bluuebunny/crossref_metadata_2025_split huggingface dataset:
Organotin(IV) complexes with tetraethyl ethylene‐ and propylene‐diphosphonates
Authors: Grigoriev E. V. , Yashina N. S. , Prischenko A. A. , Livantsov M. V. , Petrosyan V. S. , Massa W. , Harms K. , Wocadlo S. , Pellerito L. 
Published: January 2005

AbstractThe series of organotin halide complexes with tetraethyl ethylene‐ and propylene‐diphosphonates RnSnX4–n,˙L [n=0, X=Cl; n=1, R=Me, X=Cl, Br; n=1, R=Ph, X=Cl; n=2, R=Me, Et, Bu, X=Cl, Br; n=2, R=Ph, X=Cl; L=(EtO)2P(O)CH2CHR'P(O)(OEt)2, R'=H, Me] were synthesized and characterized by means of NMR and Mössbauer spectroscopy. The crystal structure of the complex of diphenyltin dichloride with propylenediphosphonate was determined. The complex consists of polymer chains with bridging bidentate ligands and an octahedral tin environment containing two types of phosphoryl fragments. All of the R2SnX2 adducts have trans‐R2SnX4 geometries of tin coordination octahedra according to the quadrupole splitting values in the Mössbauer spectra. The 31P and 119Sn NMR studies at low temperatures revealed that RSnHal3 complexes in solution form isomers with different mutual orientations of phosphoryl ligands and organic groups in the coordination sphere. The diethyltin dichloride adduct with ethylene‐diphosphonate appeared to be active against lung cancer NCI‐H522 cells.
Some applications of the diels–alder reaction in organosilicon chemistry
Authors: Qiu Huayu , Yu Weiyong , Du Zuodong 
Published: January 2005

AbstractA review of advances in the applications of the Diels–Alder reaction in organosilicon chemistry in our laboratory is presented. Using this reaction, we have synthesized a series of organosilicon monomers and polymers with polyphenyl groups and condensed rings and established a novel vulcanization system for silicone rubber. In addition, we discuss the influences of the large aromatic groups on the properties of the polymers.
Detection of polypeptide helix‐coil transitions by nuclear magnetic resonance
Authors: Goodman Murray , Masuda Yukio 
Published: December 2004

AbstractWe have studied the helix coil transition for poly‐γ‐ethyl‐L‐glutamate (PELG) in the mixed solvent system: trifluoroacetic acid‐trifluoroethanol by high resolution nuclear magnetic resonance spectroscopy. At 25°, the transition occurs over a range of solvent compositions between 62–64% trifluoroacetic acid. In addition, we have assigned τ‐values to the various types of protons in PKLG.
Theory of inactivation and reactivation of transforming DNA
Authors: Bresler S. E. , Kalinin V. L. , Perumov D. A. 
Published: December 2004

AbstractA simple quantitative theory of inactivation of transforming DNA has been formulated. It is based on the idea that genetic recombination of the molecule of transforming DNA with the bacterial genome is possible only, from the point mutation of the gene up to the nearest hit or point of damage on the corresponding DNA molecule. Our data and the data of other authors on the kinetics of inactivation by HNO2 or U.V. light or both are consistent with these theoretical predictions. Theoretical predictions were made concerning the extent of reactivation by fusion mid annealing of damaged DNA with a diluting genetically inactive DNA. These estimates have been reasonably confirmed by our experimental results.
On the discrimination between symmetric pentapeptides
Authors: Oesper Peter , Scheinok Perry A. 
Published: December 2004

AbstractPatchornik has pointed out that identification of the dipeptides AC, GC, GA, and AG is insufficient information to allow one to distinguish between the pentapeptide sequences ACGAG and AGACG. The present study proposes a probabilistic approach which will sometimes allow one to distinguish between the parent pentapeptides when all the dipeptides have been determined quantitatively. For the case when the free amino acids have also been quantitatively determined, necessary and sufficient conditions are established for unambiguous discrimination.
Contraction–relaxation of collagen fibers in LiBr–water–acetone solutions
Authors: Sherebrin M. H. , Oplatka A. 
Published: December 2004

AbstractStudies have been made of the contraction of collagen fibers in acetone due to the action of LiBr. No contraction occurs unless a small amount of water is added; on further increase of the water concentration, the fibers relax. The first contraction is irreversible. The effect of water is discussed in terms of hydrate formation.
Computation of molecular weight averages for DNA molecules containing both preformed and randomly induced single‐strand breaks
Authors: Crothers Donald M. , Spatz H.‐CH. , Elson Elliot 
Published: December 2004

