At low temperature in vitrified propylene glycol (-60%), where translational diffusion cannot occur during the lifetime of the excited state, quenching of NATA by acrylamide was observed. The Smoluchowski and RBC quenching models do not predict any quenching in the absence of translational diffusion. Hence, these frequency-domain and steady-state data indicate a through-space quenching interaction between NATA and acrylamide. The rate for quenching of NAT A by acrylamide appears to depend exponentially on the fluorophore-quencher separation distance. Comparison of the time-resolved and steady-state data provides a sensitive method to determine the distance dependence of the fluorophore-quencher interaction. The distance-dependent rate of quenching also explains the upward curvature of the Stern-Volmer plot, which is often observed for quenching by acrylamide. These results suggest that the distance-dependent quenching rates need to be considered in the interpretation of quenching data of proteins by acrylamide.     

Studies of lithium and sodium complexation by silicon podand solvents

The complex formation of lithium and sodium ions with silicon podand solvents: phenyl-tris(1,4-dioxapentyl) silane (PhSi23) and ethyl-tris(1,4-dioxapentyl) silane (EtSi23) has been studied by FTIR, ¹H-, ¹³C-, ⁷Li- and ²³Na NMR. The far FTIR spectra show that the Li⁺ cations fluctuate very fast whereas Na⁺ cations are still localised between the oxygen atoms of the oxaalkyl chains. The ⁷Li NMR spectra prove that one Li⁺ cation can be coordinated not only by one but also two silicon podand molecules. The concentration dependence of the molar conductivity of LiClO₄ in the podand solvents indicates charge transfer between ion clusters.     

The [ν]-M relationship for linear polydimethylsiloxane (PDMS)

Weight-average molecular weights for PDMS fractions were determined by light scattering and the Archibald method in the range of 3000–300,000; the [ν]-M? relationship for toluene, 25°, was found to be

$\text{[?]=1.87 × 10}{}^{\mathrm{?2}}\text{M}{}_{\mathrm{<mtext>w</mtext>}}{}^{03658}\text{,}\text{cm3/g}$

This relationship was compared with those previously used. No upward curvature of the linear relation, log $\text{[ν] = ?(}\text{M}\text{)}$, at lower molecular weights was observed. Over the whole range of molecular weights, the [ν]-values in toluene were higher than [ν]_θ-values in bromocyclohexane at 28°.     

ESI MS, spectroscopic and PM5 semiempirical studies of Colchicine complexes with lithium, sodium and potassium salts

The colchicine complexes with Li⁺, Na⁺ and K⁺ cations have been synthesized and studied by ESI MS, ¹H and ¹³C NMR, FT-IR and PM5 semiempirical methods. It has been shown that colchicine forms stable complexes of 1:1 stoichiometry with monovalent metal cations. For Li⁺ cations also formation of the 2:1 stoichiometry complexes has been detected. The most stable structures of the complexes are those in which the acetamide groups are involved in the coordination process. The structures of the colchicine complexes with Li⁺, Na⁺ and K⁺ cations are visualized and discussed in details.     

Application of Organolithium and Related Reagents in Synthesis. Part 221. Synthetic Strategies Based on Ortho-Aromatic Metallation. A Concise Regiospecific Conversion of Benzoic Acids into the 4-Pyridyl-2H-Phthalazin-1-Ones

The synthesis of phthalazin-1-ones 6, 7, 8 via the reaction of 3-hydroxyisoindolin-l-ones 3, 4, 5 with hydrazine hydrate is described. Starting compounds 3, 4, 5 were regiospecifically prepared upon the lithiation (n-BuLi) of the benzanilides 1 and subsequently the reaction of the dilithiated anilides 2 with methyl pyridinecarboxylates.     

Spectroscopic studies and PM5 semiempirical calculations of new hydrazone of gossypol with 3,6,9-trioxadecylhydrazine

A new hydrazone of gossypol with 3,6,9-trioxadecylhydrazine (GHTO) has been synthesised and its structure has been studied by ¹H NMR, ¹³C NMR, FT-IR spectroscopy and PM5 semiempirical methods. The results have shown that the newly synthesised hydrazone exists in solution in the N-imine–N-imine tautomeric form, stabilized by several intramolecular hydrogen bonds among which the O₇H N₁₆ intramolecular hydrogen bond is the strongest. The structure of GHTO is visualized by the PM5 semiempirical calculations.     

