Thermodynamic and spectroscopic study of intermolecular association in aqueous dipeptide solutions

Peculiarities of the concentration dependences of apparent molar heat capacities and volumes of aqueous solutions of dipeptides were considered. Possible interpretation of the character of these dependences on the basis of Gibson and Sheraga's model of solvation shell were shown. Three concentration regions corresponding to the existence of various molecular forms of dipeptides in solution were marked out.     

Vibrational circular dichroism of carbohydrate films formed from aqueous solutions

Vibrational circular dichroism (VCD) spectra in the entire 2000-900 cm(-1) region have been recorded, for the first time, for films of carbohydrates prepared from aqueous solutions. Eight different carbohydrates, alpha-D-glucopyranosyl-(1-->4)-D-glucose, cyclomaltohexaose, alpha-D-glucopyranosyl alpha-D-glucopyranoside, beta-D-glucopyranosyl-(1-->6)-D-glucose, beta-D-glucopyranosyl-(1-->4)-D-glucose, D-glucose, and both enantiomers of 6-deoxygalactose and of allose, were investigated. The VCD spectra obtained for films are found to be identical to the corresponding spectra obtained for aqueous solutions of carbohydrates. These measurements demonstrate several advantages of significant importance. The strong infrared absorption of water has prevented, in the past, the pursuit for routine applications of VCD in determining the structures of carbohydrates in aqueous solutions. This limitation is not present for film studies because water solvent is removed in the process of preparing the films. Also, strong infrared absorption of water at 1650 cm(-1) requires the use of very short-pathlength (6 microm) cells for measurements on aqueous solutions. This requirement and concomitant inconveniences (such as laborious assembling of a demountable liquid cell or purchasing an expensive variable pathlength liquid cell) have been eliminated for film measurements. The removal of interfering water absorption in film studies resulted in higher light throughput and better signal-to-noise ratios for VCD measurements. Another point of significance is that the amount of carbohydrate sample required for VCD measurements on films is approximately one to two orders of magnitude smaller than that required for corresponding VCD measurements on aqueous solutions. Since carbohydrate samples can now be studied as films, VCD spectroscopy becomes much more broadly applicable for carbohydrates than previously believed. The present work, in combination with other film measurements in our laboratory, indicate that VCD studies on films can be used more generally, providing a convenient and powerful approach for probing structural information for biologically important compounds.     

Intermolecular complexation and phase separation in aqueous solutions of oppositely charged biopolymers

Turbidity measurements performed at 450nm were used to follow the process of complex formation, and phase separation in gelatin-agar aqueous solutions. Acid (Type-A) and alkali (Type-B) processed gelatin (polyampholyte) and agar (anionic polyelectrolyte) solutions, both having concentration of 0.1% (w/v) were mixed in various proportions, and the mixture was titrated (with 0.01 M HCl or NaOH) to initiate associative complexation that led to coacervation. The titration profiles clearly established observable transitions in terms of the solution pH corresponding to the first occurrence of turbidity (pH(C), formation of soluble complexes), and a point of turbidity maximum (pH(phi), formation of insoluble complexes). Decreasing the pH beyond pH(phi) drove the system towards precipitation. The values of pH(C) and pH(phi) characterized the initiation of the formation of intermolecular charge neutralized soluble aggregates, and the subsequent formation of microscopic coacervate droplets. These aggregates were characterized by dynamic light scattering. It was found that Type-A and -B gelatin samples formed soluble intermolecular complexes (and coacervates) with agar molecules through electrostatic and patch-binding interactions, respectively.     

Vibrational analysis of glutathione

Infrared and Raman spectra have been obtained of crystalline glutathione and its deuterated derivative and interpreted by normal mode analysis. The force field consisted of our empirical force fields for the peptide group and NH and CO end groups, plus our ab initio force fields for the CH₂SH and CH₂COOH moieties. Observed bands are reproduced with an average error of 5 cm^?1, demonstrating that the vibrational spectrum of such a complex molecule can be understood in great depth. © 1994 John Wiley & Sons, Inc.     