AbstractWe derive equations for calculating the number‐, weight‐, and z‐average lengths between single‐strand breaks in DNA molecules. Provision is made for the existence of preformed breaks under genetic control: these are assumed to be fixed in number in each molecule, but may be variable in position. Breaks due to random degradation may be superimposed on the pre‐existing breaks. An example is given for illustrative purposes.
Equilibrium sedimentation of a polyelectrolyte in a density gradient of a low‐molecular weight electrolyte. I. DNA in CsCl
Authors: Daniel Ezra 
Published: December 2004

AbstractPrevious work had indicated that the molecular weight calculated from the equilibrium distribution of DNA in a CsCl density gradient appears to be one half the estimated value of the true molecular weight. In the present study, the sedimentation equilibrium of a polyclectrolyte in a density gradient generated by a low molecular weight electrolyte is treated in terms of the representation according to which a constant fraction of the counterions is considered permanently immobilized and the remaining fraction completely free. Equations for the preferential hydration and the molecular weight are obtained. The validity of the treatment is tested by applying it to experimental data on the equilibrium sedimentation of ϕX 174 DNA in CsCl, and it is found that the molecular weight calculated in this way is in agreement with the accepted value for this molecule. Also, recalculation of published data on density gradient centrifugation of T2 DNA in CsCl according to the present treatment brings up the molecular weight to within the range of values given by other methods.
Enthalpy of helix–coil transition from heat of solutions. III. Poly‐N‐δ‐carbobenzoxy‐L‐ornithine and poly‐N‐γ‐carbobenzoxy‐L‐α,γ‐aminobutyric acid
Authors: Giacometi G. , Turolla A. , Verdini A. S. , Snam  
Published: December 2004

AbstractThe helix‐to‐coil transition in dichloroethane–dichloroacetic acid (DCE–DCA) mixtures for poly‐N‐δ‐carbobenzoxy‐L‐ornithine (PCBO) and for poly‐N‐γ‐carbobenzoxy‐Lα,γ‐aminobutyric acid (PCBBA) have been studied by ORD and the “heat of solution” method. The results provide strong evidence for the existence of a very specific side‐chain/side‐chain interaction in PCBBA, which is discussed on the basis of a detailed structural model. The main sources of enthalpy and entropy changes in helix‐coil transitions of uncharged homopolypeptides in DCE–DCA mixtures are also discussed briefly.
Interaction between chondroitin‐6‐sulfate and poly‐L‐arginine in aqueous solution
Authors: Gelman R. A. , Glaser D. N. , Blackwell J. 
Published: December 2004

AbstractThe interactions between chondroitin‐6‐sulfate and poly‐L‐arginine in aqueous salt solution have been investigated by circular dichroism techniques. In the presence of chondroitin‐6‐sulfate, at neutral pH, poly‐L‐arginine adopts the α‐helical conformation rather than “charged coil” form observed in the absence of mucopolysaccharide. This interaction is at a maximum when the ratio of arginine to disaccharide residues is 2:1. Elevation of the temperature leads to a sharp melting transition at 76.0 ± 1.0°C. This behavior is in marked contrast to that for poly‐L‐lysine‐chondroitin‐6‐sulfate interactions, which are at a maximum at a 1:1 residue ratio and have a melting transition at 47.0 ± 1.0°C. These results indicate a stronger interaction for poly‐L‐arginine than for poly‐L‐lysine. The positive arginine side chains appear to interact with both the negative sulfate and carboxyl residues, while those of the lysines are involved only with the sulfates. Poly‐L‐ornithine at neutral pH shows no conformation directing interaction with chondroitin‐6‐sulfate, although a small proportion of α‐helix is formed on dilution of the mixture with methanol. The extent of the interaction of cationic polypeptides with chondroitin‐6‐sulfate increases in the order poly‐L‐ornithine, poly‐L‐lysine, poly‐L‐arginine, i.e., in the order of increasing side‐chain length.
Thermodynamics and kinetics of the helix‐coil transition of oligomers containing GC base pairs
Authors: Pörschke D. , Uhlenbeck O. C. , Martin F. H. 
Published: December 2004

