467 research outputs found
Origin of the OECD Principles for QSAR Validation and Their Role in Changing the QSAR Paradigm Worldwide: An Historical Overview
No full textThe discussions in the QSAR community and the steps that led to the definition of the OECD Principles for the validation of QSAR models are illustrated here, framing the process in the general framework of QSAR modeling. The individual OECD Principles are presented, commenting on them in the light of significant publications that have appeared over the years, with particular attention to the aspects of statistical validation according to the chemometric approach. It will be highlighted how and to what extent the OECD Principles have influenced the subsequent work of all QSAR modelers and have led to a significant improvement in validated QSAR modeling applicable in the regulatory field and beyond
Principles of QSAR Modeling
No full textAt the end of her academic career, the author summarizes the main aspects of QSAR modeling, giving comments and suggestions according to her 23 years’ experience in QSAR research on environmental topics. The focus is mainly on Multiple Linear Regression, particularly Ordinary Least Squares, using a Genetic Algorithm for variable selection from various theoretical molecular descriptors, but
the comments can be useful also for other QSAR methods. The need for rigorous validation, also external, and for applicability domain check to guarantee predictivity and reliability of QSAR models is particularly highlighted. The commented approach is the “predictive” one, based on chemometrics, and is usefully applied to the prioritization of environmental pollutants. All the discussed points and
the author’s ideas are implemented in the software QSARINS, as a legacy to the QSAR community
A tool for the assessment of VOC degradability by tropospheric oxidants starting from chemical structure
No full textThe tropospheric lifetime of most organic chemicals deriving from terrestrial emissions is mainly controlled by their degradation reaction with OH radicals and ozone during the daytime, and NO3 radicals at night. The studied data set, composed of 399 structurally heterogeneous volatile organic compounds (VOCs), consists of data for the three studied oxidation reactions, available either experimentally or from our herein reported quantitative structure-activity relationships (QSAR) models. A principal component analysis (PCA) model based on these principal oxidative degradations has been proposed to evaluate the overall tropospheric degradability of chemicals. The score of the first principal component can be proposed as an ATmospheric Degradability INdex (ATDIN) and is modelled by theoretical molecular descriptors to obtain a multiple linear regression (MLR) QSAR model with high predictive power, both internally and externally validated. This modelling approach allows a fast and preliminary VOC ranking according to their tendency to be degraded by atmospheric oxidants, starting only from the knowledge of their molecular structure. (C) 2004 Elsevier Ltd. All rights reserved
Aloe revisited the structure of aloeresin A
No full textAloeresin A, a major constituent of Cape Aloe, is shown to be 2′-O-(E)-p-coumaroyl aloesin (3) instead of 6′-O-(E)-p-coumaroyl aloesin (2) as accepted so far
Gramatica românească
No full textGramatica românească / alcătuită de Radu Tempea. - Sibiu : în tipografia lui Petru Bart, 1979. - [8] f., 218 p., [1] f. : frontisp., vig. ; 17,5 cm.
Text în limba română cu caractere chirilice. - Însemnări
Exploring the QSARs for OH Tropospheric Degradation of VOCs using freely available online descriptors
No full textTropospheric photochemical air pollution has impacts on scales ranging from local to global. The reactions of organic pollutants in the atmosphere with OH radicals, NO3 radicals, and ozone are of great concern from an ecological standpoint for risk assessment measurement such as degradation pathways, atmospheric lifetimes and fate of these compounds. It was already established that reactions with hydroxyl radical (-OH) is the most important pathway of day time removal of organic pollutant in atmosphere because of the reactive nature of OH radical to react practically almost with every volatile organic compounds (VOCs) in the troposphere. Due to limited availability of experimental gas phase rate constant data for chemicals, alternative theoretical approaches like QSAR/QSPRs are often practiced to predict the high risk of organic chemicals and to reduce the time consuming, expensive and difficult experimental procedures.
In this study we developed QSAR models for hydroxyl radical tropospheric degradation rate of 460 VOCs, using HOMO, LUMO from Hyperchem minimization in addition to separately freely available online molecular descriptors (from the CADASTER online platform– www.cadaster.eu) or descriptors calculated from an updated version of DRAGON software. The Genetic Algorithm as Variable Subset Selection (GA-VSS) was used to select the relevant molecular descriptors in the modeling step (Ordinary Least Squares (OLS) regression). Three splitting criteria [K-ANN, k-means cluster and random on response] were applied for verifying the external predictivity of developed QSAR models, with special emphasis on model applicability domain which was verified by the leverage approach. The statistical qualities of the models developed from the pool of online descriptors were comparable with those obtained from the DRAGON descriptors and, most importantly, the GA selected, in addition to HOMO, descriptors with comparable mechanistic meaning, from completely different pool of input descriptors.
So it can be suggested to use online freely available descriptors to increase the reproducibility of the models for the safety of environment and for REACH
A C-glucosylated 5-methylchromone from Kenya aloe
No full textA new bitter C-glucoside, aloeresin D, was isolated from a commercial sample of Kenya aloe. Its structure was assigned as 8-C-β-d-[2-O-(E)-p-coumaroyl]glucopyranosyl-2-[(R)-2-hydroxy]propyl-7-methoxy-5-methyl-chromone by spectral data and chemical transformations
WHIM descriptors of shape
No full textWHIM descriptors are 3D structural descriptors obtained from the (x,y,z)-atomic coordinates of a molecular conformation of a chemical, and are used successfully in QSAR modelling. They are built in so as to capture relevant 3D information regarding different features of molecular structure: size, shape, symmetry and atom distribution.
Different weights are used to obtain particular information for each set of descriptors.
Recently, some doubts have been raised in the literature regarding the ability of Weighted
Holistic Invariant Molecular (WHIM) descriptors of shape (the K non-directional WHIM descriptors) to describe the difference between linear and non-linear molecules. A data set of seventy aliphatic and aromatic chemicals of different shape (linear and branched
congeners) has been studied here, and is represented by K descriptors (global nondirectional WHIM descriptors of shape) based on different weights. The K descriptors range from 0 for a spherical molecule to 1 for a perfectly linear molecule. The present
study confirms the findings, already reported by the author, that the WHIM descriptors of shape (K) are unequivocally able to differentiate linear and non-linear molecules
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