RESUMO
Monovalent and divalent cations play a crucial role in living cells and for molecular techniques such as PCR. Here we evaluate DNA melting temperatures in magnesium (Mg2+) and magnesiumpotassium (Mg2++ K+) buffers with a mesoscopic model that allows us to estimate hydrogen bonds and stacking interaction potentials. The Mg2+ and Mg2++ K+ results are compared to previous calculations for sodium ions (Na+), in terms of equivalent sodium concentration and ionic strength. Morse potentials, related to hydrogen bonding, were found to be essentially constant and unaffected by cation conditions. However, for stacking interactions we find a clear dependence with ionic strength and cation valence. The highest ionic strength variations, for both hydrogen bonds and stacking interactions, was found at the sequence terminals. This suggests that end-to-end interactions in DNA will be strongly dependent on cation valence and ionic strength.
Assuntos
DNA , Magnésio , Ligação de Hidrogênio , Cátions , DNA/química , Sódio , Cátions Monovalentes/químicaRESUMO
The gas-phase structures of cytosine-Ag(+) [CAg](+) and cytosine-Ag(+)-H2O [CAg-H2O](+) complexes have been studied by mass-selected infrared multiphoton dissociation (IRMPD) spectroscopy in the 900-1800 cm(-1) spectral region using the Free Electron Laser facility in Orsay (CLIO). The IRMPD experimental spectra have been compared with the calculated IR absorption spectra of the different low-lying isomers (computed at the DFT level using the B3LYP functional and the 6-311G++(d,p) basis set for C, H, N and O atoms and the Stuttgart effective core potential for Ag). For the [CAg](+) complex, only one isomer with cytosine in the keto-amino (KA) tautomeric form and Ag(+) interacting simultaneously with the C(2)[double bond, length as m-dash]O(7) group and N(3) of cytosine was observed. However, the mono-hydration of the complex in the gas phase leads to the stabilization of a two quasi-isoenergetic structure of the [CAg-H2O](+) complex, in which Ag(+) interacts with the O atom of the water molecule and with the N(3) or C(2)[double bond, length as m-dash]O(7) group of cytosine. The relative populations of the two isomers determined from the IRMPD kinetics plot are in good agreement with the calculated values. Comparison of these results with those of protonated cytosine [CH](+) and its mono-hydrated complex [CH-H2O](+) shows some interesting differences between H(+) and Ag(+). In particular, while a single water molecule catalyzes the isomerization reaction in the case of [CH-H2O](+), it is found that in the case of [CAg-H2O](+) the addition of water leads to the stabilization of two isomers separated by small energy barrier (0.05 eV).
Assuntos
Complexos de Coordenação/química , Citosina/química , Prata/química , Cátions Monovalentes/química , Espectrometria de Massas , Modelos Moleculares , Espectrofotometria Infravermelho , Termodinâmica , Água/químicaRESUMO
In this letter, we study the effect of cation charge on anion selectivity in the pore using grand canonical Monte Carlo simulations. The mechanism of anion selectivity inside nanopores was found to be primarily a consequence of the screening of negative charges by the cations. In the case of monovalent cations, screening was not very effective and anions were rejected. We found an 'off-state' at high pH and an 'on-state' at low pH. When there are divalent cations, screening is good and there is no rejection of the anion. The concentration of anions at high pH is similar to that at low pH. The system is always in an 'on-state'. Trivalent cations show an inverse selectivity mechanism: at high pH the concentration is higher than at low pH, i.e., the pore is in the 'on-state' at high pH and in the 'off-state' at low pH.
Assuntos
Cátions/química , Nanoporos , Cátions Bivalentes/química , Cátions Monovalentes/química , Simulação por Computador , Concentração de Íons de Hidrogênio , Canais Iônicos/fisiologia , Cinética , Método de Monte Carlo , Eletricidade EstáticaRESUMO
Differential scanning calorimetry (DSC) was applied to ascertain the effect caused by Kâº, Naâº, ATP, detergent, DPPC, DPPE, and subunit γ on the thermostability of Na,K-ATPase. The enthalpy variation (ΔH) for the thermal denaturation of the membrane-bound is twice the ΔH value obtained for solubilized Na,K-ATPase. Denaturation occurs in five steps for membrane-bound against three steps for the solubilized enzyme, therefore a multi-step unfolding process. In the presence of Naâº, the melting temperature is 61.6°C, and the ΔH is lower as compared with the ΔH obtained in the presence or in the absence of Kâº. Addition of ATP does not alter the transition temperatures significantly, but the shape of the curve is modified. Subunit γ probably stabilizes Na,K-ATPase in the beginning of thermal unfolding, and different amounts of detergents in the solubilized sample change the protein stability. Reconstitution of Na,K-ATPase into a liposome shows that lipids exert a protector effect. These results reveal differences on the thermostability depending on the conformation of Na,K-ATPase. They are relevant because it allows a comparison with future studies, e.g. how the composition of the membrane interferes on the stability of Na, K-ATPase, elucidating the importance of the lipid type contained in cell membrane.
