Nicole Morel-Desrosiers

Production and consumption of methane in freshwater lake ecosystems

The atmospheric concentration of methane (CH(4)), a major greenhouse gas, is mainly controlled by the activities of methane-producing (methanogens) and methane-consuming (methanotrophs) microorganisms. Freshwater lakes are identified as one of the main CH(4) sources, as it was estimated that they contribute to 6-16% of natural CH(4) emissions. It is therefore critical to better understanding the biogeochemical cycling of CH(4) in these ecosystems. In this paper, the effects of environmental factors on methanogenic and methanotrophic rates are reviewed and an inventory of the methanogens and methanotrophs at the genus/species level in freshwater lakes is given. We focus on the anaerobic oxidation of methane, which is a still poorly known process but increasingly reported in freshwater lakes.

Complexation of basic amino acids by water‐soluble calixarene sulphonates as a study of the possiblemechanisms of recognition of calixarene sulphonates by proteins

The interactions of calixarene sulphonates with the basic amino acids arginine and lysine were studied by 1H NMR spectroscopy. Strong electrostatic binding occurs for calix[4]arene sulphonate with both lysine and arginine at pH 1 and 5. For the higher calixarenes, only weak interactions at the faces of the flattened macrocycles occur. This binding is in contrast to the inhibition of protein–protein interactions by the calixarenes where the calix[6]arene and calix[8]arene sulphonates show much stronger effects.

Intercalation of dicarboxylate anions into a Zn–Al–Cl layered double hydroxide: microcalorimetric determination of the enthalpies of anion exchange

The enthalpies of anion exchange in a chloride-intercalated zinc aluminium layered double hydroxide (LDH) in suspension in water have been studied at 25 °C. The heat effects associated with the exchange of the chloride anions for dicarboxylate anions (oxalate, succinate, adipate and tartrate) have been determined by titration microcalorimetry for different amounts of LDH. Except for tartrate, the exchange processes are endothermic, in contrast to the exchange processes involving only inorganic anions. The enthalpies of exchange vary linearly with the number of carbon atoms, suggesting that the intercalated dicarboxylate anions are perpendicular to the layers. The energetic parameters show, for the first time, hydrophobic interactions between the CH2 groups of the parallel succinate or adipate anions and hydrogen bonding between the OH groups of the parallel tartrate anions.

Interactions between cations and sugars. Part 6.—Calorimetric method for simultaneous determination of the stability constant and enthalpy change for weak complexation

A calorimetric method that allows simultaneous determination of both the association constant and the enthalpy change for weak complexation of a cation by a neutral ligand in water is presented. The theoretical and experimental aspects of the procedure are developed. The method involves the separation of the enthalpy of transfer of the electrolyte from water to an aqueous solution of the ligand into a specific contribution characterizing the interaction of the cation with the complexing site of the ligand and a non-specific contribution including all the other interactions of both the cation and the anion with the ligand. The latter contribution is estimated by considering the interactions of the same electrolyte with a non-complexing molecule very similar to the ligand. To illustrate the method the heats of mixing of aqueous solutions of MgCl2, CaCl2, SrCl2, BaCl2 and Ca(ClO4)2 with aqueous solutions of ribose, a complexing ligand, and arabinose, a non-complexing epimer, have been measured at 298.15 K. The stability constant obtained by this calorimetric method for Ca2+–ribose (β= 1.9) is in excellent agreement with that found previously by an electrochemical method.

Binding of monovalent metal cations by the p-sulfonatocalix[4]arene: experimental evidence for cation–π interactions in water

Gibbs free energies, enthalpies and entropies for the binding of Na+, K+, Rb+, Cs+, Ag+, Tl+ and NH4+ by the p-sulfonatocalix[4]arene in water are determined by microcalorimetry. Whereas no significant heat effect is detected with Na+ or Ag+, suggesting that these cations are not complexed, weak but selective binding is observed with the other cations. The whole set of thermodynamic parameters, which demonstrate that the cations bind inside the cavity of the calixarene, evidence the importance of the cation-pi interactions for these complexes in water.

Interactions between cations and sugars. Part 8.—Gibbs energies, enthalpies and entropies of association of divalent and trivalent metal cations with xylitol and glucitol in water at 298.15 K

Microcalorimetry has been used to determine the standard Gibbs energies, enthalpies and entropies of association of several divalent and trivalent metal cations, including alkaline-earth-metal and lanthanide cations, with xylitol and D-glucitol (sorbitol) in water at 298.15 K. Following a procedure previously used for small sugars, the contributions solely characterizing the specific interaction between the cation and the complexing triol of the ligand were isolated. It appears that xylitol and D-glucitol form weak complexes of comparable strength, and that all the stability constants are smaller than 10. As observed previously with D-ribose, the strongest complexes