Spectropolarimetric Titrations Research Articles

Molecular recognition studies on supramolecular systems. 32. Molecular recognition of dyes by organoselenium-bridged bis(beta-cyclodextrin)s.

A series of novel bis(beta-cyclodextrin)s tethered with organoselenium linkers, i.e., 6,6'-(o-phenylene-diseleno)-bridged bis(beta-cyclodextrin) (2), 6,6'-[2,2'-diselenobis(benzoyloxy)]-bridged bis(beta-cyclodextrin) (3), and 6,6'-[2,2'-diselenobis[2-(benzoylamino)ethylamino]]-bridged bis(beta-cyclodextrin) (4), were synthesized from beta-cyclodextrin (1). The inclusion complexation behavior of 1-4 with some dyes, such as 8-anilinonaphthalenesulfonate (ANS), Brilliant Green, Crystal Violet, Tropaeolin OO, Auramine O, and Methyl Orange, was investigated in aqueous phosphate buffer solution (pH 7.20) at 25 degrees C by UV-vis, fluorescence, and circular dichroism spectrometry, as well as fluorescence lifetime measurements. The complex stability constants (Ks) and Gibbs free energy changes (delta Go) for the stoichiometric 1:1 inclusion complexation of 1-4 with the dyes were obtained by the spectrophotometric or spectropolarimetric titrations. The bis(beta-cyclodextrin)s 2-4 showed much higher affinities toward these guest dyes than native beta-cyclodextrin 1 with fairly good molecular selectivities. The cooperative binding abilities of these bis(beta-cyclodextrin)s are discussed from the viewpoints of size/shape-fit interaction, induced-fit concept, and multiple recognition mechanism.

Molecular Recognition of Aliphatic Alcohols and Carboxylic Acid by Chromophoric Cyclodextrins

Molecular recognition behavior of eight cyclodextrin derivatives, i.e. mono(6-pyridinio-6-deoxy)-α-cyclodextrin (1α), mono(6-pyridinio-6-deoxy)-β-cyclodetrin (1β), mono(6-pyridinio-6-deoxy)-γ-cyclodextrin (1γ), mono[6-(p-picolinio)-6-deoxy]-β-cyclodextrin (2β), mono(6-anilino-6-deoxy)-β-cyclodextrin (3β), mono[6-(m-toluidino)-6-deoxy]-β-cyclodextrin (4β), mono[6-O-(8-quinolyl)]-β-cyclodextrin (5β), and novel mono[6-(2-naphthylamino)-6-deoxy]-β-cyclodextrin (6β), with a series of aliphatic alcohols and carboxylic acid has been investigated spectroscopically. Using the appended aromatic group as a spectral probe, spectroflurometric or spectropolarimetric titrations have been performed at 25°C in aqueous phosphate buffer solution (pH 7.20, 0.1 M) to determine the complex stability constants (Ks ) and Gibbs free energy changes (-δG°) for the stoichiometric 1:1 inclusion complexation of cyclodextrin derivatives with the guests. The results obtained demonstrate that the modified cyclodextrins are highly sensitive to the size/shape and hydrophobicity of guest molecules, and particularly 5β gives an excellent molecular selectivity up to 215 for 1-adamantanol/cyclohexanol. The binding ability and selectivity of the modified cyclodextrins (1α, 1β, and 1β-6β) are discussed from the view points of size/shape-fit concept, induced-fit interaction, and the multiple recognition mechanisms.

