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Relative Stereochemistry and Absolute Configuration of Farinosin, a Eudesmanolide From Encelia farinosa
Abstract
The naturally occurring eudesmanolide farinosin () is now fully characterized for the first time despite its original isolation almost half a century ago. The early assumed relative stereochemistry and absolute configuration were confirmed by vibrational circular dichroism together with evaluation of the Hooft X-ray parameters. The molecular conformation is very similar in the gas stage and in the solid state. Chirality 00:000-000, 2016. © 2016 Wiley Periodicals, Inc.
... The natural occurring eudesmanolide farinosin (4), isolated from Encelia farinosa, was fully characterized by spectral means in a recent study [1] despite its original isolation almost half a century ago. [2,3] Over the years, the compound showed relevant biological activities, as follows from studies on its antimicrobial effect [4] and on chemical ecology. ...
... For the distinction of the individual hydrogen atoms of the C-14 and C-14 0 methylene groups and the assignment of the hydrogen coupling constants of 1, we resorted to the PERCH v.2011.1 software [20][21][22][23] (PERCH Solutions Ltd., Kuopio, Finland), as done previously for farinosin (4). [1] For this purpose, the 750 MHz FID was imported into the preparation module of the PERCH shell, Fourier transformed, and phase and baseline corrected, and peak picking and integration were performed. The most stable molecular conformer of 1 was calculated in the molecular modeling portion of wileyonlinelibrary.com/journal/mrc the software and used to predict the initial calculated spectrum. ...
... The most stable molecular conformer of 1 was calculated in the molecular modeling portion of wileyonlinelibrary.com/journal/mrc the software and used to predict the initial calculated spectrum. For this purpose, all just determined chemical shifts of 1, as well as the C-1/C-2 and the cyclohexane 1 H-1 H coupling constant values, including the J 1,6α , J 9α,15 , J 1 0 ,6 0 α , and J 9 0 α,15 0 values of farinosin (4) [1] were manually fed into the table of the graphical spectral parameter editor of the PERCH software and then submitted to the iterator module of the software, by using the total-line-shape fitting method, which allowed to obtain an initial calculated spectrum. For the diastereotopic hydrogen atoms of the C-14 and C-14 0 methylene groups, the assignments were interchanged in all possible combinations, that is those of one methylene group, those of the other methylene group, and finally those of both methylene groups at once to know which total-line-shape fitting analysis gave the lowest root mean square (RMS). ...
The structure of the dimeric eudesmanolide hydroxy-bis-dihydrofarinosin (1) from Encelia farinosa followed after contrasting their 1H and 13C NMR spectra with those of encelin (6), hydroxy-bis-dihydroencelin (3), and farinosin (4). Structure 1 was verified by single crystal X-ray diffraction analysis, which further provided the streochemistry of the hydroxy group at C-4. Comparison of the experimental vibrational circular dichroism spectrum of its derived diacetate 2 with that calculated by DFT provided the absolute configuration, which resulted the same as that of its biogenetic precursor 4. Evaluation of several chemical shits differences between the two eudesmanolide fragments of 1 and 3 allows also ascertaining the yet not reported absolute configuration of the C-4 stereogenic center of 3.
... The early assumed relative stereochemistry and AC were confirmed by VCD together with evaluation of the Hooft X-ray parameters. 43 In addition, the structure of hydroxy-bisdihydrofarinosin (58), from the same plant, was verified by single crystal X-ray diffraction. The AC of derived diacetates 59 and 60 ( Figure 14) followed from the corresponding VCD studies. ...
