bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of the molecular weight of maleated polypropylenes on the melt compounding of polypropylene/organoclay nanocomposites
Authors: Wang Yeh , Chen Feng‐B. , Wu Kai‐C.
Published: May 2005

Maleic anhydride grafted polypropylene (PP‐g‐MA) and organically modified clay composites were prepared in a plasticorder. PP‐g‐MAs, including Polybond PB3150, Polybond PB3200, Polybond PB3000, and Epolene E43, with a wide range of maleic anhydride (MA) concentrations and molecular weights were used. The structure was investigated with X‐ray diffraction (XRD) and transmission electron microscopy (TEM). PP‐g‐MA compatibilizers gave rise to similar degrees of dispersion beyond the weight ratio of 3/1, with the exception of E43, which had the highest MA content and the lowest molecular weight. The thermal instability and high melt index were responsible for the ineffective modification by E43. Furthermore, PP‐g‐MA with a lower molecular weight and a higher melt index had to be compounded at a lower mixing temperature to achieve a reasonable level of torque for clay dispersion. Polypropylene/organoclay nanocomposites were then modified with different levels of PP‐g‐MA compatibilizers with a twin‐screw extruder. The polypropylene/E43/clay system, as shown by XRD patterns and TEM observations, yielded the poorest clay dispersion of the compatibilizers under investigation. The curves of the relative complex viscosity also revealed a systematic trend with the extent of exfoliation and showed promise for quantifying the hybrid structure of the nanocomposites. The mechanical properties and thermal stability were determined by dynamical mechanical analysis and thermogravimetric analysis, respectively. Although PP‐g‐MA with a lower molecular weight led to better clay dispersion in the polypropylene nanocomposites, it caused deterioration in both the mechanical and thermal properties of the hybrid systems. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1667–1680, 2005
Grafting of methyl methacrylate onto sodium alginate initiated by potassium ditelluratoargentate(III)
Authors: Liu Yinghai , Yang Lanying , Li Junbo , Shi Zengqian
Published: May 2005

Sodium alginate (SA) was graft‐copolymerized with methyl methacrylate in an alkali aqueous solution with potassium ditelluratoargentate(III) (DTA) as the initiator. Graft copolymers with both a high grafting efficiency (>90%) and a high percentage of grafting were obtained, which indicated that the DTA–SA redox pair was an efficient initiator for this grafting. The grafting parameters, including total conversion, grafting efficiency, and percentage grafting, were evaluated comparatively. The dependence of these parameters on temperature and time, monomer concentration, initiator concentration, and SA backbone concentration was also investigated. The overall activation energy of this grafting was calculated as 37.50 kJ/mol. Proof of grafting was obtained from gravimetric analysis and IR spectra. A tentative mechanism involving a two‐step, single‐electron‐transfer process of DTA is proposed to explain the generation of radicals and the initiation of grafting. Some basic properties of the grafted copolymer were studied by instrumental analyses, including thermogravimetry, X‐ray diffraction, and scanning electron microscopy. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1688–1694, 2005
Concentrated emulsion polymerization of methyl methacrylate/butyl acrylate initiated by a redox system
Authors: Zhang Hongtao , Tan Bien , Wang Anlin , Cao Jianhua
Published: May 2005

Stable concentrated emulsions of methyl methacrylate/butyl acrylate were prepared with sodium dodecyl sulfate and cetyl alcohol as the compound surfactant and poly(vinyl alcohol) as the major reinforcer of the liquid film. With a redox system based on benzoyl peroxide/N,N′‐dimethyl phenylamine introduced into the concentrated emulsions, polymer particles with different shapes and sizes were obtained by initiation of the polymerization at low temperatures. We investigated the kinetic behaviors of concentrated emulsion polymerization and drew linear regression diagrams of its time–conversion curves in a constant rate phase (conversions ranged from 20 to 70%), and the experimental results show that the variation of the concentrations of the compound surfactant and initiator, the categories of reinforcers of the liquid film, the temperatures, and so on were responsible for the polymerization stability and the polymerization rate. Finally, the kinetics equation and activation energy of the initiator were obtained. The particle size and distribution of particle diameters of latex particles were determined by photon correlation spectroscopy. The determination results reveal that concentrations of the compound surfactant, polymerization temperatures, and so on affected the shape and size of the polymer particles greatly. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1695–1701, 2005
Release kinetics of ATP in cells exposed to nonionic detergents
Authors: Köszegi Tamàs
Published: May 2005

We have previously shown that the protein binding of intracellular ATP could be examined by monitoring the ATP release kinetics from Triton X‐100 and Brij 58 nonionic detergent permeabilized cells. We have now analysed the protein binding of ATP in an isotonic medium using intact and partially ATP depleted Brij 58 treated human erythrocytes. The effects of Triton X‐100 below the critical micelle concentration (CMC) was studied in normal and tumorous tissue culture cells and human red blood cells. Our results showed that the protein association of ATP was altered in the partially ATP depleted erythrocytes. Below the CMC value, but above a critical level Triton X‐100 treatment was effective in mobilizing the intracellular ATP in both cell types. The ATP release curves were sigmoidal and an ‘all or none’ type of response was observed, especially in erythrocytes. The use of Triton X‐100 (< CMC) delays the detergent‐induced cell decomposition time thus providing a new approach to investigating the physical state of intracellular ATP.
Chemiluminescence of anthocyanins in the presence of acetaldehyde and tert‐butyl hydroperoxide
Authors: Yoshiki Yumiko , Okubo Kazuyoshi , Igarashi Kiharu
Published: May 2005

