Coordination numbers in hydrated Cu(II) ions

The potential energy surface of [Cu(H₂O)_n]²⁺ clusters with n?=?12, 16, and 18 was explored by using a modified version of the simulated annealing method. Such exploration was carried out by using the PM7 semiempirical method to obtain around 100,000 isomers, which provide candidates to be optimized with PBE0-D3, M06-2X, and BHLYP exchange-correlation functionals coupled with the 6–311++G** basis set. These methods based on the Kohn-Sham approach delivered isomers with coordination numbers of 4, 5, and 6. The analysis used to obtain coordination numbers was based on geometrical parameters and the quantum theory of atoms in molecules (QTAIM) approach. Our methodology found only one isomer with fourfold coordination and its probabilities to appear in these clusters are quite small for high temperatures. The procedure used in this article predicts important populations of fivefold and sixfold coordination clusters, in fact, the fivefold coordination dominates for PBE0-D3 and BHLYP methods, although the sixfold coordination starts to be important when the number of water molecules is increased. The nature of axial and equatorial contacts is discussed in the context of the QTAIM and the noncovalent interaction index (NCI), which gives a clear classification of such orientations. Also, these methods suggest a partial covalent interaction between the Cu²⁺ and water molecules in both positions; equatorial and axial.     

Stable isotope analysis of modern human hair collected from Asia (China,India, Mongolia,and Pakistan)

We report isotopic data (δ²H, δ¹⁸O n = 196; δ¹³C, δ¹⁵N n = 142; δ³⁴S n = 85) from human hair and drinking water (δ²H, δ¹⁸O n = 67) collected across China, India, Mongolia, and Pakistan. Hair isotope ratios reflected the large environmental isotopic gradients and dietary differences. Geographic information was recorded in H and O and to a lesser extent, S isotopes. H and O data were entered into a recently developed model describing the relationship between the H and O isotope composition of human hair and drinking water in modern USA and pre‐globalized populations. This has anthropological and forensic applications including reconstructing environment and diet in modern and ancient human hair. However, it has not been applied to a modern population outside of the USA, where we expect different diet. Relationships between H and O isotope ratios in drinking water and hair of modern human populations in Asia were different to both modern USA and pre‐globalized populations. However, the Asian dataset was closer to the modern USA than to pre‐globalized populations. Model parameters suggested slightly higher consumption of locally producedfoods in our sampled population than modern USA residents, but lower than pre‐globalized populations. The degree of in vivo amino acid synthesis was comparable to both the modern USA and pre‐globalized populations. C isotope ratios reflected the predominantly C₃‐based regional agriculture and C₄ consumption in northernChina. C, N, and S isotope ratios supported marine food consumption in some coastal locales. N isotope ratios suggested a relatively low consumption of animal‐derived products compared to western populations. Am J Phys Anthropol 2010. © 2009 Wiley‐Liss, Inc.     

Microsolvation of aminoethanol: a study using DFT combined with QTAIM

The microsolvation of aminoethanol (AE) with one, two, three or four water molecules was investigated using a density functional theory (DFT) approach. Quantum theory of atoms in molecules (QTAIM) analyses were employed to elucidate the hydrogen-bonding characteristics of AE–(H₂O) _n (n = 1–4) complexes. The results showed that AE tends to break its intramolecular OH^AE···N^AE hydrogen bond (H-bond) upon microsolvation and form intermolecular H-bonds with water molecules, while complexes that retain the intramolecular OH^AE···N^AE H-bond show reduced stabilities. The intermolecular H-bond that forms between the nitrogen atom of AE and the hydroxyl of a water molecule is the strongest one for the most stable AE–(H₂O) _n (n = 1–4) complexes, and as n increases from 1 to 4 they grow stronger. The partial covalent character of this H-bond was confirmed by QTAIM analyses. Many-body interaction analysis showed that the relaxation energies and two- and three-body energies make significant contributions to the binding energies of the complexes.     