Vibrational spectroscopic approach to the study of acetylcholine and related compounds

The present review reports the coordinated application of three spectroscopic methods (Raman, infrared(IR) and inelastic electron tunneling spectroscopy (IETS)) in the study of the conformation of Ach and some analogues (beta-MeAch, Mu and Nic) in solid state, aqueous solution and in interaction with a surface. Useful correlated information is obtained by Raman and IR spectroscopies on the conformational possibilities of these molecules in transition from solid state to aqueous solution. With this information in hand as well as on the basis of Raman and IR study of the nonenzymatic hydrolysis of Ach, the first detailed experimental investigation of the interaction of Ach and beta-MeAch adsorbed on a surface (A1203) is realised by the IETS method. The results are used to discuss an interaction analogous to that of Ach with receptor and another one analogous to that of Ach and AchE.     

Complexation of Al(III) with reduced glutathione in acidic aqueous solutions

The complexation of reduced glutathione (GSH) in its free and Al(III)-bound species in acidic aqueous solutions was characterized by means of multi-analytical techniques: pH-potentiometry, multinuclear (¹H, ¹³C and ²⁷Al) and two-dimensional nuclear Overhauser enhancement NMR spectroscopy (¹H, ¹H-NOESY), electrospray mass spectroscopy (ESI-MS), and ab initio electronic structure calculations. The following results were found. In the 25 °C 0.1 M KCl and 37 °C 0.15 M NaCl ionic medium systems, Al³⁺ coordinates with the important biomolecule GSH through carboxylate groups to form various mononuclear 1:1 (AlHL, AlH₂L and AlH₋₁L), 1:2 (AlL₂) complexes, and dinuclear (Al₂H₅L₂) species, where H₄L⁺ denotes totally protonated GSH. Besides the monodentate complexes through carboxylate groups, the amino groups and the peptide bond imino and carbonyl groups may also be involved in binding with Al³⁺ in the bidentate and tridentate complexes. The present data reinforce that the glycine carboxylate group of GSH has a higher microscopic complex formation constant than γ-glutamyl carboxylate. Compared with simple amino acids, the tripeptide GSH displays a greater affinity for the Al³⁺ ion and thus may interfere with aluminum’s biological role more significantly.     

The complexation of metal cations by D-galacturonic acid: a spectroscopic study

Solid complexes of D-galacturonic acid (GalA) with cobalt(II), copper(II), nickel(II) and oxovanadium(IV) (1-4) were prepared and characterised. The metal-to-ligand molar ratio was 1:2 for complexes 1-3 and 1:1 for complex 4. The alpha- and beta-anomers of GalA were detected in all the complexes in solid state and in solutions. An addition of small amounts of the paramagnetic complexes to the D2O solution of pure ligand led to NMR line broadening of some 1H and 13C nuclei. This broadening was sensitive to the anomeric state of GalA in the case of complexes 1 and 4. NMR and vibrational spectroscopic data indicate the formation of carboxylate complexes of all the cations, while noncarboxylic oxygens are also involved into the metal bonding in some cases. VCD spectra of complexes 1-4 in D2O and Me2SO-d6 solutions confirm that GalA carboxylic group may participate in the formation of optically active species around the metal cation. Possible ways of GalA coordination by metal cations of this study were proposed and discussed.     

An experimental study of potentiated aqueous solutions

A systematic study was undertaken of luminescent aqueous solutions of homeopathic preparation of sodium chloride at a dilution from D1 to D30, produced by "Weleda" company (Moscow) was carried out. It was shown that intensity of luminescence versus the degree of dilution is a non-monotonous function with several maxima, the main maximum corresponds to 13-14 decimal dilution. The dynamics of spectra was registered for several weeks. A systematic study of water samples (D1-D30) exposed to a similar procedure of potentization but without salt addition was also performed. The difference in the luminescence spectra of water of different stages of potentization was shown. The motility of infusoria Spirostoma ambiquum in solutions being examined was studied. A significant negative correlation between the infusoria motility and luminescence intensity was registered.     