Although demonstrated in 1975, vibrational circular dichroism (VCD) finally started to popularize during this century as a reliable tool to determine the absolute configuration (AC) of organic molecules. This research field continues to be a very dynamic one, in particular for the study of natural products which are a unlimited source of chiral molecules. It therefore turns of interest to summarize the accomplishments published in recent years and to comment on some eventual difficulties that emerged in rare cases to complete the AC determination task. Therefore the aim of this review is to update VCD results for the AC assignment of natural products published from 2015 to 2019, a period in which VCD was reported in some 126 publications involving almost 300 molecules. They are organized according the type of studied metabolite allowing an easily search. The molecules correspond to 28 monoterpenes concerning 17 papers, to 42 sesquiterpenes in 14 papers, to 51 diterpenes in 19 publications, to 5 other terpenoids in three papers, to 48 aromatic molecules in 15 reports, to 20 polyketides in 10 publications, to 27 miscellaneous formulas also in 10 papers, and to 76 nitrogen containing compounds, which include alkaloids and their synthetic analogs, in 38 articles. The landscape of reviewed molecules is quite wide as it goes from simple monoterpenes, like borneol or camphor, to very relevant biological molecules like the alkaloid cocaine or tadalafil samples to distinguish genuine and counterfeit Cialis ® . In addition, 5 natural products and a simple derivative published outside the reviewed period, were used to illustrate some aspects of density functional theory calculations.
... In turn, irradiation of the H-10 methyl signal at δ 1.85 changed various multiplicities in the spectrum, and therefore to completely define the overall spin system it was decided to evaluate it using the PERCH v. 2011.1 software 16 (PERCH Solutions, Kuopio, Finland) as we have done for other terpenoids. 17,18 The methodology used for the PERCH evaluation of the spin system of 3 is similar to that recently described for hydroxy-bis-farinosin. 19 Thus, on the one hand, the 300 MHz 1 H NMR FID data were imported into the preparation module of the PERCH shell, followed by Fourier transformation, phase and baseline correction, peak picking, and integration. ...
The absolute configuration (AC) of the naturally occurring ocimenes (−)-(3S,5Z)-2,6-dimethyl-2,3-epoxyocta-5,7-diene (1) and (−)-(3S,5Z)-2,6-dimethylocta-5,7-dien-2,3-diol (2), isolated from the essential oils of domesticated specimens of Artemisia absinthium, followed by vibrational circular dichroism (VCD) studies of 1, as well as from the acetonide 3 and the monoacetate 4, both derived from 2, since secondary alcohols are not the best functional groups to be present during VCD studies in solution due to intermolecular associations. The AC follows from comparison of experimental and calculated VCD spectra that were obtained by Density Functional Theory computation at the B3LYP/DGDZVP level of theory. Careful nuclear magnetic resonance (NMR) measurements were compared with literature values, providing for the first time systematic 1H and 13C chemical shift data. Regarding homonuclear 1H coupling constants, after performing a few irradiation experiments that showed the presence of several small long-range interactions, the complete set of coupling constants for 3, which is representative of the four studied molecules, was determined by iterations using the PERCH software. This procedure even allowed assigning the pro-R and pro-S methyl group signals of the two gem-dimethyl groups present in 3.
Secondary metabolites from land and marine (micro)organisms have been at the focus of the drug discovery process for many years. One of the reasons for this success is nature’s incredible...
The most significant new techniques that have been used in the twenty-first century for the structure elucidation of sesquiterpenes and some derivatives are reviewed in this chapter. A distinctive feature of these methodologies is the combination of accurate experimental measurements with theoretical data obtained by molecular modeling calculations that allow to visualize, understand, and quantify many structural characteristics. This has been the case for NMR spectroscopy, which has expanded its potential for solving complex structural problems by means of comparison with quantum mechanical molecular models. Ab initio and density functional theory calculations of chemical shifts, coupling constants, and residual chemical shift anisotropies have played important roles in the solution of many structures of sesquiterpenes. The assignments of their absolute configurations by evaluation of calculated and experimental chiroptical properties as electronic and vibrational circular dichroism are also reviewed. This chapter also includes the use of X-ray diffraction analysis with emphasis on calculations of the Flack and Hooft parameters, which are applicable to all molecules that crystallize in non-centrosymmetric space groups. The accurate molecular models of sesquiterpenes, validated by concordance with their experimental properties, are nowadays essential for the interpretation of the effects of these natural products on biological systems.