AbstractAbsorbance‐temperature profiles have been determined for the following self‐complementary oligonucleotides or equimolar paris of complementary oligonucleotides containing GC base pairs: A2GCU2, A3GCU3, A4GCU4, A6CG + CGU6, A8CG + CGU8, A4G2 + C2U4, A5G2 + C2U5, A4G3 + C3U4, and A5G3 + C3U5. In all cases cooperative melting transitions indicate double‐helix formation. As was found previously, the stability of GC containing oligomer helices is much higher than that of AU helices of corresponding length. Moreover, helices with the same length and base composition but different sequences also have quite different stabilites. The melting curves were andlyzed using a zipper model and the thermodynamic parameters for the AU pairs determined previously. The effect of single‐strand stacking was considered separately. According to this model, the formation of a GC pair from unstacked single strands is associated with an ethalpy change of −15 kcal/mole. Due to the high degree of single‐strand stacking at room temperature the enthalpy change for the formation of GC pairs from unstacked single strands is only −5 to −6 kcal/mole. (The corresponding parameters for AU pairs are −10.7 kcal/mole and −5 to −6 kcal/mole.) The sequence dependence of helix stability seems to be primarily entropic since no differences in ΔH were seen among the sequence isomers.The kinetics of helix formation was investigated for the same molecules using the temperature jump technique. Recombination of strands is second order with rate constants in the range of 105 to 107M−1 sec−1 depending on the chain length and the nucleotide sequence. Within a series of oligomers of a given type, the rates of recombination decrease with increasing chain length. Oligomers with the sequence AnGCUn recombine six to eight times slower than the other oligomers of corresponding chain length. The experimental enthalpies of activation of 6 to 9 kcal/mole suggest a nucleation length of one or two GC base pairs. The helix dissociation process has rate constants between 0.5 and 500 sec−1 and enthalpies of activation of 25 to 50 kcal/mole. An increase of chain length within a given nucleotide series leads to decreased rates of dissociation and increased enthalpies of activation. An investigation of the effect of ionic strength on AnGCUn helix formation showed that the rates of recombination increase considerably with increased ionic strength.
Solution properties of synthetic polypeptides. XV. Helix–coil transition in poly (ε‐carbobenzyloxy L‐lysine)
Authors: Matsuoka Mikio , Norisuye Takashi , Teramoto Akio , Fujita Hiroshi 
Published: December 2004

AbstractHelix–coil transitions of poly(ε‐carbobenzyloxy L‐lysine) (PCBL) in m‐cresol and in mixtures of dichloroacetic acid (DCA) and ethylene dichloride (EDC) were investigated by optical rotation measurements, with special interest in the molecular‐weight dependence of helical content. The data obtained were analyzed according to an approximate expression for the helical content of Okita et al. The values obtained for the cooperativity parameter σ and transition enthalpy ΔHε (at the transition temperature) were: √σ = (0.25 ± 0.06) × 10−2 and ΔHc = 930 ± 130 cal/mole in m‐cresol, and √σ = (0.49 ± 0.10) × 10−2, and ΔHc = 350 ± 150 kcal/mole in a DCA–EDC mixture (33 vol % DCA). The ΔHc value in the DCA–EDC mixture is much smaller than that derived by Giacometti et al. from heat of solution measurements. The transition in m‐cresol was extremely sharp. This was attributed to the unusually small value of √σ and the moderately large value of ΔHc obtained.
Solution properties of synthetic polypeptides. XVI. Hydrogen bonding in helix–coil transitions of poly(β‐benzyl L‐aspartate)
Authors: Norisuye Takashi , Misumi Kozo , Teramoto Akio , Fujita Hiroshi 
Published: December 2004

AbstractHelix–coil transition of polypeptides in solution involves two elementary processes: unfolding of an α‐helical polypeptide chain and interaction of the unfolded chain with solvent molecules which are capable of forming hydrogen bonds. Optical rotation data for poly(β‐benzyl L‐aspartate) in mixtures of dichloroacetic acid and carbon tetrachloride are analyzed according to the procedure developed by Sayama et al., who explicitly took these processes into theoretical account; and the enthalpy changes, ΔH0 = −650 ± 200 cal/mole and ΔHa = −5.2 ± 1.6 kcal/mole. The latter value is favorably compared with heats of association (per hydrogen bond) of various amides and lactams in CCl4. There is excellent agreement with −5.15 kcal/mole for δ‐valerolactam in CCl4. In analogy with amides in non‐polar solvents, the difference between ΔH0 and ΔHa may be attributed to the fact that the former is associated with intramolecular hydrogen bonding, whereas the latter is associated with intermolecular hydrogen bonding.
Cooperative nonenzymic base recognition. Kinetics of the binding of a base monomer to a complementary polynucleotide template
Authors: Hoffman Geoffrey W. , Pörschke Dietmar 
Published: December 2004