Light emission (chemiluminescence; CL) was observed in the reaction of anthocyanins with tert‐butyl hydroperoxide (t‐BuOOH) in the presence of acetaldehyde. The intensity of the CL of the anthocyanins was in the order of nasunin > rubrobrassicin > delphinidin > malvin = cyanidin > malvidin, indicating that glucosylation at C‐3 and C‐5 of the anthocyanin skeleton enhances the CL of the parent compound. CL intensity was enhanced at alkaline pH. The results suggest that the antioxidant effect of anthocyanins on lipid peroxidation, which is observed in the linoleic acid‐β‐carotene‐lipoxcygenase system, is at least partly due to their strong reactivity with hydroperoxides.
Synthesis and membrane action of polymyxin B analogues
Authors: Clausell Adrià , Rabanal Francesc , Garcia‐Subirats Maria , Alsina M. Asunción , Cajal Yolanda
Published: May 2005

We have designed synthetic peptides that mimic the primary and secondary structure of the cationic lipopeptide antibiotic polymyxin B (PxB) in order to determine the structural requirements for membrane action and to assess possible therapeutic potential. Two analogues with related sequences to that of PxB, but including synthetic simplifications (disulphide bridge between two cysteines in positions 4 and 10, N‐terminal nonanoic acid), have been synthesized. Peptide–lipid interactions have been studied by fluorescence resonance energy transfer between pyrene and 4,4‐difluoro‐5‐methyl‐4‐bora‐3α,4α‐diaza‐s‐indacene‐3‐dodecanoyl (BODIPY®) probes covalently linked to phospholipids, and the possibility of membrane disruption or permeabilization has been assessed by light scattering and fluorescence quenching assays. The synthetic peptide sP‐B, which closely mimics the primary and secondary structures of PxB, binds to vesicles of anionic 1‐palmitoyl‐2‐oleoylglycero‐sn‐3‐phosphoglycerol (POPG) or of lipids extracted from Escherichia coli membranes, and induces apposition of the vesicles and selective lipid exchange without permeabilization of the membrane. We conclude that sP‐B forms functional vesicle–vesicle contacts that are selective, as previously described for PxB. The second analogue, sP‐C, has a permutation of two amino acids that breaks the hydrophobic patch formed by d‐Phe and Leu residues on the cyclic part of the sequence. sP‐C lipopeptide is more effective than sP‐B in inducing lipid mixing, but shows no selectivity for the lipids that exchange through the vesicle–vesicle contacts, and at high concentrations has a membrane‐permeabilizing effect. The deacylated and non‐antibiotic derivative PxB‐nonapeptide (PxB‐NP) does not induce the formation of functional intervesicle contacts in the range of concentrations studied. Copyright © 2005 John Wiley & Sons, Ltd.
Environmental effects on the fluorescence behaviour of carbazole derivatization reagents
Authors: Romero‐Ale Emiliano E. , Olives Ana I. , Martín M. Antonia , del Castillo Benito , López‐Alvarado Pilar , Menéndez J. Carlos
Published: May 2005

The carbazole ring is the basic structure present in the fluorescence derivatization reagents 9‐chlorocarbonylcarbazole and 9‐carbazolylacetic acid. The fluorescence behaviour of these carbazole derivatives was studied in solvents with different polarities (cyclohexane, ethanol, acetonitrile, water) and at different pH values (4.5 and 8.8). The influence of the low polarity environment afforded by 2‐hydroxypropyl‐β‐cyclodextrin (HPβ‐CD) is also described. The behaviour of the fluorescent reagents is compared to the model molecules carbazole and 9‐methylcarbazole. For all derivatives studied, a bathochromic shift in the fluorescence emission maxima was observed when the solvent polarity was increased. A bathochromic shift was observed in dioxane solutions, which can be ascribed to the peculiar behaviour of this solvent. The changes in the fluorescence intensity in the case of 9‐carbazolylacetic acid can be related to the ionization of the carboxylic acid group. Inclusion into the cavity of HPβ‐CD allows emission spectra to be obtained close to those obtained in ethanolic solutions with a remarkable enhancement in the fluorescence intensity, depending on the chemical structure of the carbazole derivative included. Copyright © 2005 John Wiley & Sons, Ltd.
A study on the micelle‐sensitized Ce(IV)–Na2S2O3–norfloxacin chemiluminescence system and its applications
Authors: Xie Zenghong , Liao Sulan , Chen Guonan
Published: May 2005

A new chemiluminescence (CL) flow‐injection method was developed for the determination of norfloxacin. The method is based on the CL reaction of norfloxacin with sodium thiosulphate and Ce(IV) in sulphuric acid medium sensitized by sodium dodecylsulphate. Under optimum conditions, the CL intensity is proportional to the concentration of the norfloxacin in the range 3.89 × 10−8–7.18 × 10−6 g[sol ]mL. The detection limit (3 s[sol ]k) was 2.21 × 10−9 g[sol ]mL for norfloxacin. The method has been applied successfully to the determination of norfloxacin in pharmaceutical formulations and human urine. The mechanism for this chemiluminescence system is discussed. Copyright © 2005 John Wiley & Sons, Ltd.
Determination of carbofuran by flow‐injection with chemiluminescent detection
Authors: Xie Zenghong , Ouyang Xiaoqing , Guo Liangqia , Lin Xucong , Chen Guonan
Published: May 2005