DFT study of (17)O, (1)H and (13)C NMR chemical shifts in two forms of native cellulose, I(α) and I(β)

This computational study is intended to shed light on the crystalline and molecular structure, together with the hydrogen bonding (H-bonding) differences between two forms of native cellulose. DFT calculations were carried out to characterize the ¹⁷O, ¹H and ¹³C nuclear magnetic resonance (NMR) parameters in cellulose I_α and I_β with the B3LYP functional employing the 6–311++G7 and 6–31+G1 basis sets. Geometry optimization revealed that the average HB length is shortened by 0.01–0.08 Å when the chains are aligned, whereas the average bond angle increases by about 4–8° exhibiting the enhancement of HB strength. For the isolated cellotetramer chains, the isotropic ¹⁷O–H chemical shifts were plotted as a function of HB length. Our results indicated that as the HB length in cellotetramer I_α increases, the ¹⁷O–H chemical shift isotropy increases, but this parameter changes in the opposite direction for the other structure. Moreover, B3LYP/6–311++G7 calculations reveal that there is an acceptable correlation between the calculated ¹³C chemical shifts of the two structures and their experimental values.     

Lack of photosynthetic or stomatal regulation after 9 years of elevated [CO2] and 4 years of soil warming in two conifer species at the alpine treeline

Alpine treelines are temperature‐limited vegetation boundaries. Understanding the effects of elevated [CO₂] and warming on CO₂ and H₂O gas exchange may help predict responses of treelines to global change. We measured needle gas exchange of Larix decidua Mill. and Pinus mugo ssp. uncinata DC trees after 9 years of free air CO₂ enrichment (575 µmol mol^?1) and 4 years of soil warming (+4 °C) and analysed δ¹³C and δ¹⁸O values of needles and tree rings. Tree needles under elevated [CO₂] showed neither nitrogen limitation nor end‐product inhibition, and no down‐regulation of maximal photosynthetic rate (A_max) was found. Both tree species showed increased net photosynthetic rates (A_n) under elevated [CO₂] (L. decidua: +39%; P. mugo: +35%). Stomatal conductance (g_H2O) was insensitive to changes in [CO₂], thus transpiration rates remained unchanged and intrinsic water‐use efficiency (iWUE) increased due to higher A_n. Soil warming affected neither A_n nor g_H2O. Unresponsiveness of g_H2O to [CO₂] and warming was confirmed by δ¹⁸O needle and tree ring values. Consequently, under sufficient water supply, elevated [CO₂] induced sustained enhancement in A_n and lead to increased C inputs into this ecosystem, while soil warming hardly affected gas exchange of L. decidua and P. mugo at the alpine treeline.     

Effects of leaf water evaporative 2H‐enrichment and biosynthetic fractionation on leaf wax n‐alkane δ2H values in C3 and C4 grasses

Leaf wax n‐alkane δ²H values carry important information about environmental and ecophysiological processes in plants. However, the physiological and biochemical drivers that shape leaf wax n‐alkane δ²H values are not completely understood. It is particularly unclear why n‐alkanes in grasses are typically ²H‐depleted compared with plants from other taxonomic groups such as dicotyledonous plants and why C3 grasses are ²H‐depleted compared with C4 grasses. To resolve these uncertainties, we quantified the effects of leaf water evaporative ²H‐enrichment and biosynthetic hydrogen isotope fractionation on n‐alkane δ²H values for a range of C3 and C4 grasses grown in climate‐controlled chambers. We found that only a fraction of leaf water evaporative ²H‐enrichment is imprinted on the leaf wax n‐alkane δ²H values in grasses. This is interesting, as previous studies have shown in dicotyledonous plants a nearly complete transfer of this ²H‐enrichment to the n‐alkane δ²H values. We thus infer that the typically observed ²H‐depletion of n‐alkanes in grasses (as opposed to dicots) is because only a fraction of the leaf water evaporative ²H‐enrichment is imprinted on the δ²H values. Our experiments also show that differences in n‐alkane δ²H values between C3 and C4 grasses are largely the result of systematic differences in biosynthetic fractionation between these two plant groups, which was on average ?198‰ and?159‰ for C3 and C4 grasses, respectively.     

