Tracing the minimum-energy path on the free-energy surface

The free energy profile of a reaction can be estimated in a molecular-dynamics approach by imposing a mechanical constraint along a reaction coordinate (RC). Many recent studies have shown that the temperature can greatly influence the path followed by the reactants. Here, we propose a practical way to construct the minimum energy path directly on the free energy surface (FES) at a given temperature. First, we follow the blue-moon ensemble method to derive the expression of the free energy gradient for a given RC. These derivatives are then used to find the actual minimum energy reaction path at finite temperature, in a way similar to the Intrinsic Reaction Path of Fukui on the potential energy surface [K Fukui J. Phys. Chem. 74, 4161 (1970)]. Once the path is know, one can calculate the free energy profile using thermodynamic integration. We also show that the mass-metric correction cancels for many types of constraints, making the procedure easy to use. Finally, the minimum free energy path at 300 K for the addition of the 1,1'-dichlorocarbene to ethylene is compared with a path based on a simple one-dimensional reaction coordinate. A comparison is also given with the reaction path at 0 K.Comment: Minor revisions: Citation and Equation numbers corrected. 26 pages, 6 figures, to appear in J. Chem. Phy

Effecs of damage by wheat bug, Aelia germari [Hemiptera : Pentatomidae], on grain quality and on reproductive performance of the rice weevil, Sitophilus oryzae [Coleoptera : curculionidae]

La punaise des céréales, Aelia germari, cause des dégâts au champ, avant la maturité du grain, qui ont pour conséquence majeure une réduction de la valeur boulangère du blé destiné à la panification. Pour tenter de mettre en évidence les modifications induites par les attaques de punaises sur la valeur nutritive du grain, un essai biologique a été réalisé sur le charançon du riz Sitophilus oryzae, qui se développe dans l'endosperme du grain. Le charançon du riz a été élevé sur des échantillons de blé du cultivar 'Manondemias', dans lesquels ont été incorporés des taux différents (0, 4, 8, et 13%) de grain attaqué par la punaise phytophage A. germari. Sur les échantillons de blé punaisé à 4, 8 ou 13%, la durée moyenne de développement de S. oryzae était plus courte que sur le grain non punaisé. Cependant, le calcul des corrélations sur les variables prises deux à deux a montré que ni l'indice de sensibilité Dobie aux attaques d'insectes, ni la descendance par femelle de charançon, ni la perte de poids du grain consécutive au développement complet de S. oryzae, n'étaient corrélés avec le pourcentage de grains punaisés dans l'échantillon. À partir d'une analyse multivariable globale, associant les propriétés physicochimiques ou rhéologiques des grains avec les paramètres décrivant la réponse biologique du charançon S. oryzae, il a été montré que le blé sans attaque apparente de punaise pouvait être associé à une longue durée de développement pour S. oryzae, un nombre plus faible de descendants, une force boulangère, une teneur en azote et un temps de chute élevés, ainsi qu'à une faible teneur en cendres et en azote assimilable du grain.The wheat bug, Aelia germari, feeds on developing wheat kernels, causing a loss in baking quality of the harvested wheat (Triticum aestivum). The possible nutritional changes in the bug-damaged kernels after the harvest were tested in a bioassay using the rice weevil Sitophilus oryzae, which feeds on the kernel endosperm. The weevils were reared on baking wheat cultivar 'Manon demias', grown in Algeria, with different levels (0, 4, 8 and 13%) of wheat bug (A. germari) damaged kernels. Rice weevil reared on 4, 8 or 13% damaged wheat had a shorter development time than on wheat with no damage. However, pair-wise correlations showed that Dobie's index for susceptibility of grain for stored-products insect attack, number of progeny per female and kernel weight loss were not correlated with the percentage of damaged kernels. Using multivariate analysis, the general characteristics of high quality grain were associated with long development times for S. oryzae, low number of progeny, high baking strength, high falling number, high nitrogen, low ash content, and low assimilable nitrogen

Cross-contamination of oilseeds by insecticide residues during storage

Pesticide residues are found in oilseeds (rapeseed and sunflower) and crude oils: they are mainly organophosphate insecticides (pirimiophos-methyl, malathion when authorized) used in empty storage facilities and for direct application to stored cereal grain. Even if some secondary pests are found in stored oilseeds, French regulations do not allow use of these insecticides on stored oilseeds. These residues arise from cross-contamination from storage bins and grain handling equipment of grain stores, and not from illegal use. This uptake of insecticide residues from their storage environment by oilseeds may lead to residue contents that exceed regulatory limits. A three-year investigation in grain storage companies allowed us to follow the course of sunflower batches and rapeseed batches during storage seasons 2006-2007, 2007-2008 and 2008-2009, from reception at the storage facility to outloading. Each of these batches was sampled at outloading, and was analyzed for insecticide residues. Traceability of oilseeds established by grain-store managers allowed us to identify cross-contamination sources. The insecticides that were most commonly detected were pirimiophos-methyl, malathion, and dichlorvos (in sunflower 2006-2007), plus chlorpyriphos-methyl and deltamethrin. Pirimiophos-methyl was the most commonly detected active substance, and caused the most cases of non-accordance with regulatory levels in rapeseed. Cross-contamination could have occurred when cereal grains were treated upon receipt, when rapeseed was also delivered, especially when treatments were done systematically to the cereal grains. For sunflower, the main cross-contamination hazard resulted from treatment of cereals at the period of receipt or at their outloading, just before sunflower seeds batches were outloaded. Another situation led to cross-contamination, but generally at a lower extent: oilseeds stored in bins that contained previously treated cereals, or loaded in empty bins with handling equipment treated before the receipt of oilseeds.Keywords: Oilseed storage, Cross-contamination, Insecticide residues, Rapeseed, Sunflowe