Molecular Recognition Studies on Supramolecular Systems. 29. Anilino- and m-Toluidino-β-Cyclodextrins: Structural and Conformational Analyses and Molecular Recognition of Aliphatic Alcohols

Mono(6-anilino-6-deoxy)-β-cyclodextrin (1) and mono[6-(m-toluidino)-6-deoxy]-β-cyclodextrin (2) were synthesized and characterized. Circular dichroism and fluorescence spectral studies and fluorescence lifetime measurements have been performed to elucidate the conformations of 1 and 2 in aqueous buffer solution. 1-D and 2-D NMR spectra of 2 have been measured in D2O to deduce its structure and detailed conformation in solution. From the circular dichroism, fluorescence, and NMR spectroscopic studies, it was revealed that the substituents appended to 1 and 2 penetrate into the cyclodextrin cavity forming a stable self-inclusion complex in aqueous solution, and also that the short linkage between the m-toluidino and cyclodextrin moieties makes the cyclodextrin ring of 2 deformed to some extent. The complex stability constants (K S) of 1 and 2 for a series of aliphatic alcohols have been determined by using spectropolarimetric titrations in aqueous phosphate buffer solution (pH 7.20) at 25°C to elucidate the role of introduced substituents and the weak interactions involved in inclusion complexation by the modified cyclodextrins. The results obtained indicate that the van der Waals and hydrophobic interactions mainly contribute to the formation of complexes between the cyclodextrins and aliphatic alcohols, and the inclusion complexation process involves the induced-fit mechanism. Modified β-cyclodextrin 2 can recognize not only the size, shape, and hydrophobicity of the guest molecules, but also chiral guests, affording a moderate enantioselectivity of 1.55 for (+)/(-)-borneol.

Effect of Host Substituent upon Inclusion Complexation of Aliphatic Alcohols with Organoseleno β-Cyclodextrins

The effect of self-including host substituent upon the inclusion complexation of arylseleno β-cyclodextrins with alkanol guests has been investigated in aqueous buffer solution at 25°C, by using spectropolarimetric titrations, and the results show that the stability constants of the host–guest complexes formed are correlated with the Hammett's σ value of the host's substituent.

Molecular Recognition Studies on Supramolecular Systems. 25. Inclusion Complexation by Organoselenium-Bridged Bis(β-cyclodextrin)s and Their Platinum(IV) Complexes

A series of novel bis(β-cyclodextrin)s connected by organoselenium linkers, i.e., 6,6‘-trimethylenediseleno-bridged bis(β-cyclodextrin) (2), 2,2‘-trimethylenediseleno-bridged bis(β-cyclodextrin) (3), 6,6‘-o-phenylenediseleno-bridged bis(β-cyclodextrin) (4), and the corresponding platinum(IV) complexes (5−7) were synthesized from β-cyclodextrin (1). The inclusion complexation behavior of 1−7 with some fluorescent dyes, i.e., ammonium 8-anilino-1-naphthalenesulfonate, sodium 2-(p-toluidinyl)naphthalenesulfonate, Methyl Orange, and Mordant Orange 1, was investigated in aqueous phosphate buffer solution (pH 7.20) at 25 °C by fluorescence and circular dichroism spectrometry, as well as fluorescence lifetime measurements. The spectrofluorometric and spectropolarimetric titrations gave the complex stability constant (KS) and Gibbs free energy change (ΔG°) for the stoichiometric 1:1 inclusion complexation of 1−7 with the fluorescent dyes. The bis(β-cyclodextrin)s 2−4 showed higher affinities toward these guests t...

Molecular Recognition Study on Supramolecular Systems. 20. Molecular Recognition and Enantioselectivity of Aliphatic Alcohols by L-Tryptophan-Modified beta-Cyclodextrin.

L-Tryptophan-modified beta-cyclodextrin (L-Trp-beta-CD) has been synthesized and its molecular recognition behavior investigated through fluorescence and circular dichroism spectrometry, as well as fluorescence lifetime measurement in the presence and absence of various alcohols as guest molecules. Employing the indolyl group as a spectral probe, spectrofluorometric and spectropolarimetric titrations have been performed in aqueous phosphate buffer solution at pH 7.20 to calculate the complex stability constants for 1:1 inclusion complexation of L-Trp-beta-CD with several series of alcohols at 25 degrees C. The results obtained indicate that L-Trp-beta-CD can recognize not only size/shape and hydrophobicity but also the enantiomeric and geometrical isomers of the guest alcohols, showing a 230-fold molecular selectivity (13.5 kJ mol(-)(1)) for 2-adamantanol over cyclopentanol among the cyclic alcohols examined. Moderate enantiomeric selectivities of 1.2 and 1.9 for (-)-isomers of borneol and menthol, respectively, and geometrical selectivity of 2.0 for geraniol over nerol have also been observed.