Covering: 2015 to up to 2019
This review presents the recent progress towards elucidating the structures of chiral natural products and applications using vibrational optical activity (VOA) spectroscopy. Vibrational circular dichroism (VCD) and vibrational Raman optical activity (VROA) are two separate branches of VOA spectroscopy, providing independent and complementary structural information. While determining the absolute configuration (AC) of a given natural product is the primary goal, the determination or assessment of major conformations associated with each diastereomer is also a significant part of this enquiry. The latest developments in experimental and computational aspects of VOA spectroscopies and their applications for inferring the AC and predominant conformations of natural products are summarized. The prospects and limitations in the application of VOA spectroscopy to new natural products are summarized.
Vibrational circular dichroism (VCD) emerged during the last decade as a reliable tool for the absolute configuration (AC) determination of organic compounds. The principles, instrumentation, and methodology applied prior to early 2013 were recently reviewed by us. Since VCD is a very dynamic field, the aim of this review is to update VCD advances for the AC assignment of terpenoids, aromatic compounds, alkaloids, and other natural products for the 2013-2014 period, when VCD was applied to the AC assignment of some 70 natural products. In addition, although discovered in 2012, a brief introduction to the VCD exciton coupling approach and its applications in natural products AC assignment is presented.
Microsymposia (i) The theoretical developments made to analyse quantitatively the GISAXS patterns beyond the classical approximations, in particular to account for the profile of refraction index and the particle-particle correlations. (ii) The self similarity during the dynamic coalescence of Au/ TiO2(110) on the size distribution but also on the spatial ordering of the particles and its link with the randomness of the nucleation centers. (iii) The sintering of nanoparticles during the course of the CO oxidation reaction and the link between particle size and chemical reactivity measured by mass spectrometry.
New software,
OLEX2
, has been developed for the determination, visualization and analysis of molecular crystal structures. The software has a portable mouse-driven workflow-oriented and fully comprehensive graphical user interface for structure solution, refinement and report generation, as well as novel tools for structure analysis.
OLEX2
seamlessly links all aspects of the structure solution, refinement and publication process and presents them in a single workflow-driven package, with the ultimate goal of producing an application which will be useful to both chemists and crystallographers.
The previously described method for absolute structure determination [Hooft, Straver & Spek (2008). J. Appl. Cryst. 41, 96-103] assumes a Gaussian error distribution. The method is now extended to make it robust against poor data with large systematic errors with the introduction of the Student t-distribution. It is shown that this modification makes very little difference for good data but dramatically improves results for data with a non-Gaussian error distribution. © 2010 International Union of Crystallography Printed in Singapore - all rights reserved.
A new probabilistic approach is introduced for the determination of the absolute structure of a compound which is known to be enantiopure based on Bijvoet-pair intensity differences. The new method provides relative probabilities for different models of the chiral composition of the structure. The outcome of this type of analysis can also be cast in the form of a new value, along with associated standard uncertainty, that resembles the value of the well known Flack x parameter. The standard uncertainty we obtain is often about half of the standard uncertainty in the value of the Flack x parameter. The proposed formalism is suited in particular to absolute configuration determination from diffraction data of biologically active (pharmaceutical) compounds where the strongest resonant scattering signal often comes from oxygen. It is shown that a reliable absolute configuration assignment in such cases can be made on the basis of Cu Kalpha data, and in some cases even with carefully measured Mo Kalpha data.
This work describes the total and unambiguous assignment of the 750 MHz (1)H NMR spectrum of 3β-acetoxypregna-5,16-dien-20-one or 16-DPA (1), the well-known intermediate utilized in the synthesis of biological important commercial steroids. The task was made extracting the coupling constant values in the overlapped spectrum region by HSQC, and using these values in the (1)H iterative full spin analysis integrated in the PERCH NMR software. Comparison of the experimental vicinal coupling constants of 1 with the values calculated using Altona provides an excellent correlation. The same procedure, when applied to the published data of progesterone (2) and testosterone (3), afforded an acceptable correlation for 2 and a poor correlation for 3. In the last case, this suggested the reassignment of all four vicinal coupling constants for the methylene signals at the C-15 and C-16 positions, demonstrating the utility of this methodology.
