Mihaela Hillebrand

Interaction of kaempferol with human serum albumin: a fluorescence and circular dichroism study.

The interaction of kaempferol (kaemp), a natural flavonoid to which antioxidative, anti-inflammatory and cardio-protective biological activities have been attributed, with human serum albumin (HSA), the main in vivo transporter of exogenous substances, was investigated by steady-state, synchronous fluorescence and circular dichroism spectroscopies. The binding constant, K, and number of binding sites, n, were computed using literature models that showed satisfactory agreement and revealed a strong interaction (K approximately 3.5x10(5)M(-1), n approximately 1). The binding process was investigated at temperatures in the range 298-313K, allowing for the evaluation of the thermodynamic parameters, which indicate the occurrence of hydrogen bonding interactions. The distance between kaemp and the tryptophan residue of HSA was estimated at 2.7nm using Försters theory of nonradiative resonance energy transfer. Using circular dichroism we evidenced some degree of HSA defolding upon binding.

Bovine and Human Serum Albumin Interactions with 3-Carboxyphenoxathiin Studied by Fluorescence and Circular Dichroism Spectroscopy

The interactions of 3-carboxyphenoxathiin with Bovine Serum Albumin (BSA) and Human Serum Albumin (HSA) have been studied by fluorescence and circular dichroism spectroscopy. The binding of 3-carboxyphenoxathiin quenches the BSA and HSA fluorescence, revealing a 1:1 interaction with a binding constant of about 105 M-1. In addition, according to the synchronous fluorescence spectra of BSA and HSA in presence of 3-carboxyphenoxathiin, the tryptophan residues of the proteins are most perturbed by the binding process. Finally, the distance between the acceptor, 3-carboxyphenoxathiin, and the donor, BSA or HSA, was estimated on the basis of the Förster resonance energy transfer (FRET). The fluorescence results are correlated with those obtained from the circular dichroism spectra, which reveal the change of the albumin conformation during the interaction process.

Absorption and steady state fluorescence study of interaction between eosin and bovine serum albumin.

The interaction between eosin and bovine serum albumin in buffer solution, at pH 7.4 has been studied by means of absorption and emission spectroscopy. Applying the Scatchard model to the absorbance data a non-linear plot was obtained, reflecting a complex process. In the fluorescence spectra, two distinct effects were observed. Upon increasing the protein-dye ratio to about 0.60, the intensity of eosin emission band (544 nm) decreases to approximately 30% of its initial value. During this quenching, a small red shift is noticed. The data were rationalized in terms of two classes of binding sites. At higher protein concentrations, a new band localized at 556 nm appears, which could be assigned to a new fluorescent species. This second process corresponds to a 1:1 binding.

Spectroscopic investigations of the binding interaction of a new indanedione derivative with human and bovine serum albumins.

Binding of a newly synthesized indanedione derivative, 2-(2-hydroxy-3-ethoxybenzylidene)-1,3-indanedione (HEBID), to human and bovine serum albumins (HSA and BSA), under simulated physiological conditions was monitored by fluorescence spectroscopy. The binding parameters (binding constants and number of binding sites) and quenching constants were determined according to literature models. The quenching mechanism was assigned to a Förster non-radiative energy transfer due to the HEBID-SA complex formation. A slightly increased affinity of HEBID for HSA was found, while the number of binding sites is approximately one for both albumins. The molecular distance between donor (albumin) and acceptor (HEBID) and the energy transfer efficiency were estimated, in the view of Förster’s theory. The effect of HEBID on the protein conformation was investigated using circular dichroism and synchronous fluorescence spectroscopies. The results revealed partial unfolding in the albumins upon interaction, as well as changes in the local polarity around the tryptophan residues.

Study of the interaction between ofloxacin and human serum albumin by spectroscopic methods

The binding of ofloxacin (OFLX) to human serum albumin (HSA) was investigated by fluorescence and circular dichroism (CD) techniques. The binding parameters have been evaluated by a fluorescence quenching method. Competitive binding measurements were performed in the presence of warfarin and ibuprofen and suggest binding to the warfarin site I of HSA. The distance r between donor (HSA) and acceptor (OFLX) was estimated according to the Forsters theory of non-radiatiative energy transfer. CD spectra revealed that the binding of OFLX to HSA induced conformational changes in HSA. Molecular docking was performed and shows that for the lowest energy complex OFLX is located in site I of HSA, which correlate to the competitive binding experiments.

