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Biomedical subjects

B de Castro

Publications and source records attributed to B de Castro.

15 recordsLinked to original sources
PubMed2001

Derivative spectrophotometry as a tool for the determination of drug partition coefficients in water/dimyristoyl-L-alpha-phosphatidylglycerol (DMPG) liposomes.

The partition coefficients (K(p)) between lipid bilayers of dimyristoyl-L-alpha-phosphatidylglycerol (DMPG) unilamellar liposomes and water were determined using derivative spectrophotometry for chlordiazepoxide (benzodiazepine), isoniazid and rifampicin (tuberculostatic drugs) and dibucaine (local anaesthetic). A comparison of the K(p) values in water/DMPG with those in water/DMPC (dimyristoyl-L-alpha-phosphatidylcholine) revealed that for chlordiazepoxide and isoniazid, neutral drugs at physiological pH, the partition coefficients are similar in anionic (DMPG) and zwitterionic (DMPC) liposomes. However, for ionised drugs at physiological pH, the electrostatic interactions are different with DMPG and DMPC, with the cationic dibucaine having a stronger interaction with DMPG, and the anionic rifampicin having a much larger K(p) in zwitterionic DMPC. These results show that liposomes are a better model membrane than an isotropic two-phase solvent system, such as water-octanol, to predict drug-membrane partition coefficients, as they mimic better the hydrophobic part and the outer polar charged surface of the phospholipids of natural membranes.

Chlordiazepoxide↗
PubMed2001

Partition coefficients of beta-blockers in bile salt/lecithin micelles as a tool to assess the role of mixed micelles in gastrointestinal absorption.

The objective of this study was to develop non-invasive spectroscopic methods to quantify the partition coefficients of two beta-blockers, atenolol and nadolol, in aqueous solutions of bile salt micelles and to assess the effect of lecithin on the partition coefficients of amphiphilic drugs in mixed bile salt/lecithin micelles, which were used as a simple model for the naturally occurring mixed micelles in the gastrointestinal tract. The partition coefficients (Kp) at 25.0 +/- 0.1degreesC and at 0.1 M NaCl ionic strength were determined by spectrofluorimetry and by derivative spectrophotometry, by fitting equations that relate molar extinction coefficients and relative fluorescence intensities to the partition constant Kp. Drug partition was controlled by the: (i) drug properties, with the more soluble drug in water (atenolol) exhibiting smaller values of Kp, and with both drugs interacting more extensively in the protonated form; and by (ii) the bile salt monomers, with the dihydroxylic salts producing larger values of Kp for the beta-blockers, and with glycine conjugation of the bile acid increasing the values of Kp for the beta-blockers. Addition of lecithin to bile salt micelles decreases the values of Kp of the beta-blockers. Mixed micelles incorporate hydrophobic compounds due to their large size and the fluidity of their core, but amphiphilic drugs, for which the interactions are predominantly polar/electrostatic, are poorly incorporated in mixed micelles of bile salts/lecithin.

Adrenergic beta-Antagonists↗
PubMed2001

Spectroelectrochemical characterisation of poly[Ni(saltMe)]-modified electrodes.

Electrogenerated polymers based on the nickel(II) complex 2,3-dimethyl-N,N'-bis(salicylidene)butane-2,3-diaminatonickel(II), poly[Ni(saltMe)], were characterised by in situ FTIR and UV/Vis spectroscopy and ex-situ EPR spectroscopy in order to gain insights into film structure, electronic states and charge conduction. The role of the nickel ions during film oxidation was probed by using EPR to study naturally abundant Ni and 61Ni-enriched polymers. The data from all the spectroscopic techniques are consistent, and clearly indicate that polymerisation and redox switching are associated with oxidative ligand based processes; coulometry suggests that one positive charge was delocalised through each monomer unit. EPR provided evidence for the non-direct involvement of the metal in polymer oxidation: the polymer is best described as a polyphenylene-type compound (conducting polymer), rather than an aggregation of nickel complexes (redox polymer), and the main charge carriers are identified as polarons. An explanation for the high electrochemical stability and conductivity of poly[Ni(saltMe)] with respect to that of poly[Ni(salen)] is proposed. based on stereochemical repulsion between monomeric units; this can impose a less compact supramolecular structure on polymers with bulkier substituents.

