Comment on "Study of enantioselective metolachlor adsorption by activated carbons" by A. Gomis-Berenguer, I. Laidin, S. Rononcial and B. Cagnon, RSC Advances, 2020, 10, 40321.

A recent study on adsorption of metolachlor on activated carbons carried out by Gomis-Berenguer et al. has reported higher adsorption capacity for pure S-metolachlor compared with the adsorption capacity for the racemic mixture of this pesticide. The authors claim that the adsorption is enantioselective, with the activated carbon being more efficient for the adsorption of the S enantiomer compared with the R enantiomer. In this comment, we question this explanation (since the non-chiral surface of an activated carbon would never be selective towards one enantiomer instead of the other) and we offer some possible answers supported by theoretical calculations.

Evidence of the presence of minor tautomeric forms in selected nitroanilines.

RATIONALE: Nitroanilines can exist in several tautomeric forms: nitro-amino, nitro-imino, and aci-imino. The importance of evaluating minor tautomeric species comes from the fact that even less abundant tautomers have been proved to play important roles in reaction mechanisms. METHODS: Electron ionization mass spectra of the pesticide Pendimethalin and four related nitroanilines were recorded at 70 eV to find information about the presence of minor tautomeric forms in the gas phase. The existence of the possible tautomers was evaluated by studying specific fragmentation pathways, which were confirmed by tandem mass spectrometry (MS/MS) experiments. Further supporting information was obtained by studying the structures of some intermediate compounds by theoretical calculations at the B3LYP 6-311++G(d,p) level. RESULTS: The mass spectrum of Pendimethalin suggests the coexistence of the nitro-amine tautomer (the most stable form) with four possible less stable tautomers in equilibrium. The fragmentation routes were used to explain analogous peaks in two related compounds. However, the spectra of two other related compounds that cannot follow the proposed route of fragmentation for nitro-imine tautomers do not show the analogous peak. Theoretical calculations were used to correlate the precursor cation with the proposed fragmentation pathway. CONCLUSIONS: By the study of mass spectra and proposed fragmentation pathways it can be concluded that, although the nitro-amine is the most abundant species within the system, minor tautomers (nitro-imine and aci-imine) coexist in the gas phase.

Adsorption of bentazone and imazapyc from water by using functionalized silica: Experimental and computational analysis.

In this study, silica and functionalized silica materials (3-aminopropyl and 3-mercapto derivatives) were successfully used for the removal of the pesticides bentazone and imazapyc from aqueous solutions. Adsorbent materials were characterized by BET isotherms and FT-IR spectroscopy (confirming the functionalization), and their equilibrium adsorption capacity was evaluated at different ionic strengths. It is observed that the maximum adsorption capacities decrease in the order 3-aminopropyl-derivative > silica >3-mercaptopropyl derivative. An increase in ionic strength produces an enhancement in the removal of pesticides. All isotherms are Ib-type and follow the Langmuir model, suggesting a monolayer physical adsorption process.

Lidocaine protects neurons of the spinal cord in an excitotoxicity model.

Motor Neuron Disease disorders, described in domestic animals, are characterized by neuronal degeneration at the spinal cord. Excitotoxicity is a crucial factor for the selective loss of these neurons, being the fundamental processes involved in lesion progression after spinal cord injury, where glutamate is one of the main neurotransmitters involved. Kainic acid (KA) resembles the effects induced by the pathological release of glutamate. Lidocaine administered by different routes exerts some neuroprotective effects in the CNS. The aim of the present work was to determine whether lidocaine simultaneously injected with KA into the spinal cord could prevent the excitotoxic effects of the latter. Sprague-Dawley rats were injected by intraparenchymal route with KA or with KA plus 0.5% lidocaine into the C5 segment. Sham rats were injected with saline. Animals were motor and sensory tested at 0, 1, 2, 3, 7 and 14 post-injection days and then euthanized. Sections of the C5 segment were used for histological and immunohistochemical analysis. No KA-induced motor and sensitive impairments were observed when lidocaine was simultaneously injected with KA. Moreover, neuronal counting was statistically higher when compared with KA-injected animals. Thus, lidocaine could be considered as a neuroprotective drug in diseases and models involving excitotoxicity.

Reactive Oxygen Species-mediated Degradation of Antidiabetic Compounds: Cytotoxic Implications of Their Photodegradation Products.

