Synthesis, characterization, molecular modeling, binding energies of ß-cyclodextrin-inclusion complexes of quercetin: Modification of photo physical behavior upon ß-CD complexation.

We prepared a naturally occurring flavanoid namely quercetin from tea leaves and analyzed by Absorption, Emission, FT-IR, 1H, 13C nmr spectra and ESI-MS analysis. The inclusion behavior of quercetin in cyclodextrins like α-, ß-, γ-, per-6-ABCD and mono-6-ABCD cavities were supported such as UV-vis., Emission, FT-IR and ICD spectra and energy minimization studies. From the absorption and emission results, the type of complexes formed were found to depend on stoichiometry of Host:Guest. FT-IR data of CD complexes of quercetin supported inclusion complex formation of the substrate with α-, ß- and γ-CDs. The inclusion of host-guest complexation of quercetin with α-, ß-, γ-CDs, per-6-ABCD and mono-6-ABCDs provides very valuable information about the CD:quercetin complexes, the study also shows that ß-CD complexation improves water solubility, chemical stability and bioavailability of quercetin. Besides, phase solubility studies also supported the formation of 1:1 drug-CD soluble complexes. All these spectral results provide insight into the binding behavior of substrate into CD cavity in the order per-6-ABCD > Mono-6-ABCD > γ-CD > ß-CD > α-CD. The proposed model also finds strong support from the fact with excess CD this exciton coupling disappears indicates the formation of only 1:1 complex.

Effect of hydrophobicity and size of the ligands on the intercalative binding interactions of some metallo-surfactants containing π-conjugated systems with yeast tRNA.

The intercalative yeast t-RNA binding behavior of some metallo-surfactant complexes, Co(ip)2(TA)2](ClO4)3 (1) and [Co(dpq)2(TA)2](ClO4)3 (2) where TA = Tetradecylamine (Myristylamine), ip = imidazo[4,5-f][1,10]phenanthroline and dpq = dipyrido[3,2-d:2'-3'-f]quinoxaline containing π-conjugated systems (both below and above critical micelle concentration) have been investigated by means of absorption spectral titration, competitive binding, circular dichroism, cyclic voltammetry, and viscometry measurements. Absorption spectral titration results implicate yeast tRNA has significant effects on the binding behaviors of two surfactant complexes via intercalative mode showed a significant absorption band of hypochromicity with red shift. The intrinsic binding constant values below and above CMC were determined as Kb = 6.12 × 105 M-1, 2.31 × 106 M-1, for complex (1) and 7.23 × 105 M-1, 3.57 × 106 M-1, for complex (2). In both sets of complexes (1) and (2), the complexes bind more strongly to yeast tRNA in the above critical micelle concentration can be hydrophobic and confirm intercalation. Competitive displacement studies confirmed that complexes bind to yeast tRNA via intercalative mode. Cyclic voltammetry studies suggest the increasing amounts of yeast tRNA, the cathodic potential Epc for the two complexes shows a positive shift in peak potential indicated the process of binding via intercalation. These observations were further validated by CD, and hydrodynamic measurements. All these studies suggesting that a surfactant complex binds to yeast tRNA appear to be mainly intercalative because of hydrophobicity due to extending aromaticity of the π system of the ligand and planarity of the complex has a significant effect on tRNA binding affinity increasing in the order of complexes containing ligands ip < dpq.Communicated by Ramaswamy H. Sarma.

"Turn-on-off-on" fluorescence switching of quantum dots-cationic porphyrin nanohybrid: a sensor for DNA.

In this article, we describe a new platform for probing double stranded DNA (dsDNA) by tracing the "on-off-on" fluorescence signals of quantum dots-cationic porphyrin utilizing fluorescence and synchronous fluorescence measurements. Electrostatic interaction between the negatively charged thioglycolic acid capped CdTe quantum dots (CdTe-TGA QDs) and positively charged porphyrin surfaces leads to drastic quenching (turning off) of the donor by an effective electron transfer process. Interestingly, after the addition of calf thymus DNA (CtDNA), the porphyrins peel off from the quantum dot surface and bind to dsDNA, resulting in the restoration of fluorescence intensity of quantum dots (turning on). Consequently, this can be utilized for the selective sensing of dsDNA via optical responses. Experimental results show that the increase in fluorescence intensity is proportional to the concentration of CtDNA within the range of 6.5 × 10(-9) M to 29.6 × 10(-8) M under the optimized experimental conditions. Furthermore, the peel off mechanism was confirmed by atomic force measurement.