Ion complexation in nonactin,monactin, and dinactin: A Raman spectroscopic study

The ability of the macrotetrolide nactins to complex selectivity with a wide variety of cations makes these ionophorous antibiotics important model systems for the study of biologic ionic transport. We report a Raman spectroscopic investigation of the Na⁺, K⁺, Rb⁺, Cs⁺, Tl⁺, NH₄⁺, NH₃OH⁺, C(NH₂)₃⁺, and Ba⁺⁺ complexes of nonactin, monactin, and dinactin in 4:1 (v/v) CH₃OH/CHCl₃ and in the solid state. The nactins display characteristic spectral changes upon complexation, some of which are specific for a given cation. In the K⁺, Rb⁺, Cs⁺, NH₃OH⁺, and C(NH₂)₃⁺ complexes, which are apparently isosteric, the ester carbonyl stretch frequency is found to be linearly proportional to the cation–carbonyl electrostatic interaction energy, as calculated from a simplified model. Deviations for the Na⁺, NH₄⁺, Tl⁺, and Ba⁺⁺ complexes are interpreted as arising from additional nonelectrostatic interactions. Additional information is obtained from other spectral regions and from measurements of depolarization ratios. Spectra of the nactin complexes differ from each other more in the solid state than in solution, reflecting the effects of crystalline contact forces.     

Ruthenium red complexation with ionic polysaccharides in dilute aqueous solutions: chirooptical evidence of stereospecific interaction.

This paper presents optical and chirooptical data on the interaction of the microscopy-staining agent ruthenium red with carboxylated polysaccharides in dilute aqueous solution. The polysaccharides used are both natural (alginate and pectate) and semisynthetic (C6-oxidized cellulose and C6-oxidized amylose). A preliminary discussion of the molecular structure and conformational features which control the interaction is presented.     

Vibrational spectroscopic mapping and imaging of tissues and cells

Vibrational spectroscopic mapping (point-by-point measurement) and imaging of biological samples (cells and tissues) covering Fourier-transform infrared (FTIR) and Raman spectroscopies has opened up many exciting new avenues to explore biochemical architecture and processes within healthy and diseased cells and tissues, including medical diagnostics and drug design.     

Vibrational spectroscopic characterization of new calcium phosphate bioactive coatings

In this work calcium phosphate (CaP) compounds with different PO(3-)(4)/HPO(2-)(4) R molar ratios in the 0.65-149 range were synthesized. In fact, all these CaPs contain different amounts of HPO(2-)(4) and PO(3-)(4) ions as well as the amorphous precursors (tricalcium phosphate and octacalcium phosphate) of hydroxyapatite deposition, which was shown by in vitro and in vivo measurements. Spectroscopical IR and Raman results showed the presence of bands whose intensity ratio can be related to the molar ratio R; in particular, the Raman I(962)/I(987) and the IR I(1035)/I(1125) intensity ratios were characterized as markers of the molar ratio. For these CaP compounds a nucleation model, which was based on the ability of HPO(2-)(4) ions to form strong H bonds with PO(3-)(4) ions, was proposed.     

A kinetic study of D-glucose oxidation by bromine in aqueous solutions

The kinetics of the oxidation of D-glucose to D-gluconic acid by bromine in aqueous solution were studied using potentiometric techniques and theoretical considerations of complex bromine-bromide-pH equilibria. The pH has a strong influence on reaction rate. At pH < 8 the reaction is very slow, while in the pH range pH 8-9.5 the reaction is sufficiently fast and seems optimal for the reaction. The proposed active species at that pH region is hypobromous acid. At pH > 9.5, the reaction is further accelerated due to the formation of hypobromite. The proposed kinetics expression for gluconic acid formation, based on the determined kinetic parameters at pH 9.24, is of the form dc(GA)/dt = 160c(2)(G)c(o)(HOBr)c(o)(H(+)c(o)(Br)     

A proton magnetic relaxation study of ferrimyoglobin in aqueous ionic solutions

Proton magnetic longitudinal T₁ relaxation times have been measured for acid (horse) ferrimyoglobin solutions [0.1 M NaCl and KH₂PO₄, 2 M NaCl and 1 M MgCl₂] from 5°C to 35°C in dependence on myoglobin concentration up to 6 mM. The enhancement of the relaxation rate due to the paramagnetic haem iron. which is observed in this temperature range is compared with analogous data for the ferrihaemoglobin solution. The conclusion is that the protons exchanging from the haem pocket with bulk solvent are not those from the water molecule at the sixth ligand site of haem iron. The exchanging protons are more than 4 Å away from the haem iron being closer to it in ferrimyoglobin than in ferrihaemogiobin. This distance becomes larger in solutions with higher salt concentration, the largest difference between 0.1 M NaCl and 1 M MgCl₂ being over one Angstrom unit. This indicates a conformational change of the haem pocket, possibly its tightening.     