The binding of aryl carboxylic acid derivatives to human serum albumin—A structure-activity study

The binding interactions of some aryl carboxylic acid derivatives have been examined by circular dichroism and fluorescence spectroscopy. With specific probes, we have shown that the seven ligands under study bind primarily to the benzodiazepine site on HSA. Their association constants are in the range of 10 5–10 6 M −1 as found by spectropolarimetric titration, and appear to be closely related to some chemical features. It is concluded that the binding is enhanced by the lengthening of the carbon chain substituent with a terminating carboxyl moiety and by halogen substitution in the aromatic rings. It is further shown that hydrophilic substitutions such as hydroxyl or ketone groups in the carbon chain substituent will decrease the binding.

Binding of drugs to human serum albumin—XV: Characterization and identification of the binding sites of indomethacin

The binding of indomethacin to human serum albumin (HSA)§ has been studied with HSA immobilized in microparticles of polyacrylamide, by equilibrim dialysis and by circular dichroism. Indomethacin is bound to three sites with high affinity. The association constants are 1.5–2.1 × 10 6, 3.5 × 10 5 and 5.5 × 10 4M −1, respectively, as found by spectropolarimetric titration and from Scatchard plots of data obtained from the studies with microparticles. The evaluation of the CD-data is simplified by the different extrinsic Cotton effects engendered when indomethacin binds to the different sites. With specific probes, binding exclusively to single sites, we have shown that indomethacin primarily binds to the warfarin-(azapropazone-) site and tertiarily to the diazepam-(flurbiprofen-) site on HSA. The identity of the secondary site has not been established. Probenecid does not affect the first site of indomethacin, but improves the binding of indomethacin to the second site, probably via allosteric effects from the third site.

Crotamine conformation: Effect of pH and temperature

The pH-isomers of crotamine were characterized by spectropolarimetric titration. The neutral isomer (I) found at pH range between 4 and 8·5 corresponds to the native conformation. After heating at 80°C for 30 min in 8 M urea, it retains full biological activity and the small spectral changes induced by heating are readily reversible. Compared to the neutral isomer, the acid isomer (II) occurs at pH values lower than 2·0 and has an unmasked group presumably a carboxyl, with a p K′ of 2·6. The basic isomer (III) has two groups with ap K′ of 9·7 exposed. Also, the exposure of the N-terminal tyrosyl in II and the tyrosyl and two tryptophanyl residues in III are revealed by difference spectrophotometry. Both II and III represent partially pH-denatured conformations which undergo further structural unfolding upon heating. The unfolding ▵H° for isomers II and III were determined to be 8·2 and 6·1 kcal/mole, respectively. In addition, the spectral ▵H° calculated from the spectrophotometric data gives information about the spatial displacement of residues (mainly tryptophanyl and tyrosyl) which are more pronounced in III than in II.

Acetyl coenzyme A carbosylase. Circular dichroism studies of Escherichia coli biotin carboxyl carrier protein.

