Circular dichroism of ketoprofen complexed to serum albumins: conformational selection by the protein: a novel optical purity determination technique

Spectroscopic studies for the inclusion complexation of ketoprofen enantiomers with β-cyclodextrin

Inclusion complexes of R-ketoprofen and S-ketoprofen enantiomers with β-cyclodextrin (β-CD) in aqueous solution were studied using various spectroscopic techniques such as Raman, FTIR, UV and fluorescence. The different relative intensities and characteristic band shifts of the two enantiomers from Raman spectra suggests different interaction when complexed with β-CD. Raman experiments revealed a noticeable diminishing of the CC vibration and ring deformation, which indicate the embedding of ketoprofen inside the β-CD cavity. It's revealed that distinct differences between R- and S-ketoprofen in the presence of β-CD at neutral pH. The stoichiometry ratio and binding constant of the inclusion complexes were calculated using Benesi-Hildebrand plot. Both enantiomers showed stoichiometry ratio of 1:1 inclusion complex with β-CD. The binding constant of R-ketoprofen (4088 M^-1) is higher than S-ketoprofen (2547 M^-1). These values indicated that β-CD formed inclusion complexes more preferentially with R-ketoprofen than S-ketoprofen. Results demonstrated that β-CD can be used as a promising chiral selector for ketoprofen enantiomers.

Induced circular dichroism as a tool to investigate the binding of drugs to carrier proteins: Classic approaches and new trends

Induced circular dichroism (ICD) is a spectroscopic phenomenon that provides versatile and useful methods for characterizing the structural and dynamic properties of the binding of drugs to target proteins. The understanding of biorecognition processes at the molecular level is essential to discover and validate new pharmacological targets, and to design and develop new potent and selective drugs. The present article reviews the main applications of ICD to drug binding studies on serum carrier proteins, going from the classic approaches for the derivation of drug binding parameters and the identification of binding sites, to an overview of the emerging trends for the characterization of binding modes by means of quantum chemical (QC) techniques. The advantages and limits of the ICD methods for the determination of binding parameters are critically reviewed; the capability to investigate the binding interactions of drugs and metabolites to their target proteins is also underlined, as well as the possibility of characterizing the binding sites to obtain a complete picture of the binding mechanism and dynamics. The new applications of ICD methods to identify stereoselective binding modes of drug/protein complexes are then reviewed with relevant examples. The combined application of experimental ICD spectroscopy and QC calculations is shown to identify qualitatively the bound conformations of ligands to target proteins even in the absence of a detailed structure of the binding sites, either obtained from experimental X-ray crystallography and NMR measurements or from computational models of the complex.

Characterization of the species-dependent ketoprofen/albumin binding modes by induced CD spectroscopy and TD-DFT calculations

The stereospecificity of high-affinity biorecognition phenomena at the basis of the activity of drugs is an important topic of active research in medicinal chemistry. The binding of drugs to their targets or to carrier proteins may lead to the onset of an induced circular dichroism (ICD) signal, which can be detected experimentally. Quantum mechanical (QM) calculations based on density functional theory (DFT) and its time-dependent formulation (TD-DFT) can be used to determine the theoretical chiroptical response of all the possible conformations of drugs bound to their hosts; by comparison with the experimental ICD spectra of drug-host complexes, this approach can lead to the identification of possible binding modes in the absence of X-ray crystallography or NMR data. The present article reports the application of experimental electronic circular dichroism (ECD) spectroscopy, DFT conformational analysis and TD-DFT calculations to the investigation of the binding modes of (S)-ketoprofen to serum albumins. The peculiar species-dependent ICD spectra observed for the binding of (S)-ketoprofen to different serum albumins can be explained by the selection of different mutual arrangements of the phenyl moieties inside the binding pocket. Such structural elucidations contribute to a better understanding of the changes in the pharmacokinetic and pharmacodynamic profiles of drugs among different species.

