A dye-binding induced conformational transition of poly-(methacrylic acid) by Auramine O in aqueous solutions. I. Experimental results

Binding of auramine O to poly-(methacrylic acid) (PMA) has been established over a large range of C⁰_p/C⁰_d values using spectroscopic methods (UV absorption and visible fluorescence emission spectra), equilibrium and sedimentation dialysis, potentiometric and viscosimetric titrations. All the results show qualitative agreement with those obtained previously with the system crystal violet-PMA although the binding seems to be less strong for auramine O than for crystal violet. From dialysis experiments binding isotherms were obtained at three different degrees of neutralization α'; at α' = 0.10 the results could be fitted to a Langmuir isotherm but at α' equal to 0.40 and 0.65 deviations with respect to such an isotherm occur. The results of potentiometric and viscosimetric titrations confirm that the conformational transition which the dye-free PMA exhibits upon ionization is affected by the dye binding. The region in which the conformational transion occurs is broadened and is less sharply defined in the presence of auramine O.     

Tacticity effects on conformational structure and hydration of poly-(methacrylic acid) in aqueous solutions-a molecular dynamics simulation study

The influence of tacticity on chain dimensions, backbone and side-group conformational states and relaxation dynamics, intermolecular hydrogen bonding and its relaxation dynamics was investigated for 30 repeat unit poly(methacrylic acid) (PMA) as a function of the degree-of-neutralisation, f (i.e. charge density) [0 < f < 1 range] in explicit water with Na⁺ neutralising counter-ions, using molecular dynamics simulations. Chain expansion with increase in charge density is observed. Simulation results, where applicable, compare well with experimental results in the literature. Isotactic (i-PMA) chain exhibits the largest expansion (89% change in R_g), and syndiotactic (s-PMA-RR-0.7) chain shows 31%. For fully neutralised chain (high charge density), the probability for trans conformation at backbone bonds follows the trend i-PMA > a-PMA > s-PMA. At high charge density, a higher probability of trans state is obtained at meso dyads as compared to racemic dyads and the side group in i-PMA relative to other tacticity is conformationally more extended. RR triads impart better hydrogen bonding with water. In the COOH side group, greater rotational mobility is observed for i-PMA as compared to s-PMA. Water coordination to PMA is invariant with tacticity at low f. For f = 1, s-PMA exhibits the best level of H-bonding, due to its 3D chemical structural configuration. Binding distance between water and PMA atoms and their coordination number remain unchanged with respect to ionisation of COOH groups. Isotactic (i-PMA) exhibits the fastest relaxation of polymer-water H-bonds. While counter-ion coordination on to PMA is unaffected by tacticity, the intensity of ion condensation as well as hydrogen bond relaxation time at f = 1 varies in the order i-PMA > a-PMA > s-PMA.     

Thermally induced conformational transition of double-stranded xanthan in aqueous salt solutions.

The thermally induced conformational transition of double-stranded xanthans (degree of pyruvate substitution, DSp = 0.45) having Mw = 3.1, 5.7, and 20.3 x 10(5) has been studied in aqueous salt solutions by high-sensitivity differential scanning calorimetry (DSC). The double strandedness of these samples in the ordered conformation was ascertained by the value of mass per unit length, ML = 2090 +/- 270 g mol-1 nm-1, which was determined from the contour length obtained by electron microscopic observations and the molecular weight by light scattering measurements. The temperature at half completion of the transition T 1/2 for these samples increased linearly with the logarithm of the cation (Na+, K+) concentration. The plot of 1/T1/2 vs the natural logarithm of cation (Na+) concentration in mM for the sample with Mw = 5.7 x 10(5) (15-SX) yielded the equation 10(3)/T1/2 = 3.45-0.159 ln [Na+]. The specific enthalpy delta hcal for 15-SX, essentially independent of salt concentration above 20 mM, was 8.31 +/- 0.39 J/g (SD, n = 6). No systematic dependence of molecular weight on the transition temperature and the enthalpy was observed. Application of the Manning polyelectrolyte theory to the system using the DSC data suggested that the separation of the double strand of xanthan into two single chains was not completed at the temperature where the endothermic peak was finished. This suggestion is consistent with recent findings by light scattering measurements as a function of temperature. Our DSC study was extended to include four other samples from various sources. It was found that T1/2 and delta hcal depend on the pyruvate contents of the samples. For example, the t1/2 (t1/2/degrees C = T1/2/K - 237.15) values for samples with high pyruvate content (DSp = 0.9) and depyruvated (DSp = 0.14) in 20 mM aqueous NaCl were 48.8 and 85.3 degrees C, respectively. Two other samples showed relatively broad DSC curves having shoulders, which were resolved into two independent components. Thermodynamic parameters for each component were examined as a function of salt concentration, and the results obtained were interpreted in terms of the heterogeneity of the pyruvate content of the samples.     

