Isothermal Titration Calorimetric Technique Research Articles

Host-guest interaction of 3-hydroxyflavone and 7-hydroxyflavone with cucurbit [7]uril: A spectroscopic and calorimetric approach

The modulation of photophysical behaviour of small organic molecules in the presence of macrocycles is one of the most interesting areas of research. In this work we reported the interaction of two biologically active molecules 3-hydroxyflavone and 7-hydroxyflavone with macrocyclic host cucurbit [7]uril in aqueous medium. To investigate the change of photophysical properties of these two flavones, we have used steady state absorption, fluorescence, time resolved fluorescence emission spectroscopy and isothermal titration calorimetric technique. It is observed that on complexation with cucurbit [7]uril, the excited state proton transfer processes in both flavones have been facilitated. Isothermal titration calorimetric method was used in order to investigate the involvement of thermodynamic parameters in complexation between flavone with cucurbit [7]uril. The changes in thermodynamic properties due to the complexation of the flavones molecules with cucurbit [7]urils help to understand about the governing parameters involved in this complexation. The inclusion of flavone molecules inside the cavity of cucurbit [7]uril molecules was studied theoretically to decipher the molecular orientation of flavones in the presence of cucurbit [7]uril. The structure of HOMO and LUMO of the complexes between cucurbit [7]uril with flavones was reported. This study will be helpful to get the knowledge about the modulation of photophysical properties of the flavones molecules on addition of macrocyclic host cucurbit [7]uril. This study will be helpful for the use of cucurbit [7]uril as a potential drug delivery system.

Organic–inorganic hybrid: a novel template for synthesis of nanostructured Ag

We herein demonstrate that the amino acid arginine (Arg) can interact with phosphotungstate (Keggin) anions generating an excellent template for the synthesis of nanostructured silver wherein the Arg–PTA hybrid host plays the role of a UV-switchable reducing agent. The binding strength of the complex between arginine and phosphotungstate (PW12O40)3− i.e. the Keggin ions in aqueous medium has been assessed by the isothermal titration calorimetric technique. It is shown that the Arg–PTA complex may be used as a new class of organic–inorganic scaffolds in the synthesis of nanostructured silver. The crystalline Arg–PTA host can be photochemically reduced, resulting in electron transfer to the entrapped silver ions to form nanoparticle assemblies on the underlying colloidal template.

Microcalorimetric and SAXS Determination of PEO−SDS Interactions: The Effect of Cosolutes Formed by Ions

The effect of different ionic cosolutes (NaCl, Na(2)SO(4), Li(2)SO(4), NaSCN, Na(2)[Fe(CN)(5)NO], and Na(3)[Co(NO)(6)]) on the interaction between sodium dodecyl sulfate (SDS) and poly(ethylene oxide) (PEO) was examined by small-angle X-ray scattering (SAXS) and isothermal titration calorimetric techniques. The critical aggregation concentration values (cac), the saturation concentration (C(2)), the integral enthalpy change for aggregate formation (ΔH(agg)(int)) and the standard free energy change of micelle adsorption on the macromolecule chain (ΔΔG(agg)) were derived from the calorimetric titration curves. In the presence of 1.00 mmol L(-1) cosolute, no changes in the parameters were observed when compared with those obtained for SDS-PEO interactions in pure water. For NaCl, Na(2)SO(4), Li(2)SO(4), and NaSCN at 10.0 and 100 mmol L(-1), the cosolute presence lowered cac, increased C(2), and the PEO-SDS aggregate became more stable. In the presence of Na(2)[Fe(CN)(5)NO], the calorimetric titration curves changed drastically, showing a possible reduction in the PEO-SDS degree of interaction, possibility disrupting the formed nanostructure; however, the SAXS data confirmed, independent of the small energy observed, the presence of aggregates adsorbed on the polymer chain.

Binding and release studies of a cationic drug from a star-shaped four-arm poly(ethylene oxide)-b-poly(methacrylic acid)

Star-shape polymers possess higher densities of terminal functional groups and three-dimensional tetrahedron structure that induce significantly different association and interactions with drug compared to linear structure of identical molecular weights. Four-arm poly(ethylene oxide)-b-poly(methacrylic acid) block copolymer was synthesized by atom transfer radical polymerization technique, and it self-assembled into core-shell micelles and extended unimers at low and high pH respectively. The negatively charged carboxylate groups on the polymer chains interacted with a cationic drug through electrostatic interaction forming polymer/drug complexes stabilized by biocompatible hydrophilic PEO segments. The hydrodynamic radius (Rh) of the polymeric aggregates and polymer/drug complexes ranged from 46 to 84 nm and 32 to 55 nm at pH of 4.6 and 8.0 respectively, making them suitable for drug delivery applications. The thermodynamic parameters and interactions between polymer and drug were determined by isothermal titration calorimetric technique. The electrostatic force, hydrogen bonding and hydrophobic interactions controlled the characteristics of polymer/drug formation and complexes when the molar ratios of drug and polymer were varied. Drug selective electrode system was used to measure the dynamic release of imipramine hydrochloride (IPH) from multi-arm PEO-b-PMAA star polymer. The release exponent n was greater than 0.5 indicating a non-Fickian type diffusion behavior, where the release behavior was dominated by chain relaxation induced by ion exchange that was dependent on pH. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:782–793, 2010