Oxygen isotope analysis of human bone phosphate evidences weaning age in archaeological populations

Here we report bone phosphate oxygen (δ¹⁸O_p) values from perinates/neonates and infants (<3.5 years; n = 32); children (4–12 years; n = 12); unsexed juveniles (16–18 years; n = 2); and adult bones (n = 17) from Wharram Percy, North Yorkshire, England, in order to explore the potential of this method to investigate patterns of past breastfeeding and weaning. In prior studies, δ¹⁵N and δ¹³C analyses of bone collagen have been utilized to explore weaning age in this large and well‐studied assemblage, rendering this material highly appropriate for the testing and development of this alternative method targeting the inorganic phase of bone. Data produced reveal ¹⁸O‐enrichment in the youngest perinatal/neonatal and infant samples, and an association between age and bone δ¹⁸O_p (and previously‐published δ¹⁵N values), with high values in both these isotope systems likely due to breastfeeding. After the age of 2–3 years, δ¹⁸O_p values are lower, and all children between the ages of 4 and 12, along with the vast majority of sub‐adults and adults sampled (aged 16 to >50 years), have δ¹⁸O_p values consistent with the consumption of local modern drinking water. The implications of this study for the reconstruction of weaning practices in archaeological populations are discussed, including variations observed with bone δ¹⁵N_coll and δ¹⁸O_p co‐analysis and the influence of culturally‐modified drinking water and seasonality. The use of this method to explore human mobility and palaeoclimatic conditions are also discussed with reference to the data presented. Am J Phys Anthropol 157:226–241, 2015. © 2015 Wiley Periodicals, Inc.     

Computational benchmark for calculation of silane and siloxane thermochemistry

Geometries of model chlorosilanes, R₃SiCl, silanols, R₃SiOH, and disiloxanes, (R₃Si)₂O, R?=?H, Me, as well as the thermochemistry of the reactions involving these species were modeled using 11 common density functionals in combination with five basis sets to examine the accuracy and applicability of various theoretical methods in organosilicon chemistry. As the model reactions, the proton affinities of silanols and siloxanes, hydrolysis of chlorosilanes and condensation of silanols to siloxanes were considered. As the reference values, experimental bonding parameters and reaction enthalpies were used wherever available. Where there are no experimental data, W1 and CBS-QB3 values were used instead. For the gas phase conditions, excellent agreement between theoretical CBS-QB3 and W1 and experimental thermochemical values was observed. All DFT methods also give acceptable values and the precision of various functionals used was comparable. No significant advantage of newer more advanced functionals over ‘classical’ B3LYP and PBEPBE ones was noted. The accuracy of the results was improved significantly when triple-zeta basis sets were used for energy calculations, instead of double-zeta ones. The accuracy of calculations for the reactions in water solution within the SCRF model was inferior compared to the gas phase. However, by careful estimation of corrections to the ΔH_solv and ΔG_solv of H⁺ and HCl, reasonable values of thermodynamic quantities for the discussed reactions can be obtained.     

Effects of ionization on stability of 1-methylcytosine — DFT and PCM studies

This report present the results of natural energy decomposition analysis (NEDA), natural bond orbital (NBO), and quantum theory of atoms in molecules (QTAIM) calculations of three derivatives of biphenyl-1-aza-18-crown-6 ether and their 1:1 complexes with Cd²⁺. All calculations used the B3LYP density functional theory in combination with the 6-311G and WTBS basis sets for ligands and Cd²⁺ ion, respectively. Ligands 1 and 3 have a single 1-aza-18-crown-6, substituent; ligand 2 has two such substituents. The results show that, in the optimized geometries of the complexes, the distance between N and Cd²⁺ is greater than the distance between O and Cd²⁺. NBO and QTAIM data confirm these results. There was no stabilization energy or bond critical point for N · · · Cd²⁺ in NBO or QTAIM, respectively. Data show that the O · · · Cd²⁺ interaction is a kind of closed shell interaction. The trend of the calculated stabilization energy was similar to the experimental data. Different contributions of interaction energies for complex formation were analyzed by NEDA, and the results show that the main component of the interactions is accounted for by polarization.

     

A DFT study on the sensing behavior of a BC2N nanotube toward formaldehyde

We investigated the viability of using a BC₂N nanotube to detect formaldehyde (H₂CO) molecule by means of B3LYP and M06 density functionals. The results indicate that the molecule is weakly adsorbed on the intrinsic BC₂N nanotube releasing energy of 0.8 kcal mol^-1 (at B3LYP/6-31G(d)) without significant effect on the HOMO-LUMO energy gap and electrical conductivity of the tube. Thus, H₂CO cannot be detected using this intrinsic nanotube. To overcome this problem, a carbon atom of the tube wall was substituted by a Si atom. It was demonstrated that the Si-doped tube cannot only strongly adsorb the H₂CO molecule, but also may effectively detect its presence because of the increase in the electric conductivity of the tube.     