Journal Article↗
PubMed2001

A fast and reliable spectroscopic method for the determination of membrane--water partition coefficients of organic compounds.

Partition coefficients (Kp) between egg yolk phosphatidylcholine multilamellar vesicles and water were determined for two nonsteroidal anti-inflammatory drugs (indomethacin and acemetacin) using two independent methodologies: derivative spectrophotometry and variation of the experimental acidity constant in the presence of increasing vesicle concentration. Second-derivative spectrophotometry allowed for total elimination of background signal effects arising from lipid vesicles, without the need for separation techniques that may disturb equilibrium states. By using a model based on a simple partition, the values of K(T)p can be obtained directly; furthermore, by performing determinations at two different pH values it is possible to calculate partition coefficients for the neutral and negatively charged forms of the drugs (K(AH)p and K(A)p). In the other methodology, values of apparent acidity constants (K(app)) were determined by spectrophotometry at different pH values and different lipid concentrations, and an increase in K(app) with decreasing lipid concentration was observed for both drugs, and from this dependence it was possible to calculate K(AH)p and K(A-)p for each drug. These values were used as a check for those obtained by derivative spectroscopy, which has proven to be a reliable and more expeditious method to obtain K(AH)p and K(A-)p .

Animals↗
PubMed2001

Study of partition of nitrazepam in bile salt micelles and the role of lecithin.

The effect of trihydroxy (sodium cholate and sodium glycocholate) and dihydroxy (sodium deoxycholate and sodium glycodeoxycholate) bile salt micelles on the spectrophotometric properties and on the solubility of nitrazepam in aqueous solution, at 25.0 degrees C and at ionic strength 0.1 M in sodium chloride, has been assessed. From the results obtained it was possible to calculate the partition coefficients (Kp) of nitrazepam between aqueous and micellar phases. The partition coefficients of nitrazepam have also been determined in mixed micelles of cholate or deoxycholate with lecithin (egg yolk phosphatidylcholine), which were used as a model of the gastrointestinal tract. Drug partition was found to depend on the bile acid (number of hydroxyl groups and conjugation with glycine), and our data indicate further that addition of lecithin to bile salt micelles decreases the values of the partition coefficients in the mixed micelles at physiological pH.

Bile Acids and Salts↗
PubMed2001

Trans-[N,N'-Bis(salicydene)cyclohexane-1,2-diaminato]nickel(II)-chloroform (1/1).

In the title complex, trans-(2,2'-[cyclohexane-1,2-diylbis(nitrilomethylidyne)]diphenolato-kappa(4)O,N,N',O')nickel(II)-chloroform (1/1), [Ni(C(20)H(20)N(2)O(2))].CHCl(3), the Ni atom has a square-planar geometry, slightly tetrahedrally distorted. The Ni-N and Ni-O bonding distances are within the expected ranges for Ni-Schiff base derivatives. The diimine bridge has a gauche conformation with the cyclohexyl ring almost coplanar with the NiN(2)O(2) plane. The complex molecules pack in dimers with an Ni.Ni distance of 3.59 (1) A and form a three-dimensional structure displaying a herring-bone configuration. Channels are occupied by solvent molecules, which are involved in C-H.O hydrogen bonds with the ligand O atoms.

Journal Article↗
PubMed2000

Enzymes of hydrogen metabolism in Pyrococcus furiosus.