It was found that carbofuran enhances the chemiluminescence reaction between sodium sulphite and Ce4+ in sulphuric acid, and this formed the basis of a flow‐injection system with chemiluminescence detection for determination of carbofuran. Under optimum conditions, the enhanced chemiluminescence intensity was linear, with the concentration of carbofuran in the range 8 × 10−8–1.0 × 10−5 g[sol ]mL, with a detection limit of 2.84 × 10−8 g[sol ]mL (3 s[sol ]k). The proposed method was applied to the analysis of carbofuran in cabbage, with satisfactory results. Copyright © 2005 John Wiley & Sons, Ltd.
Determination of methyltestosterone using flow injection with chemiluminescence detection
Authors: Xie Zenghong , Ouyang Xiaoqing , Guo Liangqia , Lin Xucong , Chen Guonan
Published: May 2005

Methyltestosterone (MT) enhanced the chemiluminescence of potassium permanganate–sodium thiosulphate system in sulphuric acid medium, and this was used as the basis of a novel flow‐injection chemiluminescence method for the determination of methyltesterone. Under optimum conditions, the linear response of MT was in the range 3.0 × 10−8–8.0 × 10−6 g[sol ]mL, and the detection limit for determination of MT was 1.0 × 10−8 g[sol ]mL. The method was applied to the analysis of fish with recovery in the range 88.0–113.8% with an average RSD of 1.66% (n = 8). Copyright © 2005 John Wiley & Sons, Ltd.
Determination of oxytetracycline by flow injection with chemiluminescence detection
Authors: Guo Liangqia , Xie Zenghong , Bian Xintong , Lin Xucong , Zhang Weilin , Liu Xiaohua , Chen and Guonan
Published: May 2005

A flow‐injection chemiluminescent method for the determination of oxytetracycline was developed. The method is based on an enhancement by oxytetracycline of the chemiluminescence light emission of tris(2,2′‐bipyridine) ruthenium (II), generated by the continuous oxidation of tris(2,2′‐bipyridine) ruthenium (II) by cerium (IV) sulphate in sulphuric acid. Under the optimum conditions, the calibration curve was linear over the range 1.0 × 10−7–1.0 × 10−5 g[sol ]mL for oxytetracycline with the linear equation: ΔINT = 148.77 × C + 0.6637 (R2 = 0.9994). The detection limit was 4.52 × 10−8 g[sol ]mL. The proposed method was also successfully used to determine oxytetracycline in pharmaceutical formulations. The mean recovery of determination of oxytetracycline was 92.73%. A mechanism for the chemiluminescence enhancement by oxytetracycline of tris(2,2′‐bipyridine)–ruthenium (II) and cerium (IV) sulphate system is also proposed. Copyright © 2005 John Wiley & Sons, Ltd.
Determination of ergometrine maleate by fluorescence detection
Authors: Gong Zhengjun , Lv Yi , Huang Yuming , Zhang Zhujun
Published: May 2005

A simple, accurate, sensitive and selective fluorescence analysis method for rapid determination of trace amounts of ergometrine maleate has been developed. The method is based on the fluorescence of ergometrine maleate. The calibration graph was linear in the range of 1 × 10−4 to 0.2 µg[sol ]mL and the detection limit was 4 × 10−5 µg[sol ]mL, with a relative standard deviation of 0.32% at 0.05 µg[sol ]mL (n = 11) ergometrine maleate. The influence of foreign compounds was tested. The proposed method was applied successfully to the determination of ergometrine maleate in pharmaceutical preparations and human plasma. Copyright © 2005 John Wiley & Sons, Ltd.
Volume changes accompanying the thermal denaturation of deoxyribonucleic acid. I. Denaturation at neutral pH
Authors: Chapman R. E. , Sturtevant J. M.
Published: May 2005

Dilatometric measurements were made to determine the change in apparent specific volume φ of DNA resulting from thermal denaturation in neutral solution, φ increased continuously with temperature in the range 10–85°C. No deviations from a monotonically rising curve were observed in the φ versus temperature profile in the region of the melting temperature. The results are interpreted in terms of a partial loss of the preferentially bound DNA hydration shell. The nature of the well known buoyant density difference between native and denatured DNA was investigated by evaluating the densities in a series of cesium salt gradients at constant temperature. Extrapolation of the results to zero water activity indicates that the partial specific volumes of anhydrous native and denatured DNA are equal. The density difference at nonzero water activities is attributed to decreased hydration in the denatured state. The absence of a related change in φ accompanying the denaturation in the dilatometric experiments suggests that the probable volume change associated with loss of bound water during denaturation is accompanied by other compensatory volume effects. The possible nature of these volume effects is discussed.
Optical rotatory dispersion and circular dichroism of carbanilyl polysaccharides
Authors: Bittiger H. , Keilich G.
Published: May 2005