Studies on structures and electron affinities of the simplest alkyl dithio radicals and their anions with gaussian-3 theory and density functional theory

The equilibrium geometries and electron affinities of the R-SS/R-SS^-(R=CH₃, C₂H₅, n-C₃H₇, i-C₃H₇, n-C₄H₉, t-C₄H₉, n-C₅H₁₁) species have been studied using the higher level of the Gaussian-3(G3) theory and 21 carefully calibrated pure and hybrid density functionals (five generalized gradient approximation (GGA) methods, seven hybrid GGAs, three meta GGA methods, and six hybrid meta GGAs) in conjunction with diffuse function augmented double-ζ plus polarization (DZP++) basis sets. The geometries are fully optimized with each method and discussed. The reliable adiabatic electron affinity has been presented by means of the high level of G3 technique. With the DZP++ DFT method, three measures of neutral/anion energy differences reported in this work are the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy. The adiabatic electron affinities, obtained at the BP86, M05-2X, B3LYP, M06, B98, M06-2X, mPW1PW91, HCTH, B97-1, M05, PBE1PBE, and VSXC methods, are in agreement with the G3 results. These methods perform better for EA prediction and are considered to be reliable.     

Spin states of Mn(III) meso-tetraphenylporphyrin chloride assessed by density functional methods

The present work assessed several exchange-correlation functionals (including GGA, meta-GGA and hybrid functionals), in combination with a variety of basis sets and effective core potentials (ECP) for their ability to predict the ground spin state of Mn(III) meso-tetraphenylporphyrin chloride complex, labeled Mn(III)TPPCl, for which experimental data support the quintet high spin state. Geometry optimization of Mn(III)TPPCl was performed for three possible spin states (singlet state, LS; triplet state, IS; and quintet state, HS) at the TPSSh level using the LANL2DZ ECP for Mn and the 6-311G(d) basis set for C, N, Cl and H. Afterwards, single-point energy calculations were conducted by applying 18 exchange-correlation functionals (BLYP, B3LYP, PW91, BPW91, BP86, OLYP, OPBE, OPW91, O3LYP, PBE0, PBEh1PBE, HSEH1PBE, TPSS, TPSSh, M06 L, M06, M062X and M06HF). The influence of the basis set for the metal center was assessed using a smaller group of functionals and varying between the Pople basis set 6-31G(d), its newer formulation m6-31G(d) and the larger Def2-QZVP basis set. All functionals in combination with Pople basis sets predict the quintet state as the ground spin state. In addition, the BLYP, BP86, BPW91, PW91, PBEh1PBE, TPSS and TPSSh functionals predicted the IS lying at most ~60 kJ mol^?1 above the HS, which agrees with the reference data. Results including Def2-QZVP basis set were inconsistent, since only BLYP and B3LYP predict HS as the ground spin state. The recommended methodology for the treatment of such systems seems to be exchange-correlations functionals with few or none Hartree-Fock exchange and modest size basis sets.

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Graphical Abstract MnTPPCl molecule and the energy ordering of its spin states assessed by 18 functionals

     

Further evidence for an elongation-decarboxylation mechanism in the biosynthesis of paraffins in leaves