An acid induced conformational transition of denatured cytochrome c in urea and guanidine hydrochloride solutions.

Previous work has shown that at neutral pH ferricytochrome c (horse heart) retains certain residual structures in concentrated solutions of urea or guanidine hydrochloride (Tsong, T. Y. (1974), J. Biol. Chem. 249, 1988). Present studies reveal that cooperative unfolding of these residual structures can be achieved by acidification of the protein to pH 4 in 9 M urea but can only be partially achieved in a 6 M guanidine hydrochloride solution. The evidence that the residual structures unfold in 9 M urea upon acidification is twofold. (1) Further uncoupling of the Trp-59-heme interaction occurs; this is reflected in the intensification of the tryptophan fluorescence from 55 to 90 percent relative to that of free tryptophan in the same solvent. (2) The intrinsic viscosity of the protein solution increases from 15.0 to 21 ml/g. The acidification also induces a spin-state transformation of the heme group at pH 5 both in urea and in guanidine hydrochloride. Acidic titration of the protein in urea and guanidine hydrochloride indicates that the unfolding involves the absorption of a single proton. However, the kinetics of the spin-state transformation are triphasic. These results suggest that the displacement of the ligand His-18 by a solvent molecule and the subsequent disintegration of the residual structures are complex processes and involve at least three kinetic steps. The ineffectiveness of guanidine hydrochloride as a denaturant for ferricytochrome c is shown to be due to the presence of the high concentration of Cl minus which can stabilize certain elements of the protein structure.     

A two-state conformational transition of the extracellular ribonuclease of Bacillus amyloliquefaciens (barnase) induced by sodium dodecyl sulfate.

Barnase, the extracellular ribonuclease of Bacillus amyloliquefaciens, is shown to undergo a reversible two-state conformational transition at 0.65 mM sodium dodecyl sulfate (SDS) AAT 37 DEGREES. The prinicipal evidence is based on the equivalence of two independent values of the SDS-barnase binding ratio; about 14 mol of SDS/mol of barnase. Both were derived from fluorometric titration data, one being based on simple conservation of SDS and the other on the use of Wyman's theory of linked functions. No SDS is bound to barnase at SDS concentrations below the transition region.     

Field-dependent proton relaxation in aqueous solutions of some manganese(II) complexes: a new interpretation

Field-dependent measurements of the paramagnetic relaxation enhancement for water protons in the presence of Mn(II) complexes ( S=5/2), reported recently, are re-interpreted using theoretical models that take into consideration the fact that the relaxation of the electron spin for S>1 is multiexponential (even in the Redfield limit) and that are valid for an arbitrary relation between the electronic Zeeman interaction and the zero-field splitting in the complex.     

