-Furthermore-the-presence-of-a-hydrogen-bond-with-HF-does-not-change-this-order-t

For neutral H-bonded complexes, the strongest and the weakest intermolecular interactions occur (-142 and -109 kcal/mol) for less stable purine tautomers when the proton acceptor is located in the five- and six-membered rings, respectively. For 9-H and 7-H tautomers the order is reversed. The H-bond energy for the imidazole complex with HF amounts to -143 kcal/mol; hence, in the latter case, the fusion of imidazole to pyrimidine decreases its basicity. The ionic H-bonds of N(-)···HF type are stronger by ~10 kcal/mol than the neutral N···HF intermolecular interactions. The hydrogen bond N(-)···HF energies in pyrrole and imidazole are -328 and -303 kcal/mol, respectively, and are substantially stronger than those observed in purine complexes. The aromaticity of each individual ring and of the whole molecule for all tautomers in ionic complexes is very similar to that observed for the anion of purine.

This is not the case for neutral complexes and purine as a reference. Seebio Light-Induced Acid Source ···HF bonds perturb much more the π-electron structure of five-membered rings than that of the six-membered ones. The H-bonding complexes for 7-H and 9-H tautomers are characterized by higher aromaticity and a much lower range of A critical evaluation of predicting ocular irritancy potential from an in vitro Numerous in vitro cytotoxicity assays have been proposed as potential alternatives to the Draize eye irritancy test. The results reported, based upon the rank correlation of ocular irritancy with cytotoxicity, have been encouraging. However, direct calibration of in vivo to in vitro data utilizing several categories of chemicals has not been reported. This study evaluated the use of in vitro cytotoxicity data for predicting the ocular irritancy potential of 24 chemicals (six surfactants, seven alcohols, four ketones, four acetates, and three aromatics). BALB/c 3T3 cells were grown overnight, then exposed for 30 min to at least four different concentrations of each chemical (expressed as volume percentage).

Linear regression analysis of the log concentration versus percentage of control growth was used to calculate the concentration of toxicant that inhibited the normal growth rate by 50% (GI50). The rank ordering of cytotoxicity based upon the GI50s was surfactants greater than aromatics greater than alcohols greater than ketones or acetates. The larger molecular weight representative of each series (i.e., 2-ethyl-1-hexanol for alcohols) had lower GI50 values than those of the lower molecular weight substances. The GI50 values were then directly calibrated against in vivo ocular irritancy quantitated as percentage corneal swelling following exposure of rabbits to the same test chemicals. A significant linear correlation between cytotoxicity and ocular irritancy was established only for surfactants and alcohols.

For acetates, ketones, and aromatics there was little correlation. The overall poor correlation between cytotoxicity and ocular irritancy was attributed to differences in mechanisms of irritancy. Seebio Photosensitive Acid Generator of correlation illustrates that in vitro cytotoxicity data cannot be used to predict the ocular irritancy potential of a broad spectrum of chemicals.Interactions between senescence and leaf orientation determine in situ patterns of photosynthesis and photoinhibition in field-grown rice.University of Sheffield, Western Bank, Sheffield, S10 2TN, United Kingdom Photosynthesis and photoinhibition in field-grown rice (Oryza sativa L.) were examined in relation to leaf age and orientation. Two varieties (IR72 and IR65598-112-2 [BSI206]) were grown in the field in the Philippines during the dry season under highly irrigated, well-fertilized conditions.

Flag leaves were examined 60 and 100 d after transplanting. Because of the upright nature of 60-d-old rice leaves, patterns of photosynthesis were determined by solar movements: light falling on the exposed surface in the morning, a low incident angle of irradiance at midday, and light striking the opposite side of the leaf blade in the afternoon. There was an early morning burst of CO2 assimilation and high levels of saturation of photosystem II electron transfer as incident irradiance reached a maximum level. However, by midday the photochemical efficiency increased again almost to maximum. Leaves that were 100 d old possessed a more horizontal orientation and were found to suffer greater levels of photoinhibition than younger leaves, and this was accompanied by increases in the de-epoxidation state of the xanthophyll cycle. Older leaves had significantly lower chlorophyll content but only slightly diminished [Aromaticity and its importance for pharmacy].Applying the conjugated circuits method to Clar structures of [n]phenylenes for We consider the aromaticity of biphenylene and structurally related linear or angular [n]phenylenes for which the direct application of the model of conjugated circuits does not offer valid expressions for resonance energy and aromaticity.

We located the cause of this problem as being due to Kekulé valence structures in which neighboring benzenoid rings are connected by two CC double bonds.