In isolated tobacco leaves l-valine-U-¹⁴C gave rise to labeled even-numbered isobranched fatty acids containing 16 to 26 carbon atoms and iso C₂₉, iso C₃₁, and iso C₃₃ paraffins. l-Isoleucine-U-¹⁴C on the other hand produced labeled odd-numbered anteiso C₁₇ to C₂₇ fatty acids and anteiso C₃₀ and C₃₂ paraffins. Trichloroacetic acid inhibited the incorporation of isobutyrate into C₂₀ and higher fatty acids and paraffins without affecting the synthesis of the C₁₆ and C₁₈ fatty acids. Thus the very long branched fatty acids are biosynthetically related to the paraffins. In Senecio odoris leaves acetate-1-¹⁴C was incorporated into the paraffins (mainly n-C₃₁) only in the epidermis although acetate was readily incorporated into fatty acids in the mesophyll tissue. Similarly only the epidermal tissue incorporated acetate into fatty acids longer than C₁₈ suggesting that the epidermis is the site of synthesis of both paraffins and the very long fatty acids. In broccoli leaves n-C₁₂ acid labeled with ¹⁴C in the carboxyl carbon and ³H in the methylene carbons was incorporated into C₂₉ paraffin without the loss of ¹⁴C relative to ³H. Since n-C₁₈ acid is known to be incorporated into the paraffin without loss of carboxyl carbon these results suggest that the condensation of C₁₂ acid with C₁₈ acid is not responsible for n-C₂₉ paraffin synthesis in this tissue. Thus all the experimental evidence thus far obtained strongly suggests that elongation of fatty acids followed by decarboxylation is the most likely pathway for paraffin biosynthesis in leaves.     

Catalytic effect of (H2O)n (n = 1–3) clusters on the reaction of HNO2 + HO → H2O + NO2 under tropospheric conditions

ABSTRACT

The effect of (H₂O)_n (n?=?1–3) on the HNO₂?+?HO → H₂O?+?NO₂ reaction has been investigated theoretically at the CCSD(T)/CBS//B3LYP/6-311?+?G(3df,2pd) level of theory, coupled with rate constant calculations by using variational transition state theory. Our results show that, when (H₂O)_n (n?=?1–3) was introduced into HNO₂?+?HO → H₂O?+?NO₂ reaction, the product of the reaction did not change, but the potential energy surface became quite complex, yielding two kinds of reactions, namely HNO₂···(H₂O)_n (n?=?1–3)?+?HO and HO···(H₂O)_n (n?=?1–3)?+?HNO₂. In all catalysed reactions with (H₂O)_n (n?=?1–3), the former reaction type is favourable than the latter one with its effective rate constant respectively larger by 6–1 orders of magnitude than that of latter one. Within the temperature range of 240–320?K, the relative impacts on water monomer are much more obvious than dimer and trimer. However, the effective rate constant with water is larger by 658%–17% times of magnitude, showing that the positive water effect is obvious under atmospheric conditions.     

Isotopic evidence for the occurrence of biological nitrification and nitrogen deposition processing in forest canopies

This study examines the role of tree canopies in processing atmospheric nitrogen (N_dep) for four forests in the United Kingdom subjected to different N_dep: Scots pine and beech stands under high N_dep (HN, 13–19 kg N ha^?1 yr^?1), compared to Scots pine and beech stands under low N_dep (LN, 9 kg N ha^?1 yr^?1). Changes of NO₃‐N and NH₄‐N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines δ¹⁸O, Δ¹⁷O and δ¹⁵N in NO₃^? and δ¹⁵N in NH₄⁺, were used to assess N transformations by the canopies. Generally, HN sites showed higher NH₄‐N and NO₃‐N concentrations in RF compared to the LN sites. Similar values of δ¹⁵N‐NO₃^? and δ¹⁸O in RF suggested similar source of atmospheric NO₃^? (i.e. local traffic), while more positive values for δ¹⁵N‐NH₄⁺ at HN compared to LN likely reflected the contribution of dry NH_x deposition from intensive local farming. The isotopic signatures of the N‐forms changed after interacting with tree canopies. Indeed, ¹⁵N‐enriched NH₄⁺ in TF compared to RF at all sites suggested that canopies played an important role in buffering dry N_dep also at the low N_dep site. Using two independent methods, based on δ¹⁸O and Δ¹⁷O, we quantified for the first time the proportion of NO₃^? in TF, which derived from nitrification occurring in tree canopies at the HN site. Specifically, for Scots pine, all the considered isotope approaches detected biological nitrification. By contrast for the beech, only using the mixing model with Δ¹⁷O, we were able to depict the occurrence of nitrification within canopies. Our study suggests that tree canopies play an active role in the N cycling within forest ecosystems. Processing of N_dep within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Δ¹⁷O.     