Conformation of poly(methacrylic acid) in acidic aqueous solution studied by small angle X-ray scattering

The nature of the contracted form of poly(methacrylic acid) PMA chain in salt-free acidic aqueous solution was studied by analyzing scattering curves registered by small-angle X-ray scattering, comparing it with those of PMA in methanol at 26 degrees C and of partially neutralized PMA in aqueous solution containing added salt (the concentration of added salt, Cs=0.1 M NaF). It is shown that the distribution of segments in the contracted form as well as that of PMA in methanol is that of a random-coil in a theta medium and that this distribution of segments is stable over a fair range of degrees of ionization alpha for Cs below 0.1 M. Moreover, the persistence length of PMA at Cs=0.1 M (4+/-0.5 A) is substantially constant throughout the entire range of alpha, indicating that the contracted form of PMA changes to an expanded random-coil in a higher pH region without a significant change in the chain flexibility.     

Potentiometric titrations and the helix-coil transition of poly(L-glutamic acid) and poly-L-lysine in aqueous salt solutions

The objective have been to establish if those ions which are known to change the stability of the structure of proteins, have any influence on the properties of ionizable polypeptides. Potentiometric titrations and complementary optical rotation data are presented for aqueous solutions of poly-L -lysine (PLL) in the presence of KSCN, KCl, and KF, and for poly(L -glutamic acid) (PLGA) in the presence of KSCN, KCl, and LiCl. The following measured quantities which are affected by salt concentration were obtained: intrinsic pK (pK₀), slope of pK_app versus degree of ionization (α) curves, the degree of ionization at which the helix to coil transition occurs, and the free energy of this transition for the uncharged molecule (δG°_hel). The effects of nonspecific salts (KCl and LiCl for PLL and KSCN and KCl for PLGA) are small, and about, as expected from general electrostatic considerations. In line with the observations made with isoelectric and cat ionic collagen, specific, effects were noted with KSCN–PLL and with LiCl–PLGA. In the presence of KSCN, the poly-L -lysine helix becomes stabilized at much lower degree of ionization than in the presence of KCl, and the slope of the pK_app versus α plots is greatly reduced. However, ΔG°_hel (for the uncharged molecule) is not affected, and pK₀ is only slightly higher. We interpret these data in terms of binding of SCN^? primarily to the side-chain amino groups (both to R? NH₃⁺ and to R? NH₂) solutions. (L -glutamic acid) in LiCl solution has its transition at the same α value as in KCl solution. However, both the slopes of the pK_app versus α plots and the absolute values of ΔG°_hel are lower than in KCl solution. We interpret these results in terms of binding of Li⁺ to side chains as well as to the peptide bond.     

Biosorption of copper(II) and cobalt(II) from aqueous solutions by crab shell particles

Biosorption of each of the heavy metals, copper(II) and cobalt(II) by crab shell was investigated in this study. The biosorption capacities of crab shell for copper and cobalt were studied at different particle sizes (0.456-1.117 mm), biosorbent dosages (1-10 g/l), initial metal concentrations (500-2000 mg/l) and solution pH values (3.5-6) in batch mode. At optimum particle size (0.767 mm), biosorbent dosage (5 g/l) and initial solution pH (pH 6); crab shell recorded maximum copper and cobalt uptakes of 243.9 and 322.6 mg/g, respectively, according to Langmuir model. The kinetic data obtained at different initial metal concentrations indicated that biosorption rate was fast and most of the process was completed within 2h, followed by slow attainment of equilibrium. Pseudo-second order model fitted the data well with very high correlation coefficients (>0.998). The presence of light and heavy metal ions influenced the copper and cobalt uptake potential of crab shell. Among several eluting agents, EDTA (pH 3.5, in HCl) performed well and also caused low biosorbent damage. The biosorbent was successfully regenerated and reused for five cycles.     

Structural transitions of deoxyribonucleic acid in aqueous electrolyte solutions. I. Reference spectra of conformational limits.

