Study of alkaloid berberine and its interaction with the human telomeric i-motif DNA structure.

The alkaloid berberine presents many biological activities related to its potential to bind DNA structures, such as duplex or G-quadruplex. Recently, it has been proposed that berberine may interact with i-motif structures formed from the folding of cytosine-rich sequences. In the present work, the interaction of this alkaloid with the i-motif formed by the human telomere cytosine-rich sequence, as well as with several positive and negative controls, has been studied. Molecular fluorescence and circular dichroism spectroscopies, as well as nuclear magnetic resonance spectrometry and competitive dialysis, have been used with this purpose. The results shown here reveal that the interaction of berberine with this i-motif is weak, mostly electrostatics in nature and takes place with bases not involved in C·C+ base pairs. Moreover, this ligand is not selective for i-motif structures, as binds equally to both, folded structure, and unfolded strand, without producing any stabilization of the i-motif. As a conclusion, the development of analytical methods based on the interaction of fluorescent ligands, such as berberine, with i-motif structures should consider the thermodynamic aspects related with the interaction, as well as the selectivity of the proposed ligands with different DNA structures, including unfolded strands.

Evaluation of the effect of polymorphism on G-quadruplex-ligand interaction by means of spectroscopic and chromatographic techniques.

Guanine-rich sequences may fold into highly ordered structures known as G-quadruplexes. Apart from the monomeric G-quadruplex, these sequences may form multimeric structures that are not usually considered when studying interaction with ligands. This work studies the interaction of a ligand, crystal violet, with three guanine-rich DNA sequences with the capacity to form multimeric structures. These sequences correspond to short stretches found near the promoter regions of c-kit and SMARCA4 genes. Instrumental techniques (circular dichroism, molecular fluorescence, size-exclusion chromatography and electrospray ionization mass spectrometry) and multivariate data analysis were used for this purpose. The polymorphism of G-quadruplexes was characterized prior to the interaction studies. The ligand was shown to interact preferentially with the monomeric G-quadruplex; the binding stoichiometry was 1:1 and the binding constant was in the order of 105M-1 for all three sequences. The results highlight the importance of DNA treatment prior to interaction studies.

Spectroscopic study of the interaction of actinomycin D with oligonucleotides carrying the central base sequences -XGCY- and -XGGCCY- using multivariate methods.

The interactions of actinomycin D (ACTD) with the oligonucleotides 5'-CAAAGCTTTG-3', 5'-CATGGCCATG-3' and 5'-TATGGCCATA-3' were investigated by means of acid-base titrations and mole-ratio and melting experiments monitored by molecular absorption and circular dichroism (CD) spectroscopies. For each experiment, CD and molecular absorption spectra were recorded at each point in the experiment, and later analyzed via appropriate multivariate data analysis methods. The study of the interactions between these oligonucleotides and ACTD at 25 degrees C showed the formation of an interaction complex with a stoichiometry of 1:1 (ACTD:duplex) and values for the log(formation constant) of 5.1+/-0.3, 6.4+/-0.2, and 5.6+/-0.2, respectively. An additional interaction complex at higher temperatures was also detected, which might be related to the single-stranded forms of the oligonucleotides.

Synthesis, stability, and protonation studies of a self-complementary dodecamer containing the modified nucleoside 2'-deoxyzebularine.

The nucleoside 2'-deoxyzebularine (K) was incorporated into the self-complementary dodecamer 5'-CGTACGKGTACG-3' by solid-phase 2-cyanoethylphosphoramidite chemistry using dimethoxytrityl (DMT) as the 5'-hydroxyl protecting group. Standard synthesis cycles using trichloroacetic acid and short ammonia treatment (50 degrees C for 30 min) were found to be the optimal conditions to obtain the desired dodecamer with minimum acid and basic degradation of the acid- and base-sensitive 2-pyrimidinone residue. The protonation equilibria of the K nucleoside and of the dodecamer at 37 degrees C were studied by means of spectroscopically monitored titrations. For the K nucleoside, a pK(a) value of 3.13 +/- 0.09 was obtained. For the dodecamer, four acid-base species were found in the pH range 2-12, with pK(a) values of 9.60 +/- 0.07, 4.46 +/- 0.16, and 2.87 +/- 0.19. Melting experiments were carried out to confirm the proposed acid-base concentration profiles. Finally, kinetic experiments were also carried out at several pH values to evaluate the stability of the K nucleoside and of the dodecamer. An increased stability was shown by the K nucleoside when incorporated into the dodecamer. Multivariate methods based on both hard- and soft-modeling were applied for the analysis of spectroscopic data, allowing the estimation of concentration profiles and pure spectra.

Resolution of parallel and antiparallel oligonucleotide triple helices formation and melting processes by multivariate curve resolution.

A procedure is described for the complete resolution of concentration profiles of oligonucleotide triplexes as a function of pH and temperature. The pH and temperature ranges at which triplexes are present and the relative concentrations of all the species involved in acid-base and conformational equilibria are successfully estimated from Multivariate Curve Resolution analysis of UV absorbance spectra recorded along acid-base titrations and melting experiments of single stranded, hairpin and their mixtures. The dependence of formation constants upon pH was successfully estimated. The hairpin h26 (5'-GAAGGAGGAGA-TTTT-TCTCCTCCTTC-3'), and the single stranded oligonucleotides s11CT (5'-CTTCCTCCTCT-3'), s11AG (5'-AGAGGAGGAAG-3') and s11TG (5'-TGTGGTGGTTG-3') were synthesized and their protonation and conformational equilibria were studied in detail. The procedure was shown to be especially useful for the study of triplexes with a low hypochromism upon formation.

Study of the influence of metal ions on tRNA(Phe) thermal unfolding equilibria by UV spectroscopy and multivariate curve resolution.

The influence of metal ions (Na(+), Mg(2+) and Cd(2+)) on the thermal unfolding of phenylalanine transfer ribonucleic acid (tRNA(Phe)) was studied by UV spectroscopy-monitored melting experiments. Absorbance data were obtained during the unfolding process in the range 220-340 nm and later analyzed by a multivariate curve resolution approach (MCR-ALS) based on factor analysis. This procedure determines the number of spectroscopically distinct conformations present during the unfolding process and reveals their concentration profiles and pure spectra, without any initial assumption having to be made about the number of steps in the unfolding pathway. From the concentration profiles and pure spectra, information such as T(m) values can be recovered. The results were compared with those obtained previously in spectroscopic and calorimetric unfolding experiments, showing that the multivariate approach recovers information that complements that obtained in traditional spectroscopic melting experiments.

Three-way multivariate curve resolution applied to speciation of acid-base and thermal unfolding transitions of an alternating polynucleotide.

Analytical speciation of acid-base equilibria and thermal unfolding transitions of an alternating random polynucleotide containing cytosine and hypoxanthine, poly(C, I), is studied. The results are compared with those obtained previously for single-stranded polynucleotides, poly(I) and poly(C), and for the double-stranded poly(I). poly(C), to examine the influence of the secondary structure on the acid-base properties of bases. This study is based on monitoring acid-base titrations and thermal unfolding experiments by molecular absorption, CD, and molecular fluorescence spectroscopies. Experimental data were analyzed by a novel chemometric approach based on a recently developed three-way Multivariate Curve Resolution method, which allowed the simultaneous analysis of data from several spectroscopies. This procedure improves the resolution of the concentration profiles and pure spectra for the species and conformations present in folding-unfolding and acid-base equilibria. The results from acid-base studies showed the existence of only three species in the pH range 2-12 at 37 degrees C and 0.15M ionic strength. No cooperative effects were detected from the resolved concentration profiles, showing that equilibria concerning alternating polynucleotides like poly(C, I) are simpler than those involving poly(I). poly(C). Thermal unfolding experiments at neutral pH confirmed the existence of two transitions and one intermediate conformation. This intermediate conformation could only be detected and resolved without ambiguities when molecular absorption and CD spectral data were analyzed simultaneously.

Protonation studies and multivariate curve resolution on oligodeoxynucleotides carrying the mutagenic base 2-aminopurine.

2-Aminopurine (P) is a mutagen causing A.T to G.C transitions in prokaryotic systems. To study the base-pairing schemes between P and cytosine (C) or thymine (T), two self-complementary dodecamers containing P paired with either C or T were synthesized, and their protonation equilibria were studied by acid-base titrations and melting experiments. The mismatches were incorporated into the self-complementary sequence d(CGCPCCGGXGCG), where X was C or T. Spectroscopic data obtained from molecular absorption, circular dichroism (CD), and molecular fluorescence spectroscopy were analyzed by a factor-analysis-based method, multivariate curve resolution based on the alternating least squares optimization procedure (MCR-ALS). This procedure allows determination of the number of acid-base species or conformations present in an acid-base or melting experiment and the resolution of the concentration profiles and pure spectra for each of them. Acid-base experiments have shown that at pH 7, 150 mM ionic strength, and 37 degrees C, both C and P are deprotonated. At pH near 4, the majority of species shows C protonated and P deprotonated. Finally, at pH values near 3, the majority of species shows both protonated C and P. These results are in agreement with NMR studies showing a wobble geometry for the P x C base pair and a Watson-Crick geometry for the P x T base pair at neutral pH. Melting experiments were carried out to confirm the proposed acid-base distribution profile. For the sequence including the P x T mismatch, only one transition was observed at neutral pH. However, for the sequence including the P x C mismatch, two transitions were detected by CD but only one by molecular absorption. This behavior agrees with that observed by other authors for oligonucleotides of similar sequence and suggests the following sequence of conformational changes during melting: duplex --> hairpin --> random coil.

Study of the interaction of cis-dichloro-(1,2 diethyl-3-aminopyrrolidine)Pt(II) complex with poly(I), poly(C) and poly(I) x poly(C).

The interaction of cis-dichloro-(1,2 diethyl-3-aminopyrrolidine)platinum(II) (Ptpyrr) with the polynucleotides poly(I), poly(C) and poly(I) x poly(C) acids was studied by circular dichroism, molecular fluorescence and (1)H NMR spectroscopies. Multivariate Curve Resolution, a factor analysis method, was applied for the analysis and interpretation of spectroscopic data obtained in mole ratio and kinetics studies. This procedure allows the determination of the number of different interaction complexes present during the experiments and the resolution of both concentration profiles and pure spectra for all of them. Two different interaction complexes were observed at the experimental conditions studied. The first one, at low Ptpyrr:polynucleotide ratio (r(Ptpyrr:poly)) values, corresponds to the interaction of Ptpyrr with hypoxanthine bases in the poly(I) moiety. This interaction leads to the destabilization and dissociation of the double-stranded conformation. The second complex was observed at higher r(Ptpyrr:poly) values and corresponds to the interaction of Ptpyrr to cytosine bases in poly(C) moiety. The formation of both complexes showed that the interaction of Ptpyrr with hypoxanthine bases occurred at the first stages of the reaction and with cytosine bases at longer reaction times. The results obtained show the utility of the Multivariate Curve Resolution approach for the analysis of data obtained by monitoring spectroscopically the interaction equilibria of platinum compounds with nucleic acids.

Analytical characterization of the conformational transitions of polynucleotides by means of different molecular spectroscopies and multivariate curve resolution.

A general procedure for the study of conformational transitions of polynucleotides is described. The equilibria between different conformations induced by salt, ethidium bromide, and temperature of poly(dG-dC). poly(dG-dC) and induced by salt and temperature of poly(A). poly(U) are investigated using molecular absorption, circular dichroism, and fluorescence spectroscopies. Spectral data obtained from experiments are analyzed by means of a factor analysis method, namely, multivariate curve resolution, which allows possible intermediate states to be detected and the pure spectra and the concentration profiles of all species present in the system to be estimated. This work shows the application of this procedure for the analysis of data matrices obtained in individual experiments but also for the analysis of several data matrices simultaneously.

Effect of the reaction field electrostatic term on the molecular dynamics simulation of the activation domain of procarboxypeptidase B.

Molecular dynamics simulations of the activation domain of porcine procarboxypeptidase B (ADBp) were performed in order to examine the effects of the inclusion of a reaction field (RF) term into the calculation of electrostatics forces for highly charged proteins. Two simulations were performed with the GROMOS96 package, studying the influence of counterions on the final results. Comparison with previous results without the inclusion of the RF term (Martí-Renom, M.A., Mas,J.M., Oliva,B., Querol,E. and Avilés,F.X., Protein Engng, 1998, 11, 101-110) shows that the structure is well maintained when the RF term is included. Moreover, the analysis of the trajectories shows that simulations of solvated highly-charged proteins are sensitive to the presence of counterions, the secondary structures being more stable when their charges are neutralized.

Application of multivariate data analysis methods to comparative molecular field analysis (CoMFA) data: proton affinities and pKa prediction for nucleic acids components.

Multivariate data analysis methods (Principal Component Analysis (PCA) and Partial Least Squares (PLS)) are applied to the analysis of the CoMFA (Comparative Molecular Field Analysis) data for several nucleic acids components. The data set includes nitrogenated bases, nucleosides, linear nucleotides, 3', 5'-cyclic nucleotides and oligonucleotides. PCA is applied to study the structure of the CoMFA data and to detect possible outliers in the data set. PLS is applied to correlate the CoMFA data with either calculated AM1 proton affinities or with experimental pKa values. The possibility of making a prediction of pKa values directly from 3D structures of the monomers for polynucleotides is also shown. The influence of the superposition criteria and of conformational changes along the glycosidic bond on the pKa prediction are studied as well.

Study of the Intercalation Equilibrium between the Polynucleotide Poly(adenylic)-Poly(uridylic) Acid and the Ethidium Bromide Dye by Means of Multivariate Curve Resolution and the Multivariate Extension of the Continuous Variation and Mole Ratio Methods.

The intercalation equilibrium between the polynucleotide poly(adenylic)-poly(uridylic) acid and the dye ethidium bromide was studied using molecular absorption spectrophotometry, spectrofluorimetry, and circular dichroism. The presence of an intercalation complex, its pure spectrum, and its concentration profile were clearly deduced using Multivariate Curve Resolution and a multivariate extension of the classical continuous variation and mole-ratio methods. The value of the polynucleotide/dye ratio (r(poly/dye)) in the intercalation complex and of its apparent formation log constant were estimated to be 4 and 6.2, respectively. The proposed multivariate methodology allowed an accurate qualitative and quantitative description of the intercalation process.

[Polyps of the external auditory canal and cholesteatoma]. / Pólipos de CAE y colesteatoma.

A retrospective study of 50 patients with an aural polyp located in the external auditory canal was made. Our main objective was to identify predictive factors for underlying cholesteatoma, as well as to determine our management strategy for aural polyps. We analyzed the most important findings of otological imaging, hearing loss, radiological examination, and medical or surgical treatment. The anatomopathological examination in some cases showed a rare pathology that should be considered in our diagnostic evaluation. The presence of cholesteatoma underlying the polyp in 52% of cases indicates the importance of the polyp location and radiological signs as predictive factors for the diagnosis of cholesteatoma.

Three-way curve resolution applied to the study of solvent effect on the thermodynamic and conformational transitions related to the protonation of polycytidylic acid.

Solvent effect on the acid-base behavior of polycytidylic acid [poly(C)] is studied by means of potentiometric and spectrometric (UV and CD) procedures. Low-polarity biological environments are mimicked by using a 30% (v/v) dioxane-water mixture. Experiments performed in this medium are compared with previously reported results in aqueous solution in order to determine changes in both thermodynamic and structural aspects associated with modifications of the biomolecular surroundings. Potentiometric and spectrometric studies reveal the presence of a nonlinear polyelectrolytic effect associated with the protonation of poly(C) in the hydroorganic mixture, different from the analogous effect in aqueous solution. The curve resolution method alternating least squares is applied to the poly(C) spectrometric data. Concentration profiles and spectra of both deprotonated poly(C) and half-protonated [poly(C).poly(CH+)] are thus obtained. The fully protonated species poly(CH+) precipitates in the hydroorganic medium at pH values lower than 4. Evidence for the ordered structure of both poly(C) and [poly(C).poly(CH+)] species is seen through the comparison of the macromolecule spectra with those of the cytidine-3',5'-cyclic monophosphate monomer. Polarity decreases around the macromolecule produce significant hypochromicity in the CD spectra and hyperchromicity in the UV spectra, both signs of a disordering effect in the macromolecular structure due to the weakening of base stacking interactions.

Application of a multivariate curve resolution procedure to the analysis of second-order melting data of synthetic and natural polynucleotides.

The melting behavior of several synthetic polynucleotides and a mixture of natural tRNAs was studied by monitoring the changes in the whole UV absorbance spectrum at several pH values. The second-order absorbance data were analyzed with a soft-modeling multivariate curve resolution procedure that allows the determination of the number of different species or conformations present along the melting experiment and the calculation of the melting profile and the pure spectrum for each chemical species or conformation. The melting temperature, T(m), for each thermal transition can be calculated from the melting profiles, and structural information on the different species or conformations can be obtained from their pure spectra. The multistranded species formed at certain pH conditions show several sharp thermal transitions related to the loss of the initial highly ordered structure. For these transitions, the mixture of species obtained in the denaturing process can be resolved when several data matrices, each giving additional information, are analyzed simultaneously with the mathematical procedure proposed.

Application of a self-modeling curve resolution approach to the study of solvent effects on the acid-base and copper (II)-complexing behavior of polyuridylic acid.

The solvent effect on the acid-base and complexation behavior of the homopolynucleotide polyuridylic acid (poly(U)) has been studied by means of potentiometric and spectrometric titrations (circular dichroism and UV-VIS) in water and in 30 and 50% (v/v) dioxane-water media. The potentiometric studies revealed the absence of polyelectrolytic effects in the acid-base equilibrium, and the spectrometric experiments detected only a random coil conformation associated with both the protonated and deprotonated species. The common behavior observed in the three media seems to indicate the weakness of both intramolecular interactions, i.e., base stacking, and solute/solvent interactions, i.e., hydrogen-bonding, and consequently their small effect during the protonation process. Differences regarding the solubility of the deprotonated species in the solvents used are due to the difficult stabilization of such a charged species in the low polar environment of the dioxane-water mixtures. Complexation has been exhaustively studied in aqueous media, and no conformational changes have been noticed in the only copper(II)-poly(U) complex detected. The inclusion of the copper(II) ion in the macromolecular skeleton of the polynucleotide does not contribute to an ordination of the structure, which remains as a random coil. No comparison between this equilibrium in aqueous solution and in the hydroorganic mixtures could be carried out since the limited pH range of the soluble complex in those solvent mixtures prevented a rigorous quantitative monitoring of such a chemical process.

Application of Eigenstructure Tracking Analysis and SIMPLISMA to the Study of the Protonation Equilibria of cCMP and Several Polynucleotides.

The application of eigenstructure tracking analysis (ETA) and SIMPLISMA for the investigation of the protonation equilibria of a monomer and several polynucleotides is proposed. Both approaches have been applied in the pH and in the wavelength direction to the spectroscopic data matrices obtained in the study of each equilibrum. ETA provides information about the number of components in the system, their evolution along the titration, and the local rank. SIMPLISMA is also used to obtain the number of compounds in the system, the concentration profiles, and the unit spectrum of each compound. The results obtained with SIMPLISMA and those obtained previously with the alternating least-squares approach are compared.