Assuntos
Membrana Celular/enzimologia , Potássio/química , Dobramento de Proteína , ATPase Trocadora de Sódio-Potássio/química , Animais , Varredura Diferencial de Calorimetria , Cátions Monovalentes/química , Cátions Monovalentes/metabolismo , Potássio/metabolismo , Desnaturação Proteica , Subunidades Proteicas/química , Subunidades Proteicas/metabolismo , Força Próton-Motriz , Coelhos , ATPase Trocadora de Sódio-Potássio/metabolismo , SolubilidadeRESUMO
Potassium ions are non-essential activators of several aldehyde dehydrogenases (ALDHs), whereas a few others require the cation for activity. Two kinds of cation-binding sites, which we named intra-subunit and inter-subunit, have been observed in crystal structures of ALDHs, and based on reported crystallographic data, we here propose the existence of a third kind located in the central cavity of some tetrameric ALDHs. Given the high structural similarity between these enzymes, cation-binding sites may be present in many other members of this superfamily. To explore the prevalence of these sites, we compared 37 known crystal structures from 13 different ALDH families and evaluated the possible existence of a cation on the basis of the number, distance and geometry of its potential interactions, as well as of B-factor values of modeled cations obtained in new refinements of some reported crystal structures. Also, by performing multiple alignments of 855 non-redundant amino acid sequences, we assessed the degree of conservation in their respective families of the amino acid residues putatively relevant for cation binding. Among the ALDH enzymes studied, and according to our analyses, potential intra-subunit cation-binding sites seem to be present in most members of ALDH2, ALDH1L, ALDH4, ALDH5, ALDH7, ALDH10, and ALDH25 families, as well as in the bacterial and fungal members of the ALDH9 family and in a few ALDH1, ALDH6, ALDH11 and ALDH26 enzymes; potential inter-subunit sites in members of ALDH1L, ALDH3, ALDH4 from bacillales, ALDH5, ALDH7, ALDH9, ALDH10, ALDH11 and ALDH25 families; and potential central-cavity sites only in some bacterial and animal ALDH9s and in most members of the ALDH1L family. Because potassium is the most abundant intracellular cation, we propose that these are potassium-binding sites, but the specific structural and/or functional roles of the cation bound to these different sites remain to be investigated.
Assuntos
Aldeído Desidrogenase/química , Aldeído Desidrogenase/metabolismo , Cátions Monovalentes/química , Cátions Monovalentes/metabolismo , Bacillus subtilis/enzimologia , Bacillus subtilis/metabolismo , Sítios de Ligação , Cristalografia por Raios X/métodos , Escherichia coli/enzimologia , Escherichia coli/metabolismo , Modelos Moleculares , Alinhamento de Sequência , Staphylococcus aureus/enzimologia , Staphylococcus aureus/metabolismoRESUMO
The role of monovalent (Na+, K+) and divalent (Ca2+, Mg2+) cations in Bufo arenarum fertilization was analysed. Our results showed that the highest fertilization percentages were obtained when strings of uterine oocytes (UO) were inseminated. Under these conditions, full jelly (FJ), which represents the jelly coats surrounding the oocytes at the time of deposition, contained 68.5 +/- 7.0 mM Na+, 27.4 +/- 2.4 mM K+, 6.3 +/- 0.9 mM Ca2+ and 6.9 +/- 0.9 mM Mg2+. When the strings of oocytes were washed in deionized water, these cations diffused into the liquid medium surrounding them. There was a marked similarity between the loss of Ca2+ in the jelly and the decrease in the fertilizability of the UO. Furthermore, the use of chelating agents of divalent cations showed the importance of the Ca2+ contained in the jelly. When Ca2+ was sequestered from the jelly coats by the addition of the chelating agents to the insemination medium as well as by pretreatment of the UO strings, a decrease in fertilization percentages occurred, this effect being dose dependent and more marked with EGTA. These results demonstrate that the Ca2+ in the jelly plays a role in fertilization. Nevertheless, taking into account that during the washing of the jelly other jelly coat components were diffused and considering that the addition of Ca2+ to the insemination medium reverted significantly, but only partially, the loss of fertilizability of jellied UO (washed), the participation of other components in the fertilization mechanism is suggested.