AbstractThe kinetics of double‐helix formation by poly U and the complementary monomer N‐6,9‐dimethyladenine (m6m9A) has been measured using a new fast temperature‐jump apparatus. The cooperative binding kinetics are complicated by the extensive self‐association of the monomers, but a satisfactory analysis using average relaxation times was possible in terms of three different models. Application of a model which considers only monomer binding yields the upper limit for the binding rate constant of an m6m9A monomer next to an already bound monomer on a poly U strand: (2 ± 0.4) × 108 M−1sec−1. A lower limit is found by using a model which allows for binding of all m6m9A stacks to poly U with equal rate constants: (3 ± 0.3) × 107 M−1sec−1. A third model with “weighted” rate constants consistent with the data: (7.5 ± 1.0) × 107 M−1sec−1. The rate of cooperative binding of m6m9A to the trimer UpUpU has also been measured. The rate constants obtained with the trimer agree with those obtained with the polymer for each of the three models within experimental error.
Electro‐optical properties of carboxymethylcellulose. I. Concentration and ionic strength dependence
Authors: Hanss M. , Roux B. , Bernengo J. C. , Milas M. , Rinaudo M. 
Published: December 2004

AbstractThe experimental conditions for studying the electro‐optical properties of a natural, modified polyelectrolyte, carboxymethylcellulose (DS 1.3; DP 180) were determined. The transient Kerr effect was found to be a function of CMC concentration, field strength, and ionic strength, I. If the concentration and I were low enough (c < 20 mg.l−1), saturation was obtained for field strengths of approximately 15 kV.cm−1. The optical anisotropy was shown to be independent of I; the electrical anisotropy decreased sharply when I increased. These results are discussed in connection with polarization theories of polyelectrolytes. The molecular dimensions of carboxymethylcellulose, calculated from the birefringence kinetics, suggest that the molecule is a rigid rod.
Nucleic acid–solvent interactions: Temperature dependence of the heat of solution of thymine in water and ethanol
Authors: Alvarez José , Biltonen Rodney 
Published: December 2004

AbstractThe heats of solution of thymine in water and ethanol have been determined calorimetrically as a function of temperature. These data, along with solubility data, have been used to calculate the thermodynamic quantities (ΔGt, ΔHt, ΔSt and ΔCp,t) associated with the transfer of thymine from ethanol to water. Since ΔSt = −2 cal/mole deg and ΔCp,t = 0, it has been concluded that hydrophobic bonding does not play an important role in the thermocynamic stability of nucleic acids.However, large heat capacities of solution of thymine are observed in both solvents (ΔC°p2 = 45 ± 4 cal/mole deg). This is explained in terms of temperature variation in the degree of solvent–solute hydrogen bonding. It is our proposal that the components of macromolecules (i.e., nucleic acid bases and amino acids) do not make all possible hydrogen bonds with the solvent in the vicinity of room temperature. Thus the thermodynamic contribution of hydrogen bonding to the stability of macromolecules in aqueous solution must be reassessed.
The evaluation of standard sedimentation coefficient in band sedimentation of macromolecules
Authors: Belli Mauro 
Published: December 2004

AbstractStandardization in the calculation of the sedimentation coefficient of macromolecules by means of band techniques is discussed. When a sample of macromolecules suspended in a solvent is layered on to a denser bulk solution, the macromolecules do not sediment in this solution alone, but sediment in a mixture of bulk solution and sample solvent. This is caused by the diffusion between sample solvent and bulk solution.Experimental evidence of this process is shown during band sedimentation of ribosomes when the variation of the density and the viscosity along the cell is measured. The calculation shows that in the experimental conditions frequently used, standardization made in the usual way leads to a sedimentation coefficient which is largely overestimated; while standardization yields the correct coefficient if the diffusion effect of the sample solvent in to the bulk solution is taken into account, together with possible deuteration effects.A method to calculate the standard coefficient with the aid of a computer is proposed.
Structural components of alginic acid. II. The crystalline structure of poly‐α‐L‐guluronic acid. Results of X‐ray diffraction and polarized infrared studies
Authors: Atkins E. D. T. , Nieduszynski I. A. , Mackie W. , Parker K. D. , Smolko E. E. 
Published: December 2004

