Semi-empirical topological method for the prediction of the chromatographic retention of cis- and trans-alkene isomers and alkanes

Amazon Rainforest Hidden Volatiles-Part I: Unveiling New Compounds from Acmella oleracea (L.) R.K. Jansen Essential Oil

Motivated by the culinary and ethnopharmacological use of Acmella oleracea (L.) R.K. Jansen, this study aimed to unveil new chemical compounds from its essential oil (EO). Acmella oleracea, known for its anesthetic and spicy properties, has been used in traditional medicine and cuisine, particularly in Northern Brazil. Through a detailed GC-MS analysis, 180 constituents were identified, including 12 tentatively identified long-chain α-keto esters of various acids. Additionally, 18 new esters were synthesized for structural verification. This research expands the known chemical diversity of A. oleracea EO, providing a basis for potential pharmacological applications. The identification of new natural products, including homologs and analogs of acmellonate, underscores the EO's rich chemical profile and its potential for novel bioproduct development.

Structure–retention relationship study of diastereomeric (Z)- and (E)-2-alkylidene-4-oxothiazolidines

Quantitative structure-retention relationship (QSRR) was developed for a series of the (Z)- and (E)-2-alkylidene-4-oxothiazolidine derivatives by the multiple linear regression (MLR) analysis. Full geometry optimization based on Austin Model 1 (AM1) semiempirical molecular orbital method was carried out and a set of physicochemical molecular descriptors was calculated from the optimized structures. In order to obtain useful experimental parameters, the lipophilic character of analytes was measured by RP-TLC, and lipophilicity parameters were correlated with physicochemical structural descriptors. Statistically significant and physically meaningful structure-retention relationships were obtained.

Three-dimensional topographic index applied to the prediction of acyclic C5–C8 alkenes Kováts retention indices on polydimethylsiloxane and squalane columns

A novel approach is described for the prediction of gas chromatographic Kováts retention indices of 150 acyclic C5-C8 alkenes on two stationary phases (polydimethylsiloxane, PDMS, and squalane, SQ). The heuristic method was used to build multiple linear regression models using descriptors calculated by MODLESLAB software and CODESSA program. The resulting quantitative structure-retention relationship (QSRR) models were well-correlated, with predictive R2 values of 0.970 and 0.958 for retention indices on PDMS and SQ columns, respectively. 1Omegap, a three-dimensional (3D) topographic index, was found to play the most important role in the description of the chromatographic retention behavior of the alkenes in these two stationary phases. Moreover, this index could completely distinguish different isomers of alkene. Therefore, it can also be extended to distinguish different isomers of other compounds so that can well describe their quantitative structure-retention relationships.

Quantitative structure-(chromatographic) retention relationships

Since the pioneering works of Kaliszan (R. Kaliszan, Quantitative Structure-Chromatographic Retention Relationships, Wiley, New York, 1987; and R. Kaliszan, Structure and Retention in Chromatography. A Chemometric Approach, Harwood Academic, Amsterdam, 1997) no comprehensive summary is available in the field. Present review covers the period of 1996-August 2006. The sources are grouped according to the special properties of kinds of chromatography: Quantitative structure-retention relationship in gas chromatography, in planar chromatography, in column liquid chromatography, in micellar liquid chromatography, affinity chromatography and quantitative structure enantioselective retention relationships. General tendencies, misleading practice and conclusions, validation of the models, suggestions for future works are summarized for each sub-field. Some straightforward applications are emphasized but standard ones. The sources and the model compounds, descriptors, predicted retention data, modeling methods and indicators of their performance, validation of models, and stationary phases are collected in the tables. Some important conclusions are: Not all physicochemical descriptors correlate with the retention data strongly; the heat of formation is not related to the chromatographic retention. It is not appropriate to give the errors of Kovats indices in percentages. The apparently low values (1-3%) can disorient the reviewers and readers. Contemporary mean interlaboratory reproducibility of Kovats indices are about 5-10 i.u. for standard non polar phases and 10-25 i.u. for standard polar phases. The predictive performance of QSRR models deteriorates as the polarity of GC stationary phase increases. The correlation coefficient alone is not a particularly good indicator for the model performance. Residuals are more useful than plots of measured and calculated values. There is no need to give the retention data in a form of an equation if the numbers of compounds are small. The domain of model applicability of models should be given in all cases.