Measurements of optical rotatory dispersion (ORD) and circular dichroism (CD) have been made in the range of 600‐210 mμ for the β‐glycan carbanilates as for instance, 2,3,6‐tricarbanilylcellulose (I), 2,3,6‐tricarbanilylmannan (II), 2,3‐dicarbanilylcellulose (III), and octacarbanilylcellobiose (IV) and also for the α‐glycan carbanilates, such as 2,3,6‐tricarbanilylamylose (V), tricarbanilylpullulan (VI), 2,3‐dicarbanilylamylose (VII), and octacarbanilylmaltose (VIII). Furthermore, the 2,3,4,6‐tetracarbanilyl‐α‐methyl‐glucopyranoside (IX) and the 1,2,3,4,6‐pentacarbanilylglucose (X) have been measured in dioxane at 20°C. For the β‐glycans a small negative CD in the region of 238–240 mμ and nearly symmetrical ORD curve with a crossover point at 238–240 mμ are found; this indicates a simple negative Cotton effect. In the case of α‐glycosides, a strong negative CD with a maximum at 240–242 mμ and a strong positive CD with a maximum at 223–225 mμ were found; the ORD curves are asymmetrical and cross the abscissa in two places, at 241–243 and 220–222 mμ. With 2,3,4,6‐tetracarbanilyl‐α‐methylglucoside (IX) no CD and ORD in the ultraviolet region and with 1,2,3,4,6‐pentacarbanilyl‐glucopyranoside (X) the ORD, but not the CD, could be measured. The ORD curve is nearly symmetrical, like those of the β‐glycans but is of opposite sign. It seems impossible to discuss the striking difference of the CD and ORD spectra between the α‐and the β‐glycans in terms of contributions of single independant chromophores influenced by their individual different steric arrangements and their spatial relation to the glycosidic bond in C1. The exciton theory of Moffitt, which is suitable for explaining the ORD and CD spectra of helical polymers, has been applied to α‐ and β‐glycans. A structure with helical parts is proposed for the α‐glycans while a nearly planar arrangement is assumed for the β‐glycans.
Increasing viscosity of E. coli chromatin with increasing shear
Authors: Thompson Douglas S. , Hays John B. , Gill Stanley J.
Published: May 2005

We have observed that the viscosity of a solution of E. coli chromatin increases as the shear to which the solution is subjected is suddenly increased. This dilatant effect is opposite to the usual thixotropic non‐Newtonian effects that are observed in DNA solutions. We suggest that the observed dilatancy may be explained in terms of shear‐induced aggregation of large DNA molecules. Conditions for aggregation are discussed in the context of the “cluster theory” of condensation.
Optical activity of simple cyclic amides in solution
Authors: Greenfield Norma J. , Fasman Gerald D.
Published: May 2005

The cyclic dipeptide, L‐alanylglycyl anhydride, has been studied by optical rotatory dispersion; both L‐alanylglycyl anhydride and the lactam, L‐3‐aminopyrrolidin‐2‐one, have been studied by circular dichroism. In hydroxylic solvents the circular dichroism spectra of 3‐aminopyrrolidin‐2‐one can be attributed to an n–π* transition near 220 mμ and a π–π* transition near 190 mμ. In these solvents the optical activity of L‐alanylglycyl anhydride can be explained as being due to contributions of n–π* transitions and a split π–π* transition. In acetonitrile, however, the circular dichroism spectrum of 3‐aminopyrrolidin‐2‐one shows an additional apparent minimum near 200 mμ. The CD spectrum of the dipeptide is also quite distinctive in this solvent. The possible nature of the band at 200 mμ and the implications of these findings are discussed.
Interactions des ions métalliques avec le DNA. I. Comportement de polyélectrolyte et liaison des ions compensateurs
Authors: Daune M.
Published: May 2005

The theory of polyelectrolyte solution of Alexandrowicz: and Katchalsky is used to calculate the electrostatic potential of single stranded polynucleotides for different ionic strength. We have considered the potential of double stranded DNA as the superposition of the different potentials produced by each chain, provided the average distance between the strands is higher than an ionic strength‐dependent parameter b. For ionic strength lower than 5 × 10−2M, the assumption is no longer valid, and a cylindrical model with a uniform charge density must be used. The continuity between the two models was tested, and thus we can calculate the electrical potential at the vicinity of a phosphate group in the whole range of ionic‐strength where the double helix is stable. It was therefore possible to determine the theoretical number of ions bound electrostatically to DNA and we found an increase of ion binding with a decrease of ionic strength. Such a model was further applied to the change of specific volume in different salt solutions. Comparison is made with recent pycnometric data on Na− and Cs− salts of DNA. Agreement is good in the case of Cs+, but for Na+, cation binding is likely to be accompanied by a change of the hydration of DNA, which depends on ionic strength. With the same model we can see easily the ion‐trapping properties of DNA which play an important role in any formation of complex between heavy ions and bases.
Circular dichroism of polynucleotides: A simple theory
Authors: Johnson W. C. , Tinoco I.
Published: May 2005

A particularly simple theory of circular dichroism is described and applied to the polynucleotides. It works surprisingly well and predicts the conservative spectrum of DNA and the more intense nonconservative spectrum of RNA. The anomalously low intensity of the CD of the nucleic acids is shown to be a natural consequence of the cancellation of the rotational strengths of the many CD bands. The theory's many successes and predictions as well as its weaknesses are discussed.
Crystal and molecular structure of L‐cis‐3,6‐dimethyl‐2,5‐piperazinedione (L‐alanyl‐L‐alanyl‐2,5‐diketopiperazine)
Authors: Benedetti Ettore , Corradini Paolo , Pedone Carlo
Published: May 2005

The crystal structure of L‐cis‐3, 6‐dimethyl‐2,5‐piperazinedione (L‐alanyl‐L‐alanyl‐2,5‐diketopinerazine) has been determined. The unit cell is triclinic with a = 8.05, b = 6.08, c = 5.15 Å, α = 131.7°, β = 82.4°, γ = 106.6°. The space group is P1 with one molecule per unit cell. The six‐membered ring is found to be nonplanar with an angle between the two amide group planes of 26°. One amide group deviates slightly from planarity. The experimental molecular model is discussed in terms of its flexibility.
Intrinsic viscosity of native and single‐stranded T7 DNA and its relationship to sedimentation coefficient
Authors: Rosenberg Alan H. , Studier F. William
Published: May 2005