Vibrational spectroscopic analysis of LD-sequential, bacterial cell wall peptides: an IR and Raman study

The synthetic, zwitterionic bacterial cell wall peptides—D -Glu^γ-L-Lys, D -Glu^γ-L-Lys-D -Ala, D -Glu^γ-L-Lys-D -Ala-D -Ala, and L-Ala-D -Glu^γ-L-Lys-D -Ala-D -Ala—have been investigated in the crystalline and aqueous solution state applying ir and Raman spectroscopy. Additionally, aqueous solutions of the tetra- and pentapeptide have been investigated by CD spectroscopic techniques. Apart from the dipeptide, whose spectral features were dominated by end-group vibrations, the corresponding ir and Raman active bands of the crystalline peptides in the amide and skeletal regions were found at similar wave numbers, thus suggesting an analogous three-dimensional structure of these compounds. Dominant amide A, I, II, and III bands near 3275, 1630, 1540, and 1220–1250 cm^?1, respectively, in the ir are interpreted in favor of an intermolecularly hydrogen-bonded, β-like structure. The absence of any amide components near 1680–1690 cm^?1, together with the presence of strong amide bands near 1630 cm^?1, and weak bands near 1660 cm^?1 in the ir, which, conversely, were found in the Raman spectra as weak and strong bands, but at corresponding wave numbers, is taken as strong evidence for the presence of the unusual, parallel-arranged β-structure. On the basis of comparative theoretical considerations, a parallel-arranged, “β-type ring” conformation [P. De Santis, S. Morosetti, and R. Rizzo (1974) Macromolecules 7 , 52–58] is hypothesized. The solubilized peptides exhibited distinct similarities with their crystalline counterparts in respect to frequency values and relative intensities of the corresponding ir and Raman-active amide I/I′ components, and of some Raman bands in the skeletal region. This is interpreted in terms of residual short-range order, persisting even in aqueous solution. We concluded that the peptides show a strong propensity to form hydrated, strongly associated aggregates in water. On the basis of amide I/I′ band positions, stable, intramolecular interactions via the amide groups are discarded for the solubilized peptides. Complementarily, the CD data obtained suggest the presence of weakly bent, “open-turn”-like structures for the tetra- and pentapeptide in aqueous solution.     

The systematic study of aluminium speciation in medium concentrated aqueous solutions.

Industrial applications and environmental problems involving the aqueous chemistry of aluminium require an understanding of the speciation of this metal ion at a wide range of concentrations. The formation of polynuclear species is of special interest due to the complexity of the hydrolysis mechanisms and the diversity of the hydrolysis products. Kinetic aspects of speciation are also important considering the different stability ranges of polycationic species formed during the hydrolysis process. In the present paper we report results of systematic studies on the formation of aluminium polycations at room temperature. Automated potentiometric titrations have been used to study the hydrolysis of aluminium-ions in solutions (0.01-0.2 M) on a short time scale (2 min between titrant additions). (27)Al NMR spectroscopy and dynamic light scattering have been used for investigations on a longer time scale (24 h). The effects of alkali strength (KOH, NH(4)OH and KHCO(3)) and concentration (0.45-2.0 M), counterion identity (Cl(-), NO(3)(-), SO(4)(2-)) and ionic strength have been investigated. Optimum conditions for the generation of Al(13)-mer are proposed on short and long time scales. On a short time scale, aluminium chloride and nitrate should be used as starting materials, KOH and KHCO(3) should be used for hydrolysis and experiments conducted at low ionic strength. For solutions that have been left to age, there is a considerable hydrolysis window that can be used to generate significant quantities of the Al(13)-mer that vary little with the alkali used. Al(13)-mer species are not generated from alum as the precursor. The presence of sulphate ions alters the pathway of aluminium polymerisation to form polymeric and solid materials. On the basis of the potentiometric titration data, dynamic light scattering and (27)Al NMR measurements evidence is provided for the detrimental role of sulphate-ions in the formation of Al(13)-mer and an alternative mechanism of aluminium ion polycondensation is proposed, based on the increased stability of monomeric and oligomeric species (dimer and trimer) in the presence of sulphate-ions.