Semi-empirical topological index: a tool for QSPR/QSAR studies

The semi-empirical topological index (I(ET)), developed to predict the chromatographic retention of a series of organic compounds, is extended to predict other properties and biological activities of aliphatic alcohols. This topological index takes into account the contribution of each individual atom type to the property considered and is able to encode information about structural features of the molecules. The efficiency of this index is verified by high quality Structure - Property and structure - Activity Relationships (QSPR/QSAR) models obtained for several representative physicochemical properties, biological activities and toxicities of aliphatic alcohols. Most of the properties investigated are well modeled (r > 0.98) employing the I(ET). Cross-validation using the more general leave-one-out method demonstrates that these models are highly statistically reliable. The proposed I(ET) index promises to be a useful descriptor in the QSPR/QSAR studies.

Capillary gas chromatography–mass spectrometry of all 93 acyclic octenes and their identification in fluid catalytic cracked gasoline

The Kováts retention indices of all 93 acyclic octenes on polydimethylsiloxane and squalane as stationary phases as well as their mass spectra were measured. The means of gas chromatography-mass spectrometry (GC-MS) were used for confirmation of GC identification as well as for mass spectrometric deconvolution of the majority of gas chromatographic unseparated isomeric octene peaks. The distinction between corresponding E and Z acyclic octenes, that is either difficult or even impossible by means of GC-MS, was obtained on the basis of larger temperature coefficients of retention indices for Z isomeric octenes than for corresponding E isomers. The retention data expressed as homomorphy factors were correlated with the degree of branching, position of double bond, and position of alkyl group with respect to the double bond of acyclic octenes, and the structure-retention relationships were formulated. The 81 acyclic octenes were identified in FCC gasoline.

Methods for generating second dimension retention index data in comprehensive two-dimensional gas chromatography

Two methods of generating transportable second dimension retention data are outlined for comprehensive two-dimensional gas chromatography (GC x GC). They are both refinements of a previously outlined procedure, which adapted 'isovolatile' curves to retention prediction maps developed in our laboratory, extended to a more polar homologous series, the linear primary alcohols. The earlier work investigated maps based on alkanes and methyl ketones and methyl esters; here the method of data collection to generate the retention map has also changed, extending the retention base range. The resulting retention map permits a retention basis to be used for GC x GC data, however, a more polar retention set than alkanes is required. The calculation of 'retention indices' is aided by the generation of 'fractional reference compound' curves, by either direct manipulation of data local to the solute, or generation of a discrete curve coincident with the retention co-ordinates of the target compound.

Application of the artificial neural network in quantitative structure-gradient elution retention relationship of phenylthiocarbamyl amino acids derivatives

Quantitative structure-retention relationship(QSRR) method was used to model reversed-phase high-performance liquid chromatography (RP-HPLC) separation of 18 selected amino acids. Retention data for phenylthiocarbamyl (PTC) amino acids derivatives were obtained using gradient elution on ODS column with mobile phase of varying acetonitrile, acetate buffer and containing 0.5 ml/l of triethylamine (TEA). Molecular structure of each amino acid was encoded with 36 calculated molecular descriptors. The correlation between the molecular descriptors and the retention time of the compounds in the calibration set was established using the genetic neural network method. A genetic algorithm (GA) was used to select important molecular descriptors and supervised artificial neural network (ANN) was used to correlate mobile phase composition and selected descriptors with the experimentally derived retention times. Retention time values were used as the network's output and calculated molecular descriptors and mobile phase composition as the inputs. The best model with five input descriptors was chosen, and the significance of the selected descriptors for amino acid separation was examined. Results confirmed the dominant role of the organic modifier in such chromatographic systems in addition to lipophilicity (log P) and molecular size and shape (topological indices) of investigated solutes.

Gas chromatographic-mass spectrometric characterization of all acyclic C5-C7 alkenes from fluid catalytic cracked gasoline using polydimethylsiloxane and squalane stationary phases

Published retention indices of acyclic alkenes C5-C7 on squalane and polydimethylsiloxane as stationary phases were investigated, and reliable retention indices of alkenes from various sources were converted to separation systems used in a laboratory. Retention indices measured on available authentic commercial alkenes and on alkenic fraction of gasoline, published retention indices as well as means of GC-MS were used for verification of calculated retention indices. Retention of some gas chromatographic unseparated isomer pairs was obtained by mass spectrometric deconvolution using a specific single-ion monitoring. On the basis of these retention data, C5-C7 alkenes were identified and analyzed in the gasoline from fluid catalytic cracking. In the gasoline all 59 acyclic C5-C7 isomeric alkenes were determined at significantly different concentration levels.