One-electron properties and electrostatic interaction energies from the expectation value expression and wave function of singles and doubles coupled cluster theory

One-electron density matrices resulting from the explicitly connected commutator expansion of the expectation value were implemented at the singles and doubles coupled cluster (CCSD) level. In the proposed approach the one-electron density matrix is obtained at a little extra cost in comparison to the calculation of the CCSD correlation energy. Therefore, in terms of the computational time the new method is significantly less demanding than the conventional linear-response CCSD theory which requires additionally an expensive calculation of the left-hand solution of the CCSD equations. The quality of the new density matrices was investigated by computing a set of one-electron properties for a series of molecules of varying sizes and comparing the results with data obtained using the full configuration interaction method or higher level coupled cluster theory. It has been found that the results obtained using the new approach are of the same quality as those predicted by the linear-response CCSD method. The novel one-electron density matrices have also been applied to study the energy of the electrostatic interaction for a number of van der Waals complexes, including the benzene and azulene dimers.     

Intricate role of water in proton transport through cytochrome c oxidase

Cytochrome c oxidase (CytcO), the final electron acceptor in the respiratory chain, catalyzes the reduction of O(2) to H(2)O while simultaneously pumping protons across the inner mitochondrial or bacterial membrane to maintain a transmembrane electrochemical gradient that drives, for example, ATP synthesis. In this work mutations that were predicted to alter proton translocation and enzyme activity in preliminary computational studies are characterized with extensive experimental and computational analysis. The mutations were introduced in the D pathway, one of two proton-uptake pathways, in CytcO from Rhodobacter sphaeroides . Serine residues 200 and 201, which are hydrogen-bonded to crystallographically resolved water molecules halfway up the D pathway, were replaced by more bulky hydrophobic residues (Ser200Ile, Ser200Val/Ser201Val, and Ser200Val/Ser201Tyr) to query the effects of changing the local structure on enzyme activity as well as proton uptake, release, and intermediate transitions. In addition, the effects of these mutations on internal proton transfer were investigated by blocking proton uptake at the pathway entrance (Asp132Asn replacement in addition to the above-mentioned mutations). Even though the overall activities of all mutant CytcO's were lowered, both the Ser200Ile and Ser200Val/Ser201Val variants maintained the ability to pump protons. The lowered activities were shown to be due to slowed oxidation kinetics during the P(R) → F and F → O transitions (P(R) is the "peroxy" intermediate formed at the catalytic site upon reaction of the four-electron-reduced CytcO with O(2), F is the oxoferryl intermediate, and O is the fully oxidized CytcO). Furthermore, the P(R) → F transition is shown to be essentially pH independent up to pH 12 (i.e., the apparent pK(a) of Glu286 is increased from 9.4 by at least 3 pK(a) units) in the Ser200Val/Ser201Val mutant. Explicit simulations of proton transport in the mutated enzymes revealed that the solvation dynamics can cause intriguing energetic consequences and hence provide mechanistic insights that would never be detected in static structures or simulations of the system with fixed protonation states (i.e., lacking explicit proton transport). The results are discussed in terms of the proton-pumping mechanism of CytcO.     

A non-charged,easily polarizable,intramolecular, hydrogen bond in 2-(α-pyridyl N-oxide)ethane sulphonic acid

Non-charged intramolecular hydrogen bonds formed in 2-(α-pyridyl N-oxide)ethane sulphonic acid cause strong continuous absorption; thus, these hydrogen bonds are easily polarizable. A double minimum proton potential with a deeper well at the oxygen atom of the NO group is present in these hydrogen bonds.     

Application of sephadex to radiochemical separations

Heteropolymolybdic acid species of a variety of elements bind to Sephadex (dextran cross-linked by epichlorohydrin). Solution pH can be used as a variable to influence the formation of heteropolymolybdates and thus their affinity for Sephadex. Therefore this material may be used to chromatographically separate certain elements which have been reacted with molybdate in solution. In this paper we report the measurement of distribution coefficients on Sephadex for heteropolymolybdates of As, P, Ge, Si, and Se for solutions of varying pH. We also describe the application of the technique to the recovery of³²Si from proton-irradiated KCl targets and⁶⁸Ge from proton-irradiated molybdenum targets.