Association Behavior of Star-Shaped pH-Responsive Block Copolymer: Four-Arm Poly(ethylene oxide)-b-Poly(methacrylic acid) in Aqueous Medium

A four arm pH-responsive poly(ethylene oxide)-b-poly(methacrylic acid) block copolymer was synthesized by atom transfer radical polymerization technique. The conformation transition over the course of neutralization was investigated using a combination of potentiometric and conductometric titrations, dynamic and static light scattering, and transmission electron microscopy. The multiarm block copolymer existed as an extended unimer at high pH due to the negatively charged carboxylate groups and hydrophilic poly(ethylene oxide) segments. The block copolymers self-assembled into core-shell micelles and large spherical aggregates that flocculated at low degree of neutralization (alpha). Such behavior is controlled by the fine balance of electrostatic, hydrophobic, and hydrogen bond interactions. The hydrodynamic radius (R(h)) of the aggregates was approximately 84 nm at alpha of 0.3, and it decreased to 63 and 46 nm at alpha approximately 0.2 and 0.1, respectively, as a result of the reduced electrostatic interaction between ionized carboxylate groups. The thermodynamic parameters obtained from isothermal titration calorimetric technique in different salt concentrations indicated that the energy to extract a proton from a charged polyion was reduced by the addition of salt, which favors the neutralization process.

Energetics of the binding of phototoxic and cytotoxic plant alkaloid sanguinarine to DNA: Isothermal titration calorimetric studies

Sanguinarine is a medically important plant alkaloid with remarkable pharmacological and biological activities. DNA binding is considered to be an important aspect in its mode of action. Isothermal titration calorimetric technique was used for the first time to derive the thermodynamic aspects of the interaction of the phototoxic and cytotoxic plant alkaloid sanguinarine with natural calf thymus DNA and four synthetic DNAs of differing base pair sequences under various environmental conditions The affinity of binding of sanguinarine (iminium form) was found to be in the order of 10 5 M −1 to calf thymus DNA. The binding affinity to calf thymus DNA decreased with increase of ionic strength and temperature. The salt dependence of the binding data showed that release of 0.55 U of the cations per bound alkaloid. The binding of the iminium form was exothermic under all conditions, but the uncharged alkanolamine form showed no binding to DNA. The heat capacity changes obtained from the temperature dependence of enthalpy indicated a value of −140 cal/mol K. Sanguinarine showed high specificity to alternating purine–pyrimidine sequences with affinity of the order 10 6 M −1 and the affinity to the polynucleotides varied in the order poly(dG-dC)·poly(dG-dC) > poly(dA-dT)·poly(dA-dT) > poly(dA)·poly(dT) > poly(dG)·poly(dC). The binding to alternating GC polymer was exothermic and enthalpy driven, to the homo GC polymer was exothermic and favoured by both negative enthalpy and positive entropy changes, to the alternating AT polymer was exothermic and enthalpy driven while to the homo AT polymer was endothermic and entropy driven.

Laser light scattering and isothermal titration calorimetric studies of poly(ethylene oxide) aqueous solution in presence of sodium dodecyl sulfate

The aqueous solution of poly(ethylene oxide) (PEO) in the presence of different concentrations of sodium dodecyl sulfate (SDS) was examined by laser light scattering and isothermal titration calorimetric techniques. A small fraction of PEO aggregates were found to coexist with unimeric PEO chains in dilute solution. The presence of monovalent salt does not alter the hydrodynamic properties of PEO in aqueous solution. Addition of a monovalent anionic surfactant, such as SDS, induces cooperative binding of surfactant monomers to PEO backbones at SDS concentrations ranging from 4.0 mM (critical aggregation concentration) to 16.5 mM (saturation concentration). The hydrodynamic radius of PEO unimers decreases initially and then increases with SDS concentration, resulting from the structural reorganization of the PEO/SDS complex. Beyond the saturation concentration, the hydrodynamic radii of PEO/SDS complex are independent of SDS concentration.