Covering: 2000 to 2014This review covers conformational and configurational assignments in natural product molecules using chiroptical spectroscopy reported over the last 15 years. Special attention is given to vibrational optical activity methods associated with quantum mechanical calculations.
Even the most complex high-resolution 1-D NMR spectrum of a compound, composed of thousands of spectral lines, can be described within experimental accuracy by a relatively small number of spectral parameters, employing computerized spectral analysis based on a quantum mechanical model. The analysis can be used to explain complex spectra or spectral details, to obtain accurate spectral parameters, or to automate spectral analysis in order to save human resources. Accurate spectral parameters also provide a very efficient way to store and archive spectral information in the form of “Adaptive Spectral Libraries”, to be used, for example, in quantitative NMR. In addition, when combined with molecular modeling and spectral parameter prediction, computerized quantum mechanical spectral analysis offers a tool for automated structure verification. In this article, the principles, methods, strategies, and applications of computerized quantum mechanical NMR spectral analysis are reviewed.Keywords:NMR;quantum mechanics;spectral analysis;integral transform;total line shape;adaptive spectral library;PERCHit;PERCH software;ACA;QMTLS
The most widely used physical methods for the chemical analysis of natural products are currently well established, although in the case of vibrational circular dichroism (VCD), this is not yet the case. The first VCD experiments were performed only about 40 years ago, and it took some 20 additional years for the development of sufficiently sensitive instrumentation for these types of measurements, since VCD signals are three to four orders of magnitude less sensitive than infrared bands. In addition, it is less than 20 years since chemists started to have routine access to computers that allowed adequate quantum chemical calculations for this type of study. Thus, most applications of VCD to the study of sesquiterpenes, diterpenes, and molecules of similar size have been performed in the present century. Consequently, the main objective of this chapter is to provide some background and to show the use of circularly polarized infrared radiation for the determination of absolute configuration, and in some cases of the configuration of a specific chiral center, in natural products. This is a task that, in some cases, has been almost impossible to complete using other spectroscopic methods. VCD relies mostly on the comparison of an experimental spectrum, obtained in the mid infrared region of the electromechanical spectrum, with a spectrum obtained by density functional theory calculations, a situation that requires a detailed knowledge of the lower energy conformers of the particular molecule being studied.
Long-range coupling constants 5JHortho,OMe were measured in series of methoxyindoles, methoxycoumarins, and methoxyflavones by the modified J doubling in the frequency domain method. The COSY and NOESY spectra revealed the coupling of the –OMe group with a specific proton at the ortho position and its preferred conformation. Homonuclear 1H–1H couplings were confirmed by irradiation of the –OMe signal. Density functional theory calculations of 5JHortho,OMe using the modified aug-cc-pVTZ basis set evidenced that the Fermi contact term shows good agreement with the experimental J values. Accurate chemical shift and coupling constant values followed after iterative quantum mechanical spectral analysis using the PERCH software. Copyright © 2014 John Wiley & Sons, Ltd.
Chemical investigation of the aerial parts of Senecio polypodioides lead to the isolation of the new eudesmanoid 1β-angeloyloxyeudesm-7-ene-4β,9α-diol (1) and the known dirhamnosyl flavonoid lespidin (3), while from roots, the known 7β-angeloyloxy-1-methylene-8α-pyrrolizidine (5) and sarracine N-oxide (6), as well as the new neosarracine N-oxide (8), were obtained. The structure of 1 and 8 was elucidated by spectral means. Complete assignments of the (1) H NMR data for 5, 6, sarracine (7), and 8 were made using one-dimensional and two-dimensional NMR experiments and by application of the iterative full spin analysis of the PERCH NMR software. Copyright © 2014 John Wiley & Sons, Ltd.