The intrinsic viscosity and sedimentation coefficient, of native and single‐stranded T7 DNA have been determined at 25°C as a function of ionic strength in neutral and alkaline NaCl. The relationship between [η] and S,w is well represented by the Mandelkern‐Flory equation over the entire range of conditions between 0.0013 and 1M Na+. An apparent discrepancy between the two methods at moderate to high ionic strengths is probably due to a change in V with ionic strength. It appears that [η] is a more sensitive and reliable measure of molecular expansion for native DNA, S,w but is a better index of conformational change in single strands, since [η] becomes too small to measure conveniently at high ionic strengths. At moderate to high ionic strengths, denaturation leads to a decrease in [η], although unfolded single strands retain considerable viscosity. At sufficiently low ionic strength, the intrinsic viscosity of the single strands becomes higher than that of native DNA, and the effective volume of a single strand approaches that of the native molecule.
Stereochemistry of nucleic acids and their constituents. IV. Allowed and preferred conformations of nucleosides, nucleoside mono‐, di‐, tri‐, tetraphosphates, nucleic acids and polynucleotides
Authors: Sundaralingam M.
Published: May 2005

A uniform notation and convention is suggested to describe the torsional angles in nucleic acids and their derivatives. The torsional angle χ, relating the stereochemistry of the base with respect to the sugar, shows more variation for the β‐purine glycosides than for the β‐pyrimidine glycosides. This variation is attributed to the fact that the β‐purine derivatives may form intramolecular O(5′)‐H…N(3) hydrogen bonding. The χ values for the α‐purine and α‐pyrimidine glycosides show preference for the –syn‐clinal (or anti) conformation. The mode of puckering of the sugar also influences the χ value. The various possible conformations for the furanose ring are described by the torsional angles τ0 τ1, τ2, τ3, τ4, about the five ring bonds. From an analysis of the torsional angles (ω, ϕ, ψ, ψ′, ϕ′, ω′) about the sugar phosphate bonds in the x‐ray structures of the known nucleosides, nucleotides, phosphodiesters, nucleic acids, and related compounds, and from a consideration of molecular models, it is found that the possible conformations for the backbone of helical nucleic acids is strikingly limited. Most importantly, the preferred conformation of the nucleotide unit in poly nucleotides and nucleic acids turns out to be the same as that found for the nucleotide in the crystal structure. It is observed that base “stacking” is a consequence of the restricted backbone conformation. The torsional angles are illustrated in the form of conformational “wheels”. Interrelation between the torsion angles about successive pairs of sugar‐phosphate bonds are presented in the form of conformational maps: ω,ϕ; ϕ,ψ; ψ.ψ′; ψ′,ϕ′; ϕ′,ω′; ω′,ω. The ω′,ω map shows the perferred conformations about the inter‐nucleotide bonds of right‐ and left‐handed helices and the possible conformations of phosphodiesters. The preferred conformation of the pyrophosphate and triphosphate is that in which the phosphate oxygens display a staggered arrangement when viewed along the P–P axis. A plausible structure and conformation for the ATPM2− backbound complex is presented. This structure differs from that proposed by SzentGyorgi in that the metal (only transition metals are considered here) is not bound to the NH2 nitrogen of adenine, but rather is simultaneously bound to N(7) of the ring and three phosphates (α, β, γ), or N(7) of the ring and two phosphates (β, γ). The remaining metal coordination may be satisfied by solvent–metal or enzyme–metal bonds.
Conformation of dipeptides
Authors: Lande Saul
Published: May 2005

The amide bond in L,L‐ and L,D‐α‐chloropropionylalanine methyl ester is shown to be trans by molar polarization and infrared spectroscopy. In these dipeptide diastereoisomer analogues, therefore, differences in physical properties, i.e., melting points, crystalline forms, gas chromatographic mobilities, etc., depend on preferred molecular conformations and not peptide bond configuration. Nuclear magnetic resonance spectra of both compounds were identical, indicating that no major chemical environment differences exist, which might have resulted from dissimilar side group interactions. Based on the data reported here and those of others, most dipeptide conformations can be eliminated because of contradiction with limits set by experimental or theoretical considerations. Of the remaining conformational possibilities, a single pair accounts for observed physical differences in dipeptide diastereoisomers, free or blocked. The preferred form contains α‐hydrogens trans to each other and in the plane of the peptide bond. In this conformation, R1–R2 and amino–carboxyl distances are minimal in L,D diastereomers and maximal in L,L forms.
Polymerization of amino acid derivatives by polymer catalysts. II. Polymerization by copolymer catalysts containing a tertiary amine as a component
Authors: Suzuoki Kazuhiro , Imanishi Yukio , Higa‐Shimura Toshinobu , Okamura Seizo
Published: May 2005

The polymerizations of D,‐Lβ‐phenylalanine NCA, p–nitro‐D,L‐β‐phenylalanine NCA, and o,p‐dinitro‐D,L‐β‐phenylalanine NCA were investigated, homopolymers and copolymers of N‐vinyl‐2‐ethylimidazole or 2‐Vinylpyridine being used as catalysts. When N‐vinylpyrrolidone and N,N‐diethylacrylamide, which are capable of forming hydrogen bonds with the NCA's, were used as comonomers with N‐vinyl‐2‐ethylimidazole, the copolymer catalysts were found to bring about a faster polymerization than poly‐N‐vinyl‐2‐ethylimidazole. However, when styrene, which has no particular interaction with the NCA's, was used as a comonomer with 2‐vinylpyridine, the copolymer catalyst was found to give a slower polymerization than poly‐2‐vinylpyridine. Electronic spectroscopy showed that the charge‐transfer complex between copolymer catalysts and the NCA's plays an important role in the polymerization. The experimental results are discussed in terms of the effectiveness of the copolymer catalysts for forming hydrogen bonds or charge‐transfer complexes with the NCA's.
Polymerization of amino acid derivatives by polymer catalysts. III. Chain effect polymerization induced by poly(N‐ethylglycine) diethylamide
Authors: Sisido Masahiko , Imanishi Yukio , Okamura Seizo
Published: May 2005