Binding and photochemistry of enantiomeric 2-(3-benzoylphenyl)propionic acid (ketoprofen) in the human serum albumin environment

Global analysis of circular dichroism multiwavelength data and time resolved fluorescence was applied to investigate the interaction of R(-)- and S(+)-ketoprofen (KP) with human serum albumin (HSA) in buffer solution at neutral pH. The most stable drug:protein adducts of 1 : 1 and 2 : 1 stoichiometry were characterized as regards the stability constants and the absolute circular dichroism spectra. The spectra of the diastereomeric 1 : 1 conjugates are negative with minima at ca. 350 nm for R(-)-KP and 330 nm for S(+)-KP, those of the 2 : 1 complexes are both negative with minimum at 340 nm and quite similar in shape to each other, thereby showing that the protein loses chiral recognition capability upon multiple binding. HSA intrinsic time resolved fluorescence data obtained exciting at 295 nm point to Trp 214 being located in the secondary binding site for both KP enantiomers. The photodegradation of the S(+)- and R(-)-KP:HSA complexes was studied by steady state photolysis using lambda(irr) > 320 nm. No decrease of the photodegradation quantum yields was observed in 1 : 1 complexes. An induction time for the photodegradation course in 2 : 1 complexes was observed. Transient absorption spectroscopy at lambda(exc) = 355 nm showed that triplet KP species were formed with stereo-differentiated lifetimes and high quantum yields (0.7-0.9). Secondary transients were consistent with the occurrence of photodecarboxylation and/or photoreduction within the protein matrix.

Multiple ligand-binding properties of the lipocalin member chicken α1-acid glycoprotein studied by circular dichroism and electronic absorption spectroscopy: The essential role of the conserved tryptophan residue

Multiple ligand-binding properties of the 30-kDa chicken alpha(1)-acid glycoprotein (cAGP), a member of the lipocalin protein family, were investigated for the first time by using circular dichroism (CD) and UV/Vis absorption spectroscopy methods. By measuring induced CD (ICD) spectra, high-affinity binding (K(a) approximately 10(5)-10(6) M(-1)) of several drugs, dyes and natural compounds to cAGP was demonstrated including antimalarial agents (quinacrine, primaquine), phenotiazines (chlorpromazine, methylene blue), propranolol, non-steroidal antiinflammatory drugs (ketoprofen, diclofenac), tamoxifen, diazepam, tacrine, dicoumarol, cationic dyes (auramine O, thioflavine T, ethidium bromide), benzo[a]pyrene, L-thyroxine, bile pigments (bilirubin, biliverdin), alkaloids (piperine, aristolochic acid), saturated and unsaturated fatty acids. Analysis of the extrinsic CD spectra with the study of the covalently modified protein and CD displacement experiments revealed that a single Trp26 residue of cAGP conserved in the whole lipocalin family is part of the binding site, and it is essentially involved in the ligand-binding process via pi-pi stacking interaction resulting in the appearance of strong induced CD bands due to the non-degenerate intermolecular exciton coupling between the pi-pi* transitions of the stacked indole ring-ligand chromophore. The finding that cAGP is able to accommodate a broad spectrum of ligands belonging to different chemical classes suggests that its core beta-barrel cavity is unusually wide containing overlapping sub-sites. Significance of these new data in understanding of the ligand-binding properties of other lipocalins, especially that of human AGP, and potential practical applications are briefly discussed. Overall, cAGP serves as a simple, ultimate model to extend our knowledge on ligand-binding properties of lipocalins and to study the role of tryptophan residues in molecular recognition processes.