AbstractA structural investigation of the marine algal polysaccharide poly‐α‐L‐guluronic acid is described. The molecular chains consist of 1 → 4 diaxially linked L‐guluronic acid residues in the 1C chair conformation and are stabilized in a twofold helix conformation by an intra‐molecular O(2)H … O(6)D hydrogen‐bond. The X‐ray fiber diffraction photograph has been indexed to an orthorhombic unit cell in which a = 8.6 Å, b (fiber axis) = 8.7 Å, c = 10.7 Å.A structure corresponding to the space group P212121 is proposed, in which all intermolecular hydrogen bonds interact with water molecules and in which all oxygen atoms except for the inaccessible bridge oxygens are involed. The relationship between the shape and structure of the polyguluronic acid molecule and its biological function is discussed.
Über die Adsorption von DNA in der Grenzfläche Quecksilber–Elektrolyt. III. Reaktionen der DNA in der Grenzfläche Quecksilber–Elektrolyt
Authors: Flemming Joachim 
Published: December 2004

AbstractThe adsorption of deoxyribonucleic acid in the mercury‐electrolyte interface was investigated. The effect of this adsorption on the differential capacity of the electrical double layer at the interface between a stationary mercury drop electrode (HMDE) and a buffered aqueous sodium chloride solution was measured. The dependences of this differential capacity on potential, time, and pH was studied in the presence of native and also of denatured DNA. These results were compared with the adsorption of model compounds and with the general theory of the adsorption of polymers. The structure of the adsorbed DNA molecules corresponds to an alternating arrangement of two‐dimensional, totally adsorbed sequences and three‐dimensional loops extending into the solution. The adsorbed sequences and loops consist of several segments with a specific free‐energy change of adsorption. Essentially this energy determines the distribution of the segments between adsorbed sequences and loops. The absolute value of this energy change per segment is fairly large in the case of negatively charged poly‐electrolyte DNA at the weakly positively charged interface near the electrocapillary maximum (ECM). The fraction of totally adsorbed segments is relatively large in this potential region. The more negative the potential the lower is the absolute value of free energy change of adsorption per segment. Under the conditions unfavorable for the adsorption, only a few segments can be adsorbed. Most of the segments of the adsorbed DNA molecules extend into the solution and therefore fairly high interface concentrations can be reached. Thus, the arrangement of DNA molecules in the electrode surface is changed when the potential is altered from values near the ECM to more negative ones. This change should produce the wave on the differential capacity curves at a little more negative potential than that of ECM. At a more negative potential, intermolecular interactions between the loops extending into the solution may occur. The adsorption tendency of the resulting associates is higher than that of the isolated molecules. Therefore the isolated molecules desorb at sufficient negatively charged interface producing a round wave while the associates stay adsorbed. At this potential it is impossible for native DNA to generate associates because they are formed from the isolated molecules. This explains the hysteresis loop of the curves of differential capacity vs. potential by using the HMDE. The desorption of the associates is indicated by a sharp wave at much more negative potential. For denatured DNA the associates arise from the very few isolated adsorbed molecules at this potential; therefore, no hysteresis loop occurs. The association constant of denatured DNA must be much higher than that of the native DNA. The reasons for this are discussed.
Studies on the bacteriophage MS2. XXIII. Fixation of the MS2 RNA acid structure by formaldehyde
Authors: Slegers H. , Fiers W. 
Published: December 2004

AbstractWhen MS2 RNA is heated at low pH in the presence of formaldehyde, a fast‐sedimenting conformation is irreversibly formed. This species is homogeneous and stable at neutral pH. Its formation further requires Mg++ ions and low ionic strength. The most compact form sediments at 46S and is obtained after short reaction times at high temperature or after long reaction times at 35°C. Melting curves suggest that the specific acid conformation is not destabilized by the formaldehyde addition reaction. The pH at which this acid conformation is formed depends on the MgC12 concentration. At 10−2M MgCl2 the midpoint is pH 5.3. Removal of more than half of the bound formaldehyde has no effect on the compactness of the molecule, although most of the original secondary structure has not yet re‐formed.