The genome of Pyrococcus furiosus contains the putative mbhABCDEFGHIJKLMN operon for a 14-subunit transmembrane complex associated with a Ni-Fe hydrogenase. Ten ORFs (mbhA-I and mbhM) encode hydrophobic, membrane-spanning subunits. Four ORFs (mbhJKL and mbhN) encode putative soluble proteins. Two of these correspond to the canonical small and large subunit of Ni-Fe hydrogenase, however, the small subunit can coordinate only a single iron-sulfur cluster, corresponding to the proximal [4Fe-4S] cubane. The structural genes for the small and the large subunits, mbhJ and mbhL, are separated in the genome by a third ORF, mbhK, encoding a protein of unknown function without Fe/S binding. The fourth ORF, mbhN, encodes a 2[4Fe-4S] protein. With P. furiosus soluble [4Fe-4S] ferredoxin as the electron donor the membranes produce H2, and this activity is retained in an extracted core complex of the mbh operon when solubilized and partially purified under mild conditions. The properties of this membrane-bound hydrogenase are unique. It is rather resistant to inhibition by carbon monoxide. It also exhibits an extremely high ratio of H2 evolution to H2 uptake activity compared with other hydrogenases. The activity is sensitive to inhibition by dicyclohexylcarbodiimide, an inhibitor of NADH dehydrogenase (complex I). EPR of the reduced core complex is characteristic for interacting iron-sulfur clusters with Em approximately -0.33 V. The genome contains a second putative operon, mbxABCDFGHH'MJKLN, for a multisubunit transmembrane complex with strong homology to the mbh operon, however, with a highly unusual putative binding motif for the Ni-Fe-cluster in the large hydrogenase subunit. Kinetic studies of membrane-bound hydrogenase, soluble hydrogenase and sulfide dehydrogenase activities allow the formulation of a comprehensive working hypothesis of H2 metabolism in P. furiosus in terms of three pools of reducing equivalents (ferredoxin, NADPH, H2) connected by devices for transduction, transfer, recovery and safety-valving of energy.

Acetates↗
PubMed1999

Acid-base properties and solubility of pindolol, diazepam and chlordiazepoxide in SDS micelles.

The effect of sodium dodecyl sulphate (SDS) on the acid-base properties and on the solubility of a beta-blocker (pindolol) and of two benzodiazepines (diazepam and chlordiazepoxide) has been assessed. The study was performed by potentiometric and spectrophotometric determinations of the acidity constants and by spectrophotometric evaluation of the solubilities of the pharmaceutical drugs in aqueous solution and in solutions to which was added SDS with concentrations below and above the critical micelle concentration (cmc), at 25 degrees C and at an ionic strength 0.1 M (NaCl). The effect of the organized assemblies on the pKa values was quantified by the application of two theoretical models that differ in the inclusion of ionic exchange between positively charged species in solution. These models have allowed the determination of the binding constants for drug/micelle and yielded values in good agreement with those obtained by the solubility method, and in addition provide a more detailed picture of the effect of drug charge on its partition. The results can be taken to evidence different interaction modes of the drugs with the SDS micelles.

Adrenergic beta-Antagonists↗
PubMed1999

On the prosthetic groups of the NiFe sulfhydrogenase from Pyrococcus furiosus: topology, structure, and temperature-dependent redox chemistry.

The sulfhydrogenase complex of Pyrococcus furiosus is an alpha beta gamma delta heterotetramer with both hydrogenase activity (borne by the alpha delta subunits) and sulfur reductase activity (carried by the beta gamma subunits). The beta-subunit contains at least two [4Fe-4S] cubanes and the gamma-subunit contains one [2Fe-2S] cluster and one FAD molecule. The delta-subunit contains three [4Fe-4S] cubanes and the alpha-subunit carries the NiFe dinuclear center. Only three Fe/S signals are observed in EPR-monitored reduction by dithionite, NADPH, or internal substrate upon heating. All other clusters presumably have reduction potentials well below that of the H+/H2 couple. Heat-induced reduction by internal substrate allows, for the first time, EPR monitoring of the NiFe center in a hyperthermophilic hydrogenase, which passes through a number of states, some of which are similar to states previously defined for mesophilic hydrogenases. The complexity of the observed transitions reflects a combination of temperature-dependent activation and temperature-dependent reduction potentials.