The polymerization of D,L‐β‐phenylalanine NCA induced by preformed polyi(N‐ethylglycine) diethylamide having a terminal secondary amino group was investigated. The polymerization was much faster than the same reaction initiated by a low molecular weight amine of similar base strength. Hence, the chain effect is operative with poly (N‐ethylglycine) diethylamide as well as polysarcosine diethylamide, with which Bamford et al. found the chain‐effect mechanism. The polymerization rate was dependent on the degree of polymerization of the polymer catalyst. The rate enhancement was attributed to adsorption of the NCA onto the amide carbonyl groups of the polymer chain by the formation of hydrogen bond. In going from polysarcosine diethylamide to poly (N‐ethylglycine) diethylamide, the absorptivity of NCA on on the polymer chain was little affected. However, the reaction between an adsorbed NCA and a terminal base was more difficult with poly (N‐ethylglycine) diethylamide than with polysarcosine diethylamide. This may be related to differences in flexibility of the polymer chain.
Partial denaturation of macromolecules by chemical agents
Authors: Cohen‐Addad J. P.
Published: May 2005

The denaturation of a macromolecule such as a polypeptide is considered in the case where the total number of noncovalent internal bonds broken in the binding process of the chemical agent is only a fraction, αs, of the total number of noncovalent internal bonds involved in its helical native conformation. Starting from a two‐parameter (s, σ) model of the helix–random coil transition of polypeptides, the transition temperature is derived as a function of the fraction αs, and of the concentration and the binding constant of the chemical agent. The lower limit of the transition temperature and the corresponding slope of the transition curve are shown to depend on αs. As an illustration, existing data on the partial acid denaturation of the enzymes ribonuclease A and muromidase are analyzed. The resulting average enthalpies of a noncovalent bond in the native ribonuclease A and muromidase are found to be 1.81 kcal/mole and 1.54 kcal/mole, respectively; the corresponding entropies are 5.4 and 4.4 cal/deg./mole, respectively. As a further example, existing data on a partially methylated enzyme ribonuclease A are also considered.
Viscosity and sedimentation study of sonicated DNA–proflavine complexes
Authors: Cohen Gerald , Eisenberg Henryk
Published: May 2005

The intrinsic viscosity of sonicated calf thymus DNA (molecular weight 4–5 × 105) increases and the sedimentation constant decreases, with increasing binding of proflavine at 0. 2 ionic strength and at 25°C. The measurements correspond to a linear increase in length of the almost rodlike DNA molecules with the amount of proflavine bound; independent calculations from viscosity and sedimentation measurements yield almost identical results. Over the range of r (moles of proflavine bound per moles of nucleotides) equal to zero to r = 0.13, the length increases by about 20%. This extension is compatible with the intercalation hypothesis proposed by Lerman. Density increments at various values of r, at constant chemical potential of diffusible solutes, were determined. It was also found that, in addition to the known isosbestic point of DNA‐proflavine complexes at 455.5 mμ, an additional isosbestic point exists at 225.5 mμ; this proved extremely useful for the evaluation of binding studies.
Helix–coil transition for poly(L‐glutamic acid) in water–ethanol solutions
Authors: Conio G. , Patrone E.
Published: May 2005

Potentiometric titrations and some complementary optical rotation data are presented for solutions of poly(L‐ glutamic acid) (PGA) in several H2O–ethanol mixtures. The data allow the determination of the intrinsic pK (pK0), slope of the apparent. pK (pKapp), versus degree of ionization curves and of the enthalpy of ionization as a function of ethanol concentration. The variation of the degree of ionization at which the helix–coil transformation occurs with ethanol and temperature is also determined. Finally free energy, enthalpy, and intropy changes associated with the helix–coil transformation for the uncharged conformers are determined from the titration curves. The effect of the ethanol is to increase the stability of the helical conformation of PGA for both the charged and the uncharged forms of the polymer. The stabilization of the uncharged helix is essentially an entropic effect.
Dégradation du DNA par H2O2 en présence d'ions Cu++, Fe++ et Fe+++
Authors: Schweitz H.
Published: May 2005

The mechanism of degradation of calf thymus DNA by H2O2 in dark and light, and in the presence of either Cu++, Fe++, or Fe+++ ions has been investigated by following the decrease of molecular weight M̄w by light scattering. Both in the dark and in light, the rate of degradation decreases in the following order: Cu++>Fe++>Fe+++. In order to exploit quantitatively the variation of M̄w with time, we calculated the probability p(t) of rupture in a double stranded polymer as a function of the occurence at random of both breaks of the “first kind” (single hits) and of the “second kind” (double hits), when there are caused by any degrading agent. The value of p(t) can then be related to M̄w(t) for the present case of a randomly polydisperse sample of DNA molecules. In the dark, and in the presence of Cu++ ions, a degradation of the first kind (which takes place through the simultaneous or successive splitting of both strands of DNA at the same level) is the only one so far observed. The number of degradation sites of the first kind is equivalent to the number of bound Cu++ ions in inner sites of DNA. A model is set up to explain the successive breaks of the two strands of the DNA molecule through the formation of a complex (DNA site–Cu++‐H2O2) which exhibits peroxidative properties. The comparison of the degradation induced under these conditions in a native and a sonicated DNA, shows that the specific sites of attack of ultrasonic waves are not specific sites of H2O2 action in the presence of Cu++ ions. In the dark and in the presence of Fe++ or Fe+++ ions, breaks of the first kind and second kind are superimposed, but the last are predominant. This is ascribed to the low binding of iron ions in inner sites of DNA under these conditions. A large increase in degradation rate of the second kind occurred in the presence of light (with or without added metallic ions and) is ascribed to the action of the free radicals HO· (and HO2·) which arise from the photolysis of H2O2. These results are discussed in relation to those obtained by the action of ionizing radiations on aqueous solutions of DNA.
Interactions des ions métalliques avec le DNA. III. Stabilité et configuration des complexes Ag–DNA
Authors: Wilhelm F. X. , Daune M.
Published: May 2005