The biotin carboxyl carrier protein (BCCP) component of Escherichia coli acetyl coenzyme A carboxylase and three peptides derived from BCCP by proteolytic digestion have been examined by circular dichroism spectroscopy. BCCP, which has a peptide molecular weight of 22,500, has a spectrum typical of globular proteins with negative extrema at 222 nm and 208 nm. The two smallest peptides, BCCP(SC) and BCCP(9,100), with molecular weights of 8,900 and 9,100, respectively, exhibit unusual positive CD bands centered at 237 nm and 220 nm. BCCP(10,400), with a molecular weight of 10,400, has a CD spectrum intermediate between BCCP and that of the smallest peptides. Since d-biotin exhibits a positive CD band at 233 nm, it was suspected that the biotin prosthetic group might be the chromophore responsible for the 237 nm CD band seen in BCCP(SC) and BCCP(9,100). Enzymatic carboxylation of BCCP(SC) to form CO2-BCCP(SC) caused the CD spectrum to change with a shift of the 237 nm band to 232 nm. The positive CD band at 220 nm was unaffected by carboxylation of the biotin prosthetic group. These date suggest that the 237 nm signal may be due either to the biotin which acts as a chromophore directly or to a chromophore that is perturbed by the carboxylation of biotin. A spectropolarimetric titration was carried out to investigate the possible contribution of the single tyrosine residue of BCCP(SC) to the CD spectrum of this peptide. At pH values over 9 the CD spetrum changed with the disappearance of the 237 nm band, suggesting that tyrosine might contribute to this CD band. Denaturation of BCCP(SC) or BCCP(9,100) with 8 M urea of 6 M guanidine HCl abolished the positive CD bands and resulted in spectra typical of a random coil, whereas treatment of BCCP(SC) with 1% sodium dodecyl sulfate abolished the positive bands and left a spectrum exhibiting a shoulder at 222 nm and a negative band at 205 nm, suggestive of a high degree of ordered structure. It is concluded that the CD band at 237 nm in BCCP(SC) and BCCP(9,100) is prabably due to a noncovalent interaction of biotin with an amino acid residue(s) of the protein. It is suggested that the biotin prosthetic group is partially buried in the surface of the protein, rather than swinging free at the end of the lysine side chain through which it is covalently linked to the protein, to permit this interaction to occur.

The direct spectropolarimetric titration of palladium(II) ion with d-(-)-1,2-propylenediaminetetraacetic acid

A direct spectropolarimetric titration of palladium(II) at 40°C with the optically active ligand D-(—)-1,2-propylenediaminetetraacetic acid (D(—)PDTA) is described. The optical rotation is monitored with a modified photoelectric polarimeter, and both the palladium-D(—)PDTA complex and the titrant serve as optically active indicators. End-points are obtained by extrapolations. The proposed titration is compared with the visual back-titration method.

Determination of oxidation-reduction potentials by spectropolarimetric titration: Application to several iron-sulfur proteins

The oxidation-reduction potential ( E 0−') and the stoichiomotry of electron equivalents ( n) of several iron—sulfur proteins determined by direct potentiomctric titration in the presence of redox-mediator dyes and monitored by concomitant changes in circular dichroism are reported. The absence of circular dichroism or changes in circular dichroism by the redox mediators undergoing redox transitions forma the operational basis for the spectropolarimetric titration. The applicability of the method was explored by extensive investigations with spinach ferredoxin. The redox potential and stoichiometry values determined by spectropolarimetric titration are: (1) a plant-type (spinach) ferredoxin, E 0−' = —0.423 V, pH-independent in the range 8.0–9.5; n = 1, (2) a ferredoxin from the nonphotosynthetic bacterium, Clostridium pasteurianum, E 0−' = —0.390 V at pH 8.8, ΔE Δ pH ≅ 30 mV for pH rangc 6.8–8.8; n = 2, (3) a ferredoxin from the photosynthetic bacterium, Chromatium, E 0′ = −0.489 V at pH 9.0 and 9.5; n = 2, and (4) an iron sulfur protein designated “Protein II” isolated from Azotobactetr vinelandii, E 0− = −0.225 V at pH 7.0 and 8.0; n, = 1. An automatic-titration method with a continuously recorded titration curve was demonstrated using the azotobacter Protein II. The general applicability of the spectropolarimetric-titration technique to other biological electron carriers and some experimental factors are discussed.