Amino Acid Sequence↗
PubMed1998

Copper(II) increases bile acid binding to asparagine.

Interactions of two bile acids (cholic and glycocholic acids) with asparagine have been studied by potentiometry in aqueous solutions under conditions similar to those observed in biological fluids (37 degrees C and I = 0.15 M NaCl), and in the absence and presence of copper(II). To characterize the equilibria for the systems copper(II)/bile acid/asparagine, specifically to assess cooperative binding between bile acids and asparagine, the acidity constant of asparagine and formation constants for copper(II)/bile acid and copper(II)/asparagine were also obtained under the same conditions. The results obtained suggest cooperativity in the binding of bile acid to asparagine in the presence of copper(II).

Asparagine↗
PubMed1998

Fluorimetric and solubility studies of nadolol and atenolol in SDS micelles.

The effect of sodium dodecyl sulfate (SDS) micelles on the spectrofluorimetric intensities and on the solubility of two beta-blockers (atenolol and nadolol) were studied at 25.0 +/- 0.1 degrees C and I = 0.1 M NaCl. From the dependence of these physical properties on SDS concentration it was possible to calculate the binding constants drug-micelle, and it was found that both techniques yield similar results for the binding constants, and that are in agreement with those calculated from the effect of micelles on the apparent acidity constants of the beta-blockers.

Adrenergic beta-Antagonists↗
PubMed1998

Acid/base properties of beta-blockers and benzodiazepines in sodium dodecyl sulfate micelles. A spectrophotometric and potentiometric study.

The effect of sodium dodecyl sulfate (SDS) micelles on the acid-base properties of two family of pharmaceutical drugs (beta-blockers and benzodiazepines) at 25 degrees C and I = 0.1 M NaCl have been studied. The characterization of the several solution equilibria for the system drug/SDS micelle solution was performed by potentiometry and spectrophotometry, below and above the critical micelle concentration (cmc). Two widely used models have been applied to quantify the effect of micelles on the pKa of the drugs, and the results obtained point to different interactions of each family of drugs with the micelles.

Adrenergic beta-Antagonists↗
PubMed1995

Potentiometric determination of formation constants of copper(II)/bile acid/peptide in aqueous solution.

A potentiometric method was used to study the equilibria in aqueous solution for the systems copper(II)/bile acid salt/peptide [bile acid salt-sodium cholate or glycocholate; and peptide-glutamic acid (glutamate), glutamine or glutamylglutamic acid], specifically to assess cooperative binding between small peptides and bile acids in the presence of copper(II). The results obtained suggest large cooperativity in formation of ternary complexes with amino acids when they are coordinated to copper(II) as bidentate ligands.

Amino Acid Sequence↗
PubMed1981

Platinum interactions with nucleic acids: insights from model compounds.

A systematic review of the molecular structures of a large number of platinum(II) complexes with purine and pyrimidine bases and their nucleosides and nucleotides is presented. From these studies it is evident that the endocyclic nitrogen donor atoms of the heterocyclic purine and pyrimidine bases are dominant metal binding sites for Pt(II). Secondary interactions involving the exocyclic functional groups of the bases and the phosphate moiety of nucleotides are also noted. For Pt(II) complexes containing two adjacent (cis) bases, it is further indicated that intracomplex and intercomplex base-base interactions are of particular importance, giving rise in some cases to discontinuous helical arrays of Pt(II) complexes. These helical arrays may provide some insights into platinized-DNA polymers. Attention is also given to polynuclear species which are models for the family of mixed-valent platinum-pyrimidine blues. Finally, an attempt is made to extend the results of small molecule studies to give insights into the general area of Pt(II)-DNA chemistry and, in particular, the mode of action of the active anti-neoplastic agent cisPt.

Binding Sites↗