Critical test of some computational methods for prediction of NMR <Superscript>1</Superscript>H and <Superscript>13</Superscript>C chemical shifts

Performance of 18 DFT functionals (B1B95, B3LYP, B3PW91, B97D, BHandHLYP, BMK, CAM-B3LYP, HSEh1PBE, M06-L, mPW1PW91, O3LYP, OLYP, OPBE, PBE1PBE, tHCTHhyb, TPSSh, wB97xD, VSXC) in combinations with six basis sets (cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, aug-cc-pVTZ, IGLO-II, and IGLO-III) and three methods for calculating magnetic shieldings (GIAO, CSGT, IGAIM) was tested for predicting ¹H and ¹³C chemical shifts for 25 organic compounds, for altogether 86 H and 88 C atoms. Proton shifts varied between 1.03 ppm to 12.00 ppm and carbon shifts between 7.87 ppm to 209.28 ppm. It was found that the best method for calculating ¹³C shifts is PBE1PBE/aug-cc-pVDZ with CSGT or IGAIM approaches (mae?=?1.66 ppm), for ¹H the best results were obtained with HSEh1PBE, mPW1PW91, PBE1PBE, CAM-B3LYP, and B3PW91 functionals with cc-pVTZ basis set and with CSGT or IGAIM approaches (mae?=?0.28 ppm). We found that often larger basis sets do not give better results for chemical shifts. The best basis sets for calculating ¹H and ¹³C chemical shifts were cc-pVTZ and aug-cc-pVDZ, respectively. CSGT and IGAIM NMR approaches can perform really well and are in most cases better than popular GIAO approach.

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Graphical Abstract Mean absolute errors for ¹H and ¹³C chemical shifts and computational times of neutral toluene molecule with aug-cc-pVDZ basis set and CSGT approach

     

Dual Effect of Isoprostanes on the Release of [<Superscript>3</Superscript>H]D-Aspartate from Isolated Bovine Retinae: Role of Arachidonic Acid Metabolites

The effect of 8-isoprostanes on potassium (K⁺)-depolarization-evoked release of [³H]D-aspartate from bovine isolated retinae was investigated. Isolated bovine retinae were prepared for studies of K⁺-evoked release of [³H]D-aspartate using the Superfusion Method. Low concentrations of 8-isoPGF₂(1–100 nM) inhibited whereas higher concentrations of this 8-isoprostane (100 nM–30 M) enhanced K⁺-induced [³H]D-aspartate overflow. The excitatory effect of 8-isoPGF₂ was mimicked by thromboxane receptor agonist, U-46619 and blocked by thromboxane receptor antagonist, SQ 29,548 (10 M). Pretreatment of tissues with the cyclooxygenase (COX) inhibitor, flurbiprofen unmasked an inhibitory effect of high concentrations of 8-isoPGF₂(1–30 M) on [³H]D-aspartate release that was attenuated by AH 6809 (10 M). In conclusion, 8-isoPGF₂ exhibits a dual regulatory effect on K⁺-induced [³H]D-aspartate release in isolated bovine retinae. The inhibitory action caused by 8-isoPGF ₂ is due to the activation of EP₁/EP₂ receptors while the excitatory effects are due to the activation of thromboxane receptors.     

Structure Determination of Fatty Acids from Tunicamycin Complex

The structures of ten fatty acids, which were obtained by the hydrolysis of tunicamycin complex, were determined. GLC-mass, ¹H NMR and IR spectra showed that the major acids were trans-α, β-unsaturated iso acids with the formula C₁₄H₂₈O₂, C₁₆H₂₈O₂, C₁₆H₃₀O₂ and C₁₇H₃₂O₂. The minor acids were α, β-unsaturated normal acids and saturated normal and iso acids.     