The thermal stabilization and the configurational changes of DNA were studied during the binding process of silver ion. Melting curves of silver–DNA complexes were analyzed according to the method of Felsenfeld. An increase of Tm occurred during the formation of the first complex with a preferential stabilization of G–C pairs. In the case of DNA from H. influenzae, the transforming power was maintained after heating in presence of silver at a temperature where AT pairs were dissociated but G‐C pairs were not. Shape modifications of the molecule were followed by light scattering, both with native and ultrasonic degraded DNA. In the case of native DNA, the binding of positive ions modified the electrostatic potential and increased the flexibility of the molecule. In the case of rod shaped fragments of DNA, the progressive formation of a kind of polyampholyte was accompanied by aggregation phenomena. The formation of the second complex, with proton release, induced changes of secondary structure and possibly a tilt of the plane of the base.
Interactions des ions métalliques avec le DNA. IV. Fixation de i'ion cuivrique sur le DNA
Authors: Schreiber J. P. , Daune M.
Published: May 2005

The binding of cupric ion (Cu++) to DNA was followed by spectrophotometry, melting profiles, and hydrodynamic techniques, in 0. 1M NaClO4 and at pH 5. 6. A small amount of Cu++ is bound specifically to bases (about 1 Cu++ per 20 nucleotides), in agreement with polarographic and EPR data. A preferential stabilization of G–C pairs and only a slight increase of the flexibility of the molecule were observed. In 5 × 10−3M NaClO4, a higher number of nonhomogeneous binding sites is found by spectrophotometry. It is concluded that at least two types of sites are available for Cu++. The first one, where Cu++ is chelating N7 of purines to phosphate, is observed only at low ionic strength and destabilizes the double helix. The second exists mainly at 0, 1M or higher ionic strength. All the sites are identical and could be attributed to two successive guanine residues in the same strand. Similar behavior was found for other divalent cations, e. g., Fe++, Mn++, and Co++.
Raman spectroscopy of poly‐L‐proline in aqueous solution
Authors: Smith M. , Walton A. G. , Koenig J. L.
Published: May 2005

The laser‐excited Raman spectra have been obtained of poly‐L‐proline in the solid and aqueous solution. In the solid state, the Raman spectra are of the form II which is the left‐handed helical form with the peptide bonds in the trans configuration. Dissolution of this form in water produced only minor changes in the Raman spectra. The differences in frequencies produced by dissolving the polymer are associated with vibrational modes of the pyrrolidine ring as indicated by comparison with the Raman spectrum of L‐proline. The similarity of the Raman spectra dictates that, the conformations of poly‐L‐proline chain are similar in the solid and aqueous solution.
Reversibility of thermal transitions in proteins: Racemization and aggregation as factors in the reversible denaturation of a soluble keratin derivative (SCMKA)
Authors: Harrap B. S.
Published: May 2005

The reversibility of the thermal denaturation of a low‐sulfur fraction of solubilized wool keratin (SCMKA) has been studied under a variety of conditions of time, protein concentration, and pH. Two types of irreversibility for the transition have been encountered. One of these is associated with an aggregation of the protein on denaturation to give a product which may contain elements of a β conformation. This type of irreversibility is favored by high protein concentration, and the original conformation may in fact be regained if the aggregated structure is broken down by a solvent such as 8M urea and the urea subsequently removed by dialysis. The other type of irreversibility appears to be due to racemization of the protein. It does not seem to be dependent on protein concentration and is apparent only at temperatures beyond the actual transition range (∼40–65°C) at pH values below 11, At pH 12, however, racemization appears to proceed slowly even at 4°C. The thermal transition at pH 9 and pH 10 has been shown to be multistage in nature. Over the pH range 9–12 there is a progressive decrease in thermal stability with increase of pH. Addition of NaCl at pH 10 leads to an increase in thermal stability of the molecule.
Studies of the optical properties of the proflavine–DNA complex
Authors: Li Hsueh Jei , Crothers Donald M.
Published: May 2005

We report studies of the optical properties of the proflavine–DNA complex, using absorbance and circular dichroism spectroscopy. From comparison of the absorption spectra of proflavine complexed with calf thymus and T2 DNA, we conclude that stacking of the dyes external to the double helix is comparatively much weaker with T2 DXA, probably because of its glucosylation. Several sources are found for the circular dichroism induced in proflavine when it is complexed with DNA. There is a relatively weak circular dichroism induced when the dye is infinitely dilute on the DNA lattice; this presumably arises from the environmental asymmetry of the binding site. Stronger circular dichroism effects are induced by interaction of intercalated and stacked dyes; studies with T2 DNA, for which stacking seems to be blocked, permit a tentative resolution of effects due to the two modes of binding. One recurring theme of these studies is the observation that the optical properties are quite dependent on environment. The most dramatic example is a strong variation with salt concentration of the amplitude of the circular dichroism induced in the isolated (intercalated) monomer by the surrounding DNA. This suggests that the structure of the intercalated complex is quite sensitive to external conditions.
Studies on the helical conformational of amylose and possible interconversions
Authors: Sundararajan P. R. , Rao V. S. R.
Published: May 2005