Spectropolarimetric determination of copper(II), nickel(II) and iron(III) ions with n-carboxymethylpyrrolidine-2-carboxylic acid

N-Carboxymelhyl pyrrolidine-2-carboxylic acid (CMPCA) is suggested as an optically active titrant. The values of the acidity constants of the ligand were determined and the order of magnitude of the stability constants of the complexes formed by CMPCA with some metal ions was evaluated. In order to determine the best conditions for the spectropolarimetric titrations, the dependence of the molar rotation of CMPCA and its complexes on wavelength and pH was examined. The spectropolarimetric titrations ofcopper(II), nickel(II) and iron(III) ions were carried out successfully.

The direct spectropolarimetric titration of gallium (III) with d-(—)-1,2-propylenediaminetetraacetic acid

A direct spectropolarimetric titration was developed for gallium(III) at ambient temperatures with D-(—)-1,2-propylenediaminetetraacetic acid (D(—)-PDTA). The optical rotation was monitored with a modified photoelectric polarimeter to determine the end-point. The gallium-D(—)PDTA complex and the titrant, D(—)PDTA, are optically active and serve as self-indicators. Since D(—)-PDTA is stereospecific in its reactions with gallium(III), maximum utilization of the optical rotation is achieved. The optical rotation is linearly dependent on the concentration of the gallium-D(—)PDTA complex and the D(—)PDTA titrant, hence straight-line extrapolations are possible. The proposed titration is compared with the visual compleximetric method.

Sequential spectropolarimetric analysis of mixtures of barium-calcium, cadmium-copper and cadmium-zinc

A direct sequential spectropolarimetric titration method based on the stereospecific ligand D-(—)-1,2-propylenediamine tetraacetic acid was developed for the following three mixtures: barium-calcium, cadmium-copper, and cadmium-zinc. In a single titration, without changing the pH or other environmental conditions, mixtures that cannot be sequentially titrated visually in aqueous solution, can be successfully titra¨ted. Because the optically active titrant and the stereospecifically formed complexes have molecular rotations significantly different from each other, they serve as self-indicators for the two titration end-points. Both the precision and accuracy of the sequential spectropolarimetric titrations are excellent and the range of the average deviations was 0.0 to 0.37%.

The direct spectropolarimetric titration of aluminum. indium, and thallium with D-(—)-1,2-propylenediaminetetraacetic acid

A direct spectropolarimetric titration method of analysis was developed for aluminum(III), indium(III) and thallium(III), with the stereospecific ligand, D-(—)-1, 2-propylenediaminetetraacetic acid (D(—)PDTA). The optical rotation of the solution was monitored with a photoelectric polarimeter to determine the end-point of the titration. The metal complexes and titrant are optically active and serve as self-indicators. Since D(—)PDTA is stereospecific in its reaction with these metals, maximum utilization of the optical rotation of these complexes is achieved. The observed optical rotation is linear with respect to concentration of the metal complexes; consequently, it is possible to perform straight-line extrapolations to the end-point. The effects of some parameters were investigated and optimum conditions were established. The range of the average deviations from visual chelometry for the three metals determined was 0.11–0.18%.

Determination of Zirconium and Thorium by Spectropolarimetric Titrimetry UsingD-(-)-1,2-PRopylenediaminetetraacetic Acid

Abstract A direct spectropolarimetric titration method of analysis was developed for zirconium and thorium with the stereospecific ligand D-(-)-1,2-propylene-diaminetetraacetic acid (D(-)PDTA). The optical rotation of the solution was monitored with a photoelectric polarimeter to determine the end point of the titration. The titrant and the metal complex formed are optically active and serve as self-indicators. The observed optical rotation is linear with respect to the concentrations-of the metal complex and the titrant; consequently graphical straight line extrapolations are performed to determine the end point. Particular advantage of the spectropolarimetric titrimetric technique is illustrated by the analysis of zirconium, which was analyzed directly at ambient temperatures.