Experimental and theoretical study of the inclusion complexes of 3-carboxycoumarin acid with β- and 2-hydroxypropyl-β-cyclodextrins

The association process of a host-guest system, cyclodextrins (CyD) - 3-carboxycoumarin acid (3CCA) was followed by means of UV-vis, circular dichroism and steady-state fluorescence spectroscopies in buffer solution at pH=1. The experimental data were analyzed in order to get information on the stoichiometry, the equilibrium constants and the geometry of the inclusion complexes. In the circular dichroism spectra, a positive induced signal was obtained reflecting that the guest penetrates the cavity in such a way that the transition moment of the electronic band is quasi parallel to the host main axis. The experimental data are supported by the DFT and TDDFT (B3LYP/6-31G) calculations performed on the isolated ligand and by TDDFT (ZINDO) calculations carried out on the supramolecular ligand-cyclodextrin system.

Theoretical study of the excited states of 3-phenyl- and 3-thiophenyl-coumarins

Abstract The ground and excited states of phenyl( I )- and thiophenyl( II )–coumarin were characterized by solvent dependent semiempirical calculations using the AMSOL program. Potential energy surfaces (PES) were built in terms of the torsion about the coumarin-substituent bond in order to get an insight on the possibility of twisted intramolecular charge transfer (TICT) excited states formation. For I the results predict a quasi-planar conformation of S 1 , thus the first transition is presumably the Franck–Condon one. For II the most stable conformation was the twisted one and an electronic charge transfer from thiophenyl, the donor (D), to coumarin, the acceptor (A), was predicted, supporting the hypothesis of a TICT state. The absorption and fluorescence FC transitions were calculated considering the relative order of states at the optimised geometry of S 0 and S 1 , respectively. The absorption maxima are predicted in a satisfactory way, while the fluorescence maximum is well predicted for I , but not for II .

Kaempferol-human serum albumin interaction: characterization of the induced chirality upon binding by experimental circular dichroism and TDDFT calculations.

The experimental induced circular dichroism (ICD) and absorption spectra of the achiral flavonoid kaempferol upon binding to human serum albumin (HSA) were correlated to electronic CD and UV-vis spectra theoretically predicted by time-dependent density functional theory (TDDFT). The neutral and four anionic species of kaempferol in various conformations were considered in the calculations. The appearance of the experimental ICD signal was rationalized in terms of kaempferol binding to HSA in a distorted, chiral, rigid conformation. The comparison between the experimental and simulated spectra allowed for the identification of the kaempferol species that binds to HSA, namely the anion generated by deprotonation of the hydroxyl group in position 7. This approach constitutes a convenient method for evidencing the binding species and for determining its conformation in the binding pocket of the protein. Its main advantage over the UV-vis absorption method lays in the fact that only the bound ligand species gives an ICD signal.

Excited states properties of some phenoxathiin derivatives

Abstract Several properties of the excited singlet state (natural lifetimes, fluorescence quantum yields, solvatochromicity, etc.) of some 3-substituted phenoxathiin derivatives (methyl, formyl, acetyl) were investigated by means of absorption and steady-state fluorescence spectroscopy. The results point out that the emission properties are dependent on the type of the substituent, the most efficient being 3-acetylphenoxathiin. The effect of the solvent polarity on the fluorescence maxima of 3-formyl- and 3-acetylphenoxathiin, rationalized in terms of the Lippert–Mataga equation, points at an increase in the dipole moment in the excited state. A specific behavior in protic solvents was also noticed. AMI calculations for the ground and excited states support the experimental data and explain the higher efficiency of the carbonyl containing derivatives in terms of a different nature of the first excited singlet. It was established that the enhanced radiative decay for these compounds is due to the presence of new low-lying vacant π -molecular orbital situated between the frontier orbitals of phenoxathiin. The theoretical results reflect also the charge transfer character of the excited states in agreement with the solvatochromicity already discussed.

Hydroacridines: part 29. 15N NMR chemical shifts of 9-substituted 1,2,3,4,5,6,7,8-octahydroacridines and their N-oxides-Taft, Swain-Lupton, and other types of linear correlations.

The 15N NMR chemical shifts of 1,2,3,4,5,6,7,8‐octahydroacridine, 12 of its 9‐substituted derivatives, and of the corresponding N‐oxides were measured and examined in terms of the 9‐substituent effects and the effects of N‐oxidation. For the 9‐substituent effects, good linear correlations were found with the Taft and Swain‐Lupton substituent constants, for both octahydroacridines and their N‐oxides. The 15N chemical shifts of both octahydroacridines and their N‐oxides also correlate well, linearly with the 13C chemical shifts of the para‐carbons in analogously substituted benzene derivatives.