Diastereoselectivity and Site Dependency in the Photochemistry of Ketoprofen in the Bovine Serum Albumin Matrix†

The photodegradation of the S(+)- and R(-)-ketoprofen (KP) enantiomers in the bovine serum albumin matrix was studied by steady-state photolysis with the use of lambda(irr) > 320 nm and transient absorption spectroscopy with lambda(exc) = 355 nm, at 1/1 and 2/1 KP/BSA molar ratios. R(-)-KP was found to be more labile than S(+). Triplet ketoprofen species were evidenced with lifetimes of 400 ns for S(+) and 600 ns for R(-)-KP. Further longer-lived transients with lifetimes of 2.6 and 6.0 mus for S(+) and R(-), respectively, were detected. On the basis of the binding constants of the drug enantiomers to the two main binding sites of the protein, obtained from circular dichroism experiments, the individual disappearance quantum yields of the 1:1 and 2:1 diastereomeric KP:BSA complexes could be estimated. The photoreactivity in the BSA matrix was rationalized on the basis of diastereoselective photodecarboxylation in the two main protein sites.

Complexation with albumins of chiral aromatic substrates and their chemistry in ground and excited states. Catalytic and chirality recognition properties of the protein in the cases of binaphthol, its photoisomers, and ketoprofen

The reactivity of organic molecules can be modified upon complexation with proteins: these changes can be different and more significant when the substrate is in an electronically excited state. Here we review UV, CD, and fluorescence spectroscopy studies on the photochemistry and on the chemistry of atropisomeric binaphthols and of ketoprofen, complexed to serum albumins. The chemical and photochemical properties of the organic substrates, complexed to the albumins or free in common solvents, are different. The role of the protein complexation is also evidenced in photoresolution processes of racemate-protein complexes. Catalytic effects due to serum albumins are also reported. In particular, the Arrhenius parameters for the rate of thermal isomerization of a metastable photoproduct of binaphthol in common solvents are compared with those of the bovine serum albumin catalyzed isomerization.

Enantioselective inhibition of the binding of rac-profens to human serum albumin induced by lithocholate

The reversible binding of lithocholate to human serum albumin determines a decrease of the binding of rac-ketoprofen. The process was followed by displacement chromatography using increasing concentrations of the competitor, i.e., lithocholate, in the mobile phase. The inhibition of rac-ketoprofen binding resulting was enantioselective and greater displacement was observed for the (S) enantiomer. The displacement process resulting was competitive in nature, the two enantiomers of ketoprofen binding to the same binding site as the modifier. The investigation was extended to other nonsteroidal antiinflammatory drugs. The enantioselective binding inhibition was larger in the case of rac-naproxen and rac-suprofen with respect to the phenomenon observed in the case of rac-ketoprofen. The difference in circular dichroism spectroscopy was also used to characterize the binding of lithocholate to human serum albumin. This bile acid was proven to bind to site II on human serum albumin. The results, as obtained by displacement chromatography and difference circular dichroism spectroscopy, strongly support the hypothesized role of bile acids in inducing the enantioselective inhibition of ketoprofen binding to human serum albumin in patients suffering from liver diseases.

Use of non-covalent labeling in illustrating ligand binding to human serum albumin via affinity capillary electrophoresis with near-infrared laser induced fluorescence detection

This paper demonstrates the use of a near-infrared (NIR) dye as a non-covalent label for human serum albumin (HSA). The dye is a water soluble, heptamethine cyanine dye. The utility of the dye as a tracer illustrating the binding of various drugs to HSA is demonstrated via affinity capillary electrophoresis with near-infrared laser-induced fluorescence detection (ACE-NIR-LIF). Additionally, the factors affecting the separation of relevant species were investigated. The change in quantum yield of the dye upon complexation with HSA was calculated. Spectrophotometric measurements were conducted to study the stoichiometry of the dye albumin complex.