Larvae of the monophagous herbivore,Trirhabda geminata, selectively eat particular plants and plant parts of its natural host,Encelia farinosa. Measurements of leaf damage and larval positions on branches through time support this observation. Time-lapse movie photography revealed that larvae are sufficiently mobile to search most of a plant in a 48-hr period and that aggregations were the result of larval activity and not directly the result of oviposition. Experiments withT. geminata larvae on artificial diets containing a range of natural concentrations of chemical extracts fromE. farinosa leaves showed that the larvae grew significantly slower and had a lower overall survivorship at the high concentration. Combining the results of all choice tests, larvae appeared unable to distinguish between high- and low-concentration agar diets. Considered individually, larval preferences for natural production concentrations changed as the season progressed. Early-season larvae preferred low-concentration leaves, while late-season larvae preferred high-concentrations. Measurements of chemical and nitrogen content of leaves selected by larvae in the field confirmed this pattern. Percent parasitism in field-collected larvae increased with season as the larval population decreased. This combination of slowed growth and increasing parasitism and predation is a putative defense strategy ofEncelia farinosa to prevent adaptation by a specialist herbivore to the total range of compounds elaborated.
The principal component of Encelia californica Nutt., encecalin, is not present in Encelia farinosa Gray, and the principal components of the latter, encelin and farinosin, are absent from the former. An F1 hybrid of the two species contains encecalin and farinosin and, in addition, a new lactone, structurally related to farinosin, which is absent from the parental species.
The components of the two major classes of natural products of the composite Encelia farinosa, sesquiterpene lactones and benzopyrans/benzofurans (chromenes), were resolved and quantified by high performance liquid chromatography. Geographic variation was examined over a range of mesic and xeric populations from California and Arizona (USA). The concentration of farinosin, a sesquiterpene lactone, varies significantly between individuals and seasonally within individuals. Ontogenetic comparisons revealed interpopulation variations within sesquiterpene lactones and chromenes. A geographic survey suggests the presence of two putative chemical races which vary in the amounts of sesquiterpene lactones and chromenes present. The adaptive significance of this quantitative variation is discussed.
Age-specific concentrations of farinosin, a sesquiterpene lactone, and encecalin and euparin, both chromenes, were measured in two semi-desert populations of Encelia farinosa in California over a three month period. All three natural products differed significantly in concentration. The average concentration of encecalin is 10 times that of euparin and eight times that of farinosin. Farinosin concentrations did not differ between the two populations, but both euparin and encecalin were in higher amounts in the UC Riverside population than in the Home Gardens population. Seasonal changes in natural products differed at the 0.05 level for all populations and compounds except euparin in the UC Riverside population. Percent nitrogen content of dried leaves was similar in both populations, but differed between collection dates. Water content did not vary significantly with leaf age on one collection date. The significance of these results is discussed in the light of herbivore history.
Three major sesquiterpenes (including an acid derivative as well as the two eudesmanolide lactones encelin and farinosin) from Encelia actoni and E. asperifolia were studied for toxicity and antifeedant activity against larvae of the polyphagous Spodoptera littoralis (Noctuidae). The sesquiterpene lactone encelin that contained an α-methylene-γ-lactone moiety was the most active compound encountered; the other two compounds showed only weak activity. When neonate larvae were kept on an artificial diet spiked with those concentrations of encelin that are present in the plant, high mortality of the larvae as well as significant growth reduction and a pronounced increase in the duration of the larval period were observed. The deleterious effects of encelin were due to toxicity as well as to antifeedant activities. Toxicity of encelin against larvae of S. littoralis was demonstrated by injecting the lactone into the haemolymph of final instar larvae. The LD50 was observed at 60 μg per larva corresponding to 111 mg kg−1. When encelin was incorporated into artificial diet at a concentration of 1 μmol g−1 fr. wt and offered to second instar larvae, larval weight gain as well as the amount of diet consumed were significantly reduced compared to controls indicating antifeedant activity.