The circular dichroism properties of calf thymus DNA have been examined at 27 degrees over the wavelength range of 215-300 nm in aqueous solutions of NaCl, KCl, LiCl, CsCl, and NH4Cl at pH 7. The concentrations of these electrolytes were varied from 0.01 to ca. 5-10 m. The spectral changes induced by changes in concentration of NaCl and KCl and all but the highest concentrations of NH4Cl as well as lower concentrations of Cstcl and LiCl could be represented by a common two-state transition involving the conversion of the typical conservative spectrum commonly seen in dilute solutions of these salts to a nonconservative spectrum similar to that obtained by Tunis-Schneider and Maestre ((1970), J. Mol. Biol. 52, 521) for the C form of DNA. At higher concentrations of CsCl, LiCl, and NH4Cl, an additional component, resembling an A type spectrum, was required to account for the observed CD changes with changing concentration of electrolyte. Relying on the published spectra of the B, the C, and the A forms of DNA by Tunis-Schneider and Maestre for identification and approximate values of the molecular ellipticities of these forms, we have analyzed these spectral transitions by two least mean squares methods in order to obtain accurate reference spectra of aqueous "B", C, and "A" conformations of calf thymus DNA. The results obtained suggest that although the C form in solution is identical with that obtained in film, the aqueous B conformational limit is not identical with the crystallographic Watson-Crick structure. In addition, the A form generated in solution under our experimental conditions appears to be more similar to that assumed by low molecular weight Escherichia coli DNA at 75% relative humidity rather than calf thymus DNA at the same relative humidity.     

Oxidation of methyl linoleate in micellar solutions induced by the combination of iron(II)/ascorbic acid and iron(II)/H2O2

The oxidation of methyl linoleate (ML) was studied in the presence of Fe(II) alone and its combination with either ascorbic acid (AsAH(2)) or hydrogen peroxide (H(2)O(2)) at different molar ratios. Reactions were carried out in micellar solutions of TTAB (tetradecyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate), respectively, and were monitored by UV spectroscopy and electrospray ionization mass spectrometry (ESI-MS). Fe(II) alone was able to catalyze the oxidation of ML in micellar solutions of TTAB, but not in those of SDS. The combination of H(2)O(2) with Fe(II) showed catalytic effect only in the TTAB medium, leading to different ML and Fe(II) oxidation kinetics compared to the Fe(II)-only catalyzed reactions. The AsAH(2)/Fe(II) combination demonstrated to be a good catalyst for the oxidation of ML in SDS micellar solutions, but not in TTAB micellar solutions; the activity of the catalyst was dependent on the AsAH(2)/Fe(II) molar ratio. The obtained results confirm that, for the ML oxidation to be initiated, the presence of a Fe(II)/Fe(III) couple is essential, which is related to the pH of micellar solutions. The catalytic properties of the AsAH(2)/Fe(II) combination were explained by taking into account the anti-oxidant and pro-oxidant properties of AsAH(2), as well as the possible formation of an iron/ascorbate complex as the initiator of the ML oxidation.     

Physicochemical properties of pectic acid. I. Thermodynamic evidence of a pH-induced conformational transition in aqueous solution

On the basis of measurements of enthalpy of dissociation and of dilution, an interamolecular conformational transition induced by pH change is shown for pectic acid in aqueous solution. Additional evidence is given by potentiometic, viscometric, and chiroptical results. The transition from a more rigid, probably H-bonded, structure prevailing at low pH to a more extended one at around neutrality is accompanied by a ΔH value of about 500 cal/equiv and a ΔS value of 1.6 cal/equiv K in water at 25°C. The addition of salts increases the stability of the rigid conformation without changing the general features of the phenomenon. Dilatometric measurements suggest that the transition is accompanied by practically no change in the overall solvation of the polymer chain.     

Conformational transition of poly(L-glutamic acid) in aqueous solution

Further direct evidences are given that a clear correlation exists between potentiometric and spectroscopic measurements in monitoring the poly(L-glutamic, acid) helix⁺ coil transtition. Specific Li⁺ ion poly (L -glUtamic acid) interactions have been observed, suresting that Li⁺ ions may exert a distinct destabilizing action on the helical conformation of the polyelectrolyte.     