Hydrogen isotope response to changing salinity and rainfall in Australian mangroves

Hydrogen isotope ratios (²H/¹H, δ²H) of leaf waxes covary with those in precipitation and are therefore a useful paleohydrologic proxy. Mangroves are an exception to this relationship because their δ²H values are also influenced by salinity. The mechanisms underlying this response were investigated by measuring leaf lipid δ²H and leaf and xylem water δ²H and δ¹⁸O values from three mangrove species over 9.5 months in a subtropical Australian estuary. Net ²H/¹H fractionation between surface water and leaf lipids decreased by 0.5–1.0‰ ppt^?1 for n‐alkanes and 0.4–0.8‰ ppt^?1 for isoprenoids. Xylem water was ²H depleted relative to surface water, reflecting ²H discrimination of 4–10‰ during water uptake at all salinities and opportunistic uptake of freshwater at high salinity. However, leaf water ²H enrichment relative to estuary water was insensitive to salinity and identical for all species. Therefore, variations in leaf and xylem water δ²H values cannot explain the salinity‐dependent ²H depletion in leaf lipids, nor the 30‰ range in leaf lipid δ²H values among species. Biochemical changes in direct response to salt stress, such as increased compatible solute production or preferential use of stored carbohydrates, and/or the timing of lipid production and subsequent turnover rates, are more likely causes.     

Prototropic tautomerism and basic molecular principles of hypoxanthine mutagenicity: an exhaustive quantum-chemical analysis

The molecular structures, relative stability order, and dipole moments of a complete family of 21 planar hypoxanthine (Hyp) prototropic molecular–zwitterionic tautomers including ylidic forms were computationally investigated at the MP2/6–311++G(2df,pd)//B3LYP/6–311++G(d,p) level of theory in vacuum and in three different surrounding environments: continuum with a low dielectric constant (??=?4) corresponding to a hydrophobic interface of protein–nucleic acid interactions, dimethylsulfoxide (DMSO), and water. The keto-N1HN7H tautomer was established to be the global minimum in vacuum and in continuum with ??=?4, while Hyp molecule exists as a mixture of the keto-N1HN9H and keto-N1HN7H tautomers in approximately equal amounts in DMSO and in water at T?=?298.15?K. We found out that neither intramolecular tautomerization by single proton transfer in the Hyp base, nor intermolecular tautomerization by double proton transfer in the most energetically favorable Hyp·Hyp homodimer (symmetry C _2h ), stabilized by two equivalent N1H…O6 H-bonds, induces the formation of the enol tautomer (marked with an asterisk) of Hyp with cis-oriented O6H hydroxyl group relative to neighboring N1C6 bond. We first discovered a new scenario of the keto–enol tautomerization of Hyp?·?Hyp homodimer (C _2h ) via zwitterionic near-orthogonal transition state (TS), stabilized by N1⁺H…N1^? and O6⁺H…N1^? H-bonds, to heterodimer Hyp^??·?Hyp (C _s ), stabilized by O6H…O6 and N1H…N1 H-bonds. We first showed that Hyp^??·?Thy mispair (C _s ), stabilized by O6H…O4, N3H…N1, and C2H…O2 H-bonds, mimicking Watson–Crick base pairing, converts to the wobble Hyp?·?Thy base pair (C _s ), stabilized by N3H…O6 and N1H…O2 H-bonds, via high- and low-energy TSs and intermediate Hyp?·?Thy^?, stabilized by O4H…O6, N1H…N3, and C2H…O2 H-bonds. The most energetically favorable TS is the zwitterionic pair Hyp⁺?·?Thy^? (C _s ), stabilized by O6⁺H…O4^?, O6⁺H…N3^?, N1⁺H…N3^?, and N1⁺H…O2^? H-bonds. The authors expressed and substantiated the hypothesis, that the keto tautomer of Hyp is a mutagenic compound, while enol tautomer Hyp^? does not possess mutagenic properties. The lifetime of the nonmutagenic tautomer Hyp^? exceeds by many orders the time needed to complete a round of DNA replication in the cell. For the first time purine–purine planar H-bonded mispairs containing Hyp in the anti-orientation with respect to the sugar moiety – Hyp?·?Ade _syn , Hyp?·?Gua^? _syn , and Hyp?·?Gua _syn , that closely resembles the geometry of the Watson–Crick base pairs, have been suggested as the source of transversions. An influence of the surrounding environment (??=?4) on the stability of studied complexes and corresponding TSs was estimated by means of the conductor-like polarizable continuum model. Electron-topological, structural, vibrational, and energetic characterictics of all conventional and nonconventional H-bonds in the investigated structures are presented. Presented data are key to understanding elementary molecular mechanisms of mutagenic action of Hyp as a product of the adenine deamination in DNA.