Encelia farinosa Gray (Asteraceae: Heliantheae), a common shrub of the Sonoran desert, contains both sesquiterpene lactones and chromenes. Chloroform extracts of E. farinosa leaves were deterrent to larvae of Heliothis zea (Boddie) in choice tests, but farinosin, a eudesmanolide sesquiterpene lactone from E. farinosa, was not. Five sesquiterpene lactones from different Asteraceous genera were examined for their effect on the relative growth rate (RGR) of H. zea at 0.1% fresh weight in an artificial diet. Only dchydroleucodin, a guaianolide sesquiterpene lactone from Artemisia tridentata spp. vaseyana, reduced growth. Three closely related chromenes (precocene 1 and 2 and encecalin) were examined for their effects on survivorship (SR) and RGR of H. zea at five different concentrations. Precocene 1 and 2 exhibited the greatest activity on survivorship and growth rates, whereas encecalin was less active.
Thirty-six sesquiterpene lactones and related compounds were evaluated for antimicrobial activity against six strains of bacteria. The results obtained show that the beta unsubstituted cyclopentenone ring moiety contributes to moderate antimicrobial activity against Gram positive bacteria. The corresponding saturated compounds gave a more than ten-fold decrease in activity. The significant antimicrobial activity appears to be independent of the presence or absence of an α-methylene-γ-lactone moiety. A more than ten-fold diminution in antimicrobial activity was also observed when the beta position of the cyclopentenone ring was substituted. A similar result was found when the beta unsubstituted enone system was present in a six-membered ring. Enhanced activity was obtained by esterification of the hydroxyl group of helenalin as well as epoxidation of mexicanin-A.
The structure of a minor sesquiterpene lactone isolated from Gaillardia aristata Pursh. has been shown to be 3-epiisotelekin (2a). In the course of the structure proof evidence has been accumulated which requires revision of the structure of farinosin, a sesquiterpene lactone from Encelia farinosa Gray, to 13.
Two sesquiterpene lactones isolated from Encelia farinosa Gray (Compositae, tribe Heliantheae), farinosin and encelin, have been found to have the structures 2 and 5, respectively. Encelin, dehydrofarinosin, is also obtained by dehydration of farinosin. Farinosin is the principal lactonic constituent of the leaves while encelin is found in the green stems. As the plant becomes senescent and scarious the compounds disappear.
Chemical analysis of leaves from 12 different localities of Encelia farinosa (including var. phenicodonta and var. radians) collected on the peninsula of Baja California (Mexico) revealed the presence of various chemotypes that differed with regard to the concentrations of chromenes and sesquiterpene lactones. Localities of E. farinosa collected in the northern part of Baja California were characterized by high concentrations of the chromene encecalin (up to 252 μmol g−1 dry wt.), whereas the sesquiterpene lactone farinosin was not detected. Localities of E. farinosa collected at the southern tip of the peninsula lacked encecalin, but were shown to accumulate farinosin (up to 85 μmol g−1 dry wt.) instead. On the mainland of Mexico, as well as in Arizona (U.S.A.), farinosin concentrations varied from 18 to 44 μmol g−1 dry wt. for 10 different localities analyzed. Chromenes were not detected or present only in minor amounts (up to 13 μmol g−1 dry wt.), when compared to the samples from northern Baja California. Chemical variation within localities was small when compared to variation between different localities. Accumulation of encecalin and aridity seem to coincide at least on the peninsula of Baja California, as localities of E. farinosa that receive the least amount of rainfall contained the largest amounts of encecalin in their leaves. Leaves of E. farinosa that contained sufficiently large amounts of either encecalin or farinosin were both detrimental to neonate larvae of the polyphagous pest insect Spodoptera littoralis as shown by addition of the respective crude leaf extracts to artificial diet. Possible advantages of the observed intraspecific chemical variability of E. farinosa with regard to adaptation by generalist insect herbivores are discussed.