Role of arg-410 and tyr-411 in human serum albumin for ligand binding and esterase-like activity

Recombinant wild-type human serum albumin (rHSA), the single-residue mutants R410A, Y411A, Y411S and Y411F and the double mutant R410A/Y411A were produced using a yeast expression system. The recombinant proteins were correctly folded, as they had the same stability towards guanidine hydrochloride and the same CD spectrum as HSA isolated from serum (native HSA). Thus the global structures of the recombinant proteins are probably very similar to that of native HSA. We investigated, by ultrafiltration and CD, the high-affinity binding of two representative site II ligands, namely ketoprofen and diazepam. According to the crystal structure of HSA, the residues Arg-410 and Tyr-411 protrude into the centre of site II (in subdomain 3A), and the binding results showed that the guanidino moiety of Arg-410, the phenolic oxygen and the aromatic ring of Tyr-411 are important for ketoprofen binding. The guanidino moiety probably interacts electrostatically with the carboxy group of ketoprofen, the phenolic oxygen could make a hydrogen-bond with the keto group of the ligand, and the aromatic ring may participate in a specific stacking interaction with one of or both of the aromatic rings of ketoprofen. By contrast, Arg-410 is not important for diazepam binding. The two parts of Tyr-411 interact favourably with diazepam, and probably do so in the same way as with ketoprofen. In addition to its unique ligand binding properties, HSA also possesses an esterase-like activity, and studies with p-nitrophenyl acetate as a substrate showed that, although Arg-410 is important, the enzymic activity of HSA is much more dependent on the presence of Tyr-411. A minor activity could be registered when serine, but not alanine or phenylalanine, was present at position 411.

Chiral discrimination in the transport of ketoprofen and ibuprofen esters through an aqueous phase mediated by various serum albumins

Serum albumins that act as carriers discriminated between enantiomers of alkyl esters of ketoprofen and ibuprofen in transport in the O/W/O (oil/water/oil) system using a U-shaped cell. The transport rate and the preferred enantiomer of the esters were substantially affected by pH, temperature, and species of albumin. Among five serum albumins studied, bovine serum albumin (BSA) showed the largest rate constant and rat serum albumin (RSA) manifested the highest enantioselectivity. Regarding enantiomer selectivity in transport overall, it is anticipated that the ester uptake step plays an important role for BSA, whereas the ester release is the key step for RSA.

Helix 6 of subdomain III A of human serum albumin is the region primarily photolabeled by ketoprofen, an arylpropionic acid NSAID containing a benzophenone moiety

It is well known that the subdomain III A (site II) of human serum albumin (HSA) binds a variety of endogenous and exogenous substances. However, the nature of the microenvironment of the binding site remains unclear. Ketoprofen (KP), an arylpropionic acid NSAID which contains a benzophenone moiety, was used as a photoaffinity labeling agent to label the binding region. Subsequent CNBr cleavage of the photolabeled HSA revealed that the 11.6 kDa and 9.4 kDa fragments contained most of the incorporated radioactivity. Competition experiments showed that the 11.6 kDa fragment contains the common binding region for site II ligands. This fragment was redigested with Achromobacter lyticus protease I (AP-I) and the amino acid sequence of the photolabeled peptide was determined to be XCTESLVNRR, which corresponds to the sequence 476C-485K of HSA. The complete amino acid sequence of the corresponding AP-I digested HSA peptide encompasses residues 476 to 499, which form helices 5 and 6 of subdomain III A. The HSA-Myr X-ray crystallography data showed that helix 5 is involved to the least extent in ligand binding. A docking model provided further support that helix 6 represents the photolabeled region of KP.

Stereoselective binding of ketoprofen enantiomers to human serum albumin studied by high-performance liquid affinity chromatography

A chiral stationary phase based on immobilized human serum albumin (HSA) was used to study the stereoselective binding of ketoprofen enantiomers by means of high-performance liquid affinity chromatography. The technique of zonal elution was applied together with a novel mathematical approach describing attachment to more than one type of binding site. Phenylbutazon (PBZ) and diazepam (DAZ) were used as markers for the major believed binding regions on HSA. Both R- and S-ketoprofen (KTR and KTS) display high affinity to the primary PBZ- and DAZ-binding sites and low-affinity to the secondary DAZ sites. The binding to high-affinity regions is accepted to be a stepwise process initiated by the binding to the primary DAZ sites and followed by the attachment to the primary PBZ sites. The chiral recognition is attributed to the high-affinity PBZ-binding sites and to the low-affinity DAZ-binding sites.