Reactive oxygen species (ROS) have been described in their double physiological function, helping in the maintenance of health as well as contributing to oxidative stress. Diabetes mellitus is a chronical disease nearly related to oxidative stress, whose treatment (in type II variant) consists in the administration of antidiabetic compounds (Andb) such as Gliclazide (Gli) and Glipizide (Glip). In this context, as Andb may be exposed to high ROS concentrations in diabetic patients, we have studied the potential ROS-mediated degradation of Gli and Glip through photosensitized processes, in the presence of Riboflavin (Rf) vitamin. We found that singlet oxygen (O2 (1 ∆g )) participated in the Rf-sensitized photodegradation of both Andb, and also superoxide radical anion in the case of Gli. Two principal products derived from O2 (1 ∆g )-mediated degradation of Gli were identified and their chemical structures characterized, through HPLC mass spectrometry. O2 (1 ∆g )-mediated degradation products and their toxicity was assayed on Vero cell line. These studies demonstrated that neither Gli nor its photoproducts caused cytotoxic effect under the experimental conditions assayed. Our results show strong evidences of ROS-mediated Andb degradation, which may involve the reduction or loss of their therapeutic action, as well as potential cytotoxicity derived from their oxidation products.

Tautomerism of uracil and related compounds: A mass spectrometry study.

It has been demonstrated that uracil has a preponderant tautomeric form, but it is also known that different tautomers co-exist in this equilibrium. In this work, mass spectrometry is used as a helpful tool to analyse the equilibria, using derivative compounds to forbid the presence of some tautomers and ion trap mass spectrometry to follow relevant fragmentation pathways. Theoretical calculations were performed to confirm tautomers abundance by energy minimization in gas phase. Analysis of mass spectra of uracil, three methyl-substituted uracils, 2-thiouracil and three benzouracils suggest that uracil exists mainly as three tautomers in gas phase: one major structure that corresponds to the classical structure of uracil (pyrimidine-2,4(1H,3H)-dione) bearing two carbonyls and two NH moieties, and two minor enolic forms (4-hydroxypyrimidin-2(1H)-one and 2-hydroxypyrimidin-4(1H)-one). Such tautomeric distribution is supported by theoretical calculations, which show that they are the three most stable tautomers.

Comment on "Structural and vibrational studies on 1-(5-Methyl- [1,3,4] thiadiazol-2-yl)-pyrolidin-2-ol" [Spectrochimica Acta Part A, 152 (2016) 252-261]. The importance of intramolecular OH⋯N hydrogen bonding in the conformational properties of thiadiazol-pyrrolidin-2-ol bearing species.

The title paper [1] reports a study on the spectroscopic and physicochemical properties of 1-(5-methyl- [1,3,4]thiadiazol-2-yl)-pyrrolidin-2-ol (MTPN) based on experimental and theoretical data. The latter ones are based on the computed molecular structure for a rather unusual conformer. Here, after a careful analysis of the conformational space of MTPN, the most stable conformation was determined for the molecule isolated in a vacuum, which results to be 21.9kJ/mol more stable than the conformer reported previously. Our study also includes the closely related species 1-(5-trifluoromethyl- [1,3,4]thiadiazol-2-yl)-pyrrolidin-2-ol (FMTPN). An intramolecular OH⋯N hydrogen bond determines the conformational behavior of the [1,3,4]thiadiazol-2-yl)-pyrrolidin-2-ol group as demonstrated by Natural Bond Orbital population analysis.

Determination of heats of tautomerization nitrile-ketenimine by mass spectrometry.

Tautomerism of some nitriles has been studied by mass spectrometry. The analysis of the corresponding mass spectra has allowed to assign some fragmentations to specific tautomers and to determine heats of tautomerization through temperature effects and electron energy studies. Experimental determinations are supported by theoretical calculations. The joint analysis of mass spectrometry and DFT-B3LYP data indicate that this tautomeric equilibrium can be studied by the experimental spectrometric strategy employed.

The evidence for the occurrence of tautomeric structures for selected aldehydes and thioaldehydes.

Mass spectra of selected aldehydes and thioaldehydes have been analyzed and specific fragmentation assignments have been done to keto and enol tautomers, although many peaks can be assigned to both forms (i.e. mass spectra are superimposed on one another). The enolization rate for aldehydes is generally favored by the increase in the steric effect caused by α substitution to the carbonyl group. The analysis of the corresponding mass spectra has allowed to establishing an acceptable correlation between selected ion abundances ratios and approximate enolization equilibrium constants (carried out by means of DFT calculations). The influence of temperature on the enol/keto selected fragments abundance ratios (for different aldehydes and thioaldehydes and for different pair of ions of the same compound whenever possible) is studied in order to estimate the enthalpy difference for the tautomeric equilibria. The results indicate that the thioketo-thioenol equilibrium can be studied by mass spectrometry and the ionization in the ion source have negligible effect on the position of that equilibrium.