Bioactivity-guided fractionation of the methanolic root bark extract of Leucophyllum frutescens led to the identification of leubethanol (1), a new serrulatane-type diterpene with activity against both multi-drug-resistant and drug-sensitive strains of virulent Mycobacterium tuberculosis. Leubethanol (1) was identified by 1D/2D NMR data, as a serrulatane closely related to erogorgiane (2), and exhibited anti-TB activity with minimum inhibitory concentrations in the range 6.25-12.50 μg/mL. Stereochemical evidence for 1 was gleaned from 1D and 2D NOE experiments, from 1H NMR full spin analysis, and by comparison of the experimental vibrational circular dichroism (VCD) spectrum to density functional theory calculated VCD spectra of two diastereomers.
Spectral comparison is an important part of the assignment of the absolute configuration (AC) by vibrational circular dichroism (VCD), or equally by Raman optical activity (ROA). In order to avoid bias caused by personal interpretation, numerical methods have been developed to compare measured and calculated spectra. Using a neighbourhood similarity measure, the agreement between a computed and measured VCD or ROA spectrum is expressed numerically to introduce a novel confidence level measure. This allows users of vibrational optical activity (VOA) techniques (VCD and ROA) to assess the reliability of their assignment of the AC of a compound. To that end, a database of successful AC determinations is compiled along with neighbourhood similarity values between the experimental spectrum and computed spectra for both enantiomers. For any new AC determination, the neighbourhood similarities between the experimental spectrum and the computed spectra for both enantiomers are projected on the database allowing an interpretation of the reliability of their assignment.
The VCD spectrum of the monoterpene (-)-myrtenal (1) was compared with theoretical spectra using ab initio density functional theory (DFT) calculations at the B3LYP/6-31G(d,p), B3LYP/6-31G+(d,p), B3LYP/6-311G+(d,p), B3LYP/DGDZVP, and B3PW91/DGTZVP levels of theory. Conformational analysis of 1 indicated that the lowest energy conformer was s-trans-C2-C10, which contributes more than 98.5% to the total conformational population regardless of the employed level of theory. The use of a recently developed confidence level algorithm demonstrated that VCD spectra calculated for the main conformer, using the indicated hybrid functionals and basis set, gave no significant changes, from where it follows that B3LYP/DGDZVP calculations provide a superior balance between computer cost and VCD spectral accuracy. The DGDZVP basis set demanded around a quarter the time than the 6-311G+(d,p) basis set while providing similar results. The spectral comparison also provided evidence that the levorotatory enantiomer of myrtenal has the 1R absolute configuration.
The impact of acidic deposition on interactions between the plant Encelia farinosa and the herbivorous beetle Trirhabda geminata (Chrysomelidae) was determined under greenhouse conditions. Acidic fogs (pH 2.75) did not significantly affect the overall foliar concentrations of water or soluble protein as compared with control fogs (pH 5.6). Nonetheless, E. farinosa foliage was altered by exposure to three 3-h acidic fogs such that growth and biomass gain by T. geminata increased by more than 30% as compared to beetles feeding on control-fogged plants. Thus, previous indications that changes in soluble proteins or water content were responsible for increased biomass gain and growth of T. geminata cannot be substantiated by this study. Additionally, changes in the plant defensive chemistry were not responsible for increased herbivore growth, as farinosin, encecalin, and euparin foliar concentrations did not vary significantly between fog treatments. Significant increases in CO2 assimilation rates of E. farinosa exposed to acidic fogs were documented at 3, 7, and 21 days following treatment, suggesting that carbohydrate-based products of increased plant metabolism may have played a role (e.g. soluble carbohydrates). However, the key factors responsible for increasing herbivore performance on acidic-fogged E. farinosa remain largely unknown.
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