Assuntos
Bufo arenarum/fisiologia , Matriz Extracelular/química , Fertilização , Metais Alcalinos/química , Metais Alcalinoterrosos/química , Animais , Cátions Bivalentes/química , Cátions Monovalentes/química , Quelantes/farmacologia , Ácido Edético/farmacologia , Ácido Egtázico/farmacologia , Feminino , Fertilização/efeitos dos fármacos , MasculinoRESUMO
We determined the energies for ro-vibrational transitions of fluoromethylidynium (CF(+)) using a numerical variational approach and a Potential Energy Function calculated with the internally contracted multireference configuration interaction method including also the Davidson correction (MRCI+Q). For this purpose, all the CSFs built the full valence space have been selected as multireferential space and all the valence electrons have been correlated for the ground state X(1) summation operator(+) of CF(+). The rotational transitions observed experimentally toward the Orion Bar have been calculated to be 101.2 (102.6)GHz, 202.9 (205.2) GHz and 304.0 (307.7)GHz (experimental values in parentheses) respectively for the J=1-->0, J=2-->1 and J=3-->2 transitions. From the manifold of transitions data, it is shown how to calculate the spectroscopic parameters as well as the coefficients for the Dunham expansion.
Assuntos
Compostos de Flúor/química , Cátions Monovalentes/química , Eletroquímica , Análise Espectral , TermodinâmicaRESUMO
The substituent effect on the reactivity of the C-N bond of molecular ions of 4-substituted N-(2-furylmethyl)anilines toward two dissociation pathways was studied. With this aim, six of these compounds were analyzed by mass spectrometry using electron ionization with energies between 7.8 and 69.9 eV. Also, the UB3LYP/6-31G (d,p) and UHF/6-31G (d, p) levels of theory were used to calculate the critical energies (reaction enthalpies at 0 K) of the processes that lead to the complementary ions [C(5)H(5)O](+) and [M - C(5)H(5)O](+), assuming structures that result from the heterolytic and homolytic C-N bond cleavages of the molecular ions, respectively. A kinetic approach proposed in the 1960s was applied to the mass spectral data to obtain the relative rate coefficients for both dissociation channels from ratios of the peak intensities of these ions. Linear relationships were obtained between the logarithms of the relative rate coefficients and the calculated critical energies and other thermochemical properties, whose slopes showed to be conditioned by the energy provided to the compounds within the ion source. Moreover, it was found that the dissociation that leads to [C(5)H(5)O](+) is a process strongly dependent upon the electron withdrawing or donating properties of the substituent, favored by those factors that destabilize the molecular ion. On the contrary, the dissociation that leads to [M - C(5)H(5)O](+) is indifferent to the polar electronic effects of the substituent. The abundance of both products was governed by the rule of Stevenson-Audier, according to which the major ion is the one of less negative electronic affinity.
Assuntos
Compostos de Anilina/química , Furanos/química , Gases/química , Compostos Heterocíclicos com 2 Anéis/química , Algoritmos , Cátions Monovalentes/química , Cromatografia Gasosa-Espectrometria de Massas , Modelos Químicos , Estrutura Molecular , TermodinâmicaRESUMO
The influence of chloride salts of Na+, Rb+ and Cs+ at concentrations from 0.15 to 1.2M was studied with bovine albumin, trypsin, ovoalbumin and lysozyme partitioning in an aqueous two-phase system formed by polyethyleneglycol 1500 and potassium phosphate at pH 7.4. Monovalent cations favoured the protein transfer to the polyethyleneglycol rich phase in the following order: Rb+ > Na+ > Cs+. Structure making cations as Na+ induced a poor loss of structured water, producing little diminution of the molar partial specific volume of polyethyleneglycol, while Rb+ and Cs+, structure breaking cations, induced a significant decrease in the specific volume of the polyethylene glycol. The increase of available solution free volume in the top phase favours the protein transfer to the polyethyleneglycol rich phase. Na+ and Rb+ induced a slight decrease in the alpha helix content of the proteins, while Cs+ increased the secondary structure for all the proteins. All the cations induced a decrease in the hydrophobic surface of the proteins, this effect was more significant in the presence of Cs+.