Insecticidal properties of whole meal or protein extracts of the bean seeds Phaseolus vulgaris L. on juvenile stages of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae)

Callosobruchus maculatus is a pest that causes serious damage to Cicer arietinum (chickpea) stored seeds, but that does not develop in seeds of other legumes such as Phaseolus vulgaris or Pisum sativum. The bean seed is rich in antinutritional compounds known to inhibit the development of C. maculatus. In an integrated approach to protect stocks of Cicer arietinum against attacks of this weevil, this study had the main objective to assess the potential of using bean flours from a wild bean Vigna caracalla, four varieties of P. vulgaris, and of a crude extract from P. vulgaris lectins seed. The extraction method was chosen to extract lectin-like protein compounds. The biological effects of bean flour or protein extracts were observed on artificial seeds composed from C. arietinum flour enriched with P. vulgaris whole flour or extracts incorporated at different percentages. The antinutritional activity either of bean-seed whole meal or of lectin-like extracts was determined by the analysis of different biological parameters. Incorporation of bean flour mixed with chickpeas decreased fertility and fecundity of female C. maculatus and caused longer development times of juvenile stages. Peptide extracts of the P. vulgaris reduced fecundity and survival of C. maculatus. Keywords: Callosobruchus maculatus, Phaseolus vulgaris, Lectin-like extract, Insecticidal properties, Artificial see

A systemic approach of qualitative changes in the stored wheat ecosystem: prediction of deterioration risks in unsafe storage conditions in relation to relative humidity, infestation by Sitophilus oryzae (L.), and variety influence

A multidimensional laboratory trial was carried out to identify how key overall quality traits of different common wheat varieties change during storage, to understand their interactions in the process of deterioration, and finally to reveal underlying trends of critical storage conditions that may endanger grain quality. A large set of qualitative criteria were followed on grain batches of three wheat varieties with various qualities for food processing, which were stored for 160 d at 22-23°C, under two different relative humidities (r.h.), and with or without infestation by the rice weevil Sitophilus oryzae. All variables involved in quality components assessment were recorded at 42-d periods. From the Pearson’s product moment correlation matrix, it was observed that the quality traits that correlated significantly to biotic variables (insect and fungal species dynamics) were: moisture content, hL mass, seed viability, and fat acidity of extracted flour. The interactions between biotic deteriorative factors and qualitative trait changes revealed through principal component analysis (PCA) were significant between four factors explaining the major part of qualitative criteria variance: storage duration; moisture content; hidden infestation density; fungal contamination level. The rate of increase of insect population was significantly different among varieties. PCA revealed that the significant difference in qualitative deterioration pattern among the three varieties was not related to their hardness, but to a different r.h. affinity. The germination rate was the qualitative criteria the more early declining during storage. The technological properties of extracted flour from each variety were significantly affected only when insect density exceeded 1000 insects per kg, a situation only observed in hot-spots. This work highlighted the trends of variation in quality traits of wheat varieties when stored under critical conditions. It can be used in IPM approaches to predict the susceptibility of a wheat variety to insect and fungi damage during storage. Keywords: Common wheat, Variety, Qualitative change, Insect pest, Fungal spoilage, Multivariate analysi

Multivariate analysis of the temporal changes of fungal communities in unsafe storage conditions of some common wheat varieties in relation to relative humidity level and rice weevil infestation

Fungal colonization of stored grain bulks is a major threat for mycotoxin contamination and reduction in viability of grain when stored under unsafe conditions, e.g. under high r.h. and insect presence. An investigation was carried out to identify the trends of the changes in the fungal species communities during storage of wheat grain under these unsafe storage conditions. The distribution change of fungi genera was monitored on small grain samples of three wheat varieties with different kernel size and hardness (soft, medium-hard and hard), during 160 d storage at constant temperature of 22-23°C, at two r.h. levels, and with or without an infestation by the rice weevil . According to their behavioral differences related to grain water activity affinity, fungi genera were classified in three groups: i/ The hydrophilic group of field fungi (Fusarium, Geniculifera, Sepedonium, and Chrysogenum); ii/ The intermediate semi-xerophilic fungi (Alternaria, Mucor, Ulocladium, Epicoccum, and Arthrobotrys); iii/ The storage xerophilic fungi (Penicillium and Aspergillus). Temporal abundance of these three groups with grain storage time and condition was observed in weak relation with wheat variety and insect presence. The multivariate comparison of the different experimental situations revealed a difference in the susceptibility of varieties to fungal species colonization in close relationship with the final equilibrium level between ambient r.h. and grain moisture content which was observed variety-dependent. This difference was not related to grain hardness but rather to a different r.h. affinity. For one variety (Apache), the germination rate was declining more rapidly than for the two others with storage time. Any significant relation between sound and infested grain condition and the contamination rate by storage fungi could be found. The susceptibility of the three wheat varieties to critical storage conditions and fungal colonization may lead in one variety to a hot-spot formation.Keywords: Common wheat, Variety, Fungal microflora, Insect pest, Fungi abundance chang

A valence bond view of isocyanides' electronic structure

International audienceHigh level Valence Bond calculations support a predominantly carbenic electronic structure for isocyanides, with a secondary zwitterionic character, despite their linear geometry. This geometry results from the significant energetic stabilization due to nitrogen π lone pair donation. Results are not changed by substitution or solvation effects