The nonbonded energy (van der Waals) was computed for isolated helical amylose chains as a function of the dihedral angles (ϕ,ψ), i.e., the relative orientations of the glucose residues in the polysaccharide chain. In conformity with x‐ray data, different helical conformations have been proposed for different crystalline modifications of amylose. Right‐handed helical conformations were preferred for 71.13, 61.33, and 42.65‐type helices from minimum energy considerations. The hydrogen bond search reveals the possibility of formation of strong OH ⃛O type hydrogen bonds between contiguous residues in the polymer chain for 61.33 and 71.13 helices. Smooth transition from a 71.13 to a 61.13 helix is possible with small changes in the values of (ϕ,ψ) and without the loss of intramolecular hydrogen bonds, whereas all the intramolecular hydrogen bonds will be broken during the conversion from 61.33 to 42.65 helix.
Configurational statistics of polysaccharide chains. Part I. Amylose
Authors: Rao V. S. R. , Yathindra N. , Sundararajan P. R.
Published: May 2005

The conformational energy of amylose molecule was computed as a function of rotations ϕ and ψ about interunit glycosidic bonds. The characteristic ratio 〈r2〉0/Nl of the mean‐square end‐to‐end distance of the unperturbed chain to the product of n, the number of glucose residues in the chain and the square of the virtual bond length lV = 4.2 Å calculated for amylose chain is about 6.9, which is in good agreement with the experimental value (6.6 ± 0.2). The value of this ratio calculated for free rotation about the inter unit glycosidic bond is 1.1. The conformational map reveals that the rotations of pyranose units about the interunit glycosidic bonds are highly restricted; this tends to lock these units in a preferred orientation. From the probability of occurrence of the glucose residue conformations it is concluded that amylose exists as a random coil with short helical sequences in neutral aqueous solutions.
Circular dichroism of the “random” polypeptide chain
Authors: Tiffany M. Lois , Krimm S.
Published: May 2005

The circular dichroism (CD) spectrum of an unordered polypeptide chain does not correspond, as has been assumed heretofore, to that of a charged chain such as poly‐L‐glutamic acid or poly‐L‐lysine. The latter have been shown to have locally ordered structures with characteristic CD spectra. We have now obtained CD spectra of the unordered forms of the above synthetic, polypeptides, as well as of two fibrous proteins (collagen and feather keratin) and a globular protein (myoglobin). These spectra are all similar to that of unordered polyproline, having a negative band in the vicinity of 2000 mμ and no additional bands at longer wavelengths. The lack of structural uniqueness of the unordered polypeptide chain is emphasized by these studies.
Polynucléotides protonés. VII. Transitions thermiques entre differents complexes de l'acide polyinosinique et de l'acide polycytidylique en milieu acide
Authors: Thiele Danielle , Guschlbauer Wilhelm
Published: May 2005

The interaction between poly (I) and poly (C) in acid medium has been studied by potentiometric titration, mixing curves and thermal denaturation. Phase diagramms as a function of ionic strength, pH, and temperature have been established. From these data it is shown that the acid titration of the complex poly (I) · poly (C) passes through a triple‐stranded intermediate poly (I) · poly (C) · poly (C+) to yield finally the protonated double‐helical complex poly (I) · poly (C+). The mixing curves indicate the sole presence of the three‐stranded complex in the intermediate zone. On the basis of the pK's the coexistence between the three‐stranded complex with the neighboring double‐stranded structure is demonstrated in a narrow rang of pH and ionic strength. The geometry of the base arrangements, their conformation and the sense of the strands are discussed in the light of the data presented. A Hoogsteen‐type pairing between the bases for poly (I) · poly (C+) is favored, although the reverse Hoogsteen pair cannot be excluded.
Hydrodynamic relaxation times of DNA from decay of flow dichroism measurements
Authors: Callis Patrik R. , Davidson Norman
Published: May 2005

The relaxation of the flow dichorism signal of native DNA molecules after rapid (∼0.004 sec) stoppage of the flow has been studied in the narrow‐channel flow dichroism apparatus previously described. The flow dichroism signal decays with a spectrum of relaxation times. The slowest relaxation time accounts for about 0.40 of the total signal for shear gradients in the range of 100–300 sec−1 and for about 0.25 of the total signal for shear gradients in the range of 5,000– 20,000 sec−1. This longest relaxation time in dilute aqueous salt solution at 25°C for T4‐DNA molecules is measured as 0.45 sec; in the molecular weight range from 125 × 106 to 12 × 106, the relaxation time varies as. M(1.6±0.1). The relaxation time is proportional to η/T for large (sixfold) changes in solvent viscosity and for temperature changes between 25 and 0°C, showing that the molecular relaxation is hydrodynamically limited. An accurate analysis of the data at short times to determine the shorter relaxation times was not possible. A theoretical prediction based on the Zimm‐Rouse theory of polymer dynamics for the longest relaxation time is in good agreement with the observed value, but the theory predicts that 87%, of the total signal should decay with this relaxation time, whereas experimentally only a smaller fraction of the total signal does so. This discrepancy occurs because the theory is applicable to low hydrodynamic shear stress flows and the experiments were made at higher shear stresses.