The removal of Cu(II) from aqueous solutions by Ulothrix zonata

In this work, adsorption of copper(II) ions on alga has been studied by using batch adsorption techniques. The equilibrium biosorption level was determined as a function of contact time at several initial metal ion concentrations. The effect of adsorbent concentration on the amount adsorbed was also investigated. The experimental adsorption data were fitted to the Langmuir adsorption model. The free energy change (deltaG0) for the adsorption process was found to be -12.60 kJ/mol. The results indicated that the biomass of Ulothrix zonata is a suitable biosorbent for both the removal and recovery of heavy metals from wastewater.     

Theoretical conformational analysis of phospholipids. II. Role of hydration in the gel to liquid crystal transition of phospholipids

To obtain a satisfactory agreement between computed transition temperatures and those determined experimentally, we introduce explicitly water molecules which hydrate the polar headgroup of dipalmitoylphosphatidylethanolamine molecules. The calculated free energy curves as a function of the intermolecular interchain distance and the degree of hydration of the polar groups permit the determination of the transition of the phospholipid system from the gel to the liquid crystalline phase. The detailed structure of the hydration shell is defined using the supermolecular approach.     

A dye-binding assay for measurement of the binding of Cu(II) to proteins

We analysed the theory of the coupled equilibria between a metal ion, a metal ion-binding dye and a metal ion-binding protein in order to develop a procedure for estimating the apparent affinity constant of a metal ion:protein complex. This can be done by analysing from measurements of the change in the concentration of the metal ion:dye complex with variation in the concentration of either the metal ion or the protein. Using experimentally determined values for the affinity constant of Cu(II) for the dye, 2-(5-bromo-2-pyridylaxo)-5-(N-propyl-N-sulfopropylamino) aniline (5-Br-PSAA), this procedure was used to estimate the apparent affinity constants for formation of Cu(II):transthyretin, yielding values which were in agreement with literature values. An apparent affinity constant for Cu(II) binding to α-synuclein of ∼1 × 10⁹ M⁻¹ was obtained from measurements of tyrosine fluorescence quenching by Cu(II). This value was in good agreement with that obtained using 5-Br-PSAA. Our analysis and data therefore show that measurement of changes in the equilibria between Cu(II) and 5-Br-PSAA by Cu(II)-binding proteins provides a general procedure for estimating the affinities of proteins for Cu(II).     

Theoretical conformational analysis of phospholipids II. Role of hydration in the gel to liquid crystal transition of phospholipids

To obtain a satisfactory agreement between computed transition temperatures and those determined experimentally, we introduce explicitly water molecules which hydrate the polar headgroup of dipalmitoylphosphatidylethanolamine molecules. The calculated free energy curves as a function of the intermolecular interchain distance and the degree of hydration of the polar groups permit the determination of the transition of the phospholipid system from the gel to the liquid crystalline phase. The detailed structure of the hydration shell is defined using the supermolecular approach.     

A ratiometric fluorescent detection of Zn(II) in aqueous solutions using pyrene-appended histidine

A fluorescent chemosensor, Py-His, based on histidine was easily synthesized in solid phase synthesis. Py-His displayed a highly sensitive ratiometric response to Zn(II) with potent binding affinity (K_a = 1.17 × 10¹³ M^?2) in aqueous solutions. The detection limit of Py-His for Zn(II) was calculated as 80.8 nM. Moreover, Py-His distinguished Zn(II) and Hg(II) by different ratiometric response type; the chemosensor showed a more enhanced increase of excimer emission intensity to Zn(II) than Hg(II). Upon addition of Ag(I) and Cu(II), Py-His showed a turn-off response mainly due to the quenching effect of these metal ions. The binding stoichiometry (2:1 or 1:1) of Py-His to target metal ions played a critical role in the fluorescent response type (ratiometric and turn off response) to target metal ions. The role of imidazole group of Py-His for ratiometric detection of Zn(II) was proposed by pH titration experiments.