A. de Juan

Chemometrics applied to unravel multicomponent processes and mixtures: Revisiting latest trends in multivariate resolution

Abstract Progress in the analysis of multicomponent processes and mixtures relies on the combination of sophisticated instrumental techniques and suitable data analysis tools focused on the interpretation of the multivariate responses obtained. Despite the differences in compositional variation, complexity and origin, the raw measurements recorded in a multicomponent chemical system can be very often described with a simple model consisting of the composition-weighted sum of the signals of their pure compounds. Multivariate resolution methods have been the tools designed to unravel this pure compound information from the non-selective mixed original experimental output. The evolution of these chemometric approaches through the improvement of exploratory tools, the adaptation to work with complex data structures, the ability to introduce chemical and mathematical information in the algorithms and the better quality assessment of the results obtained is revisited. The active research on these chemometric area has allowed the successful application of these methodologies to chemical problems as complex and diverse as the interpretation of protein folding processes or the resolution of spectroscopic images.

Assessment of new constraints applied to the alternating least squares method

Abstract The introduction of constraints in multivariate curve resolution methods, such as the Alternating Least Squares (ALS), is commonly used to limit the span of possible solutions, guiding the iterative process to a final result as close as possible to the true situation. In the present work, two modifications of the unimodality constraint and a new constraint for chromatographic concentration profiles related to the prevention of fronting have been checked. Simulated data sets as well as real data have been used to evaluate the effect of these new constraints in the resolution results. The parameters measured to assess the goodness of the constraints are related to the recovery of the concentration profiles and the quality of the data fit.

Resolution and segmentation of hyperspectral biomedical images by multivariate curve resolution-alternating least squares.

MCR-ALS is a resolution method that has been applied in many different fields, such as process analysis, environmental data and, recently, hyperspectral image analysis. In this context, the algorithm provides the distribution maps and the pure spectra of the image constituents from the sole information in the raw image measurement. Based on the distribution maps and spectra obtained, additional information can be easily derived, such as identification of constituents when libraries are available or quantitation within the image, expressed as constituent signal contribution. This work summarizes first the protocol followed for the resolution on two examples of kidney calculi, taken as representations of images with major and minor compounds, respectively. Image segmentation allows separating regions of images according to their pixel similarity and is also relevant in the biomedical field to differentiate healthy from non-healthy regions in tissues or to identify sample regions with distinct properties. Information on pixel similarity is enclosed not only in pixel spectra, but also in other smaller pixel representations, such as PCA scores. In this paper, we propose the use of MCR scores (concentration profiles) for segmentation purposes. K-means results obtained from different pixel representations of the data set are compared. The main advantages of the use of MCR scores are the interpretability of the class centroids and the compound-wise selection and preprocessing of the input information in the segmentation scheme.

Solvatochromic Parameters for Binary Mixtures and a Correlation with Equilibrium Constants. Part I. Dioxane-Water Mixtures

The values of the solvatochromic parameters α and β were determined at 25°C for dioxane-water mixtures from 0 to 100% of dioxane. These values as well as those of the Reichardt polarity parameter ET(30) and the polarity-polarizability π* are correlated with acid dissociation constants and other equilibrium constants in solvent mixtures of the same composition. As a general rule, two linear zones with different slopes are obtained, one zone covering water-rich solutions, and the other dioxane-rich solutions. The change in behavior takes place at about 55% (v/v) dioxane for all equilibria studied. A fit of pK to an equation of the multiparametric form proposed by Kamlet and Taft shows in most cases a linear dependence on π* alone, in other cases a dependence on π* and β.

A soft-modeling approach to interpret thermodynamic and conformational transitions of polynucleotides.

Multivariate outputs from the experimental monitoring of biochemical processes are usually difficult to interpret applying methods based on a priori chemical models. Curve resolution methods are model-free procedures, generally known as soft-modeling methods, which obtain the concentration profiles and instrumental responses of each individual species involved in a multivariate monitored process without making any kind of external assumption. Of the curve resolution methods available, the alternating least squares (ALS) is proposed here because of its ability to operate on one or on several matrices. Furthermore, ALS allows the introduction of information related to the internal data structure and to the general features of the concentration profiles and instrumental responses through the input of suitable constraints in the iterative resolution procedure. The ALS potential is tested on several data sets coming from the multivariate spectrometric monitoring of polyuridylic (polyU), polycytidylic (polyC), and polyadenylic (polyA) protonation equilibria in dioxane/water 30% (v/v). Information concerning the evolution of the concentration profiles and the spectra of each individual species involved in the acid-base equilibria, the presence and pattern of polyelectrolyte effects, and the presence of conformational transitions associated or not with the proton uptake process is presented.

Factor analysis applied to the study of the effects of solvent composition and nature of the inert electrolyte on the protonation constants in dioxane—water mixtures

Abstract The effect of solvent properties on protonation constants was studied for water-dioxane mixtures covering a wide range of solvent compositions, from data sets on several solutes with a variety of functional groups. The establishment of the model was carried out using different approaches: the use of a classical procedure, where the best multiparametric fit to the Kamelt and Taft equation was evaluated for each substance; and the application of combined factor analysis and target factor analysis to quantify and identify the factors affecting the variation of the whole data sets, without the need to postulate any a priori hypothetical model. Further, a preliminary overview of the influence of different inert electrolytes on the values of the solute protonation constants in this solvent mixture was also obtained through factor analysis.

Combining multiset resolution and segmentation for hyperspectral image analysis of biological tissues.

Hyperspectral images can provide useful biochemical information about tissue samples. Often, Fourier transform infrared (FTIR) images have been used to distinguish different tissue elements and changes caused by pathological causes. The spectral variation between tissue types and pathological states is very small and multivariate analysis methods are required to describe adequately these subtle changes. In this work, a strategy combining multivariate curve resolution-alternating least squares (MCR-ALS), a resolution (unmixing) method, which recovers distribution maps and pure spectra of image constituents, and K-means clustering, a segmentation method, which identifies groups of similar pixels in an image, is used to provide efficient information on tissue samples. First, multiset MCR-ALS analysis is performed on the set of images related to a particular pathology status to provide basic spectral signatures and distribution maps of the biological contributions needed to describe the tissues. Later on, multiset segmentation analysis is applied to the obtained MCR scores (concentration profiles), used as compressed initial information for segmentation purposes. The multiset idea is transferred to perform image segmentation of different tissue samples. Doing so, a difference can be made between clusters associated with relevant biological parts common to all images, linked to general trends of the type of samples analyzed, and sample-specific clusters, that reflect the natural biological sample-to-sample variability. The last step consists of performing separate multiset MCR-ALS analyses on the pixels of each of the relevant segmentation clusters for the pathology studied to obtain a finer description of the related tissue parts. The potential of the strategy combining multiset resolution on complete images, multiset segmentation and multiset local resolution analysis will be shown on a study focused on FTIR images of tissue sections recorded on inflamed and non-inflamed palatine tonsils.

Assessment of solvent parameters and their correlation with protonation constants in dioxane-water mixtures using factor analysis

Abstract The effect of the composition of dioxane—water mixtures on wavelength shifts of different solvatochromic indicators and on p K a values of different carboxylic acids are studied using factor analysis. Target factor analysis is subsequently applied to find out which are the physicochemical parameters affecting the observed data variation. For wavelength shifts of the selected solvatochromic indicators three main sources of variation are found: solvent polarity, π*, the hydrogen-bond donor acidity of solvent, α, and the hydrogen-bond acceptor basicity of the solvent, β. For p K a values of carboxylic acids the main source of variation is identified as being the polarity of the mixed solvent.

Correlation of acid-base properties of solutes with the polarity parameters and other solvatochromic parameters of dioxane-water mixtures

Abstract The behaviour of water-dioxane mixtures as solvents is studied paying special attention to the correlations which may exist among the solvatochromic parameters of these mixtures, as well as to the correlations among some of these parameters and the acid-base properties of model solutes (whose acidity constants are determined). Values of the polarity parameter π . , proposed by Kamlet and Taft, are determined from measurements of shifts in λ max of 2-nitroanisole, 4-nitroanisole and 4-ethylnitrobenzene, proposed as reference dyes for amphiprotic solvents. The correlations of π . values with some bulk properties and different microscopic parameters of the working solvents are established. Acid dissociation constants for propionic acid, chosen as a model for aliphatic carboxylic acids, and salicylic acid, for both aromatic carboxylic and phenolic hydroxylic acids, in water-dioxane mixtures are determined from e.m.f. data (at a constant electrolyte concentration 0.2 M and at 25 °C). The solvent composition range studied comprises from 10 to 70% dioxane (vol./vol.). Correlations obtained among the values of acid dissociation constants and the solvatochromic parameters for the solvent mixtures can be useful in helping to explain the variation of the acid-base character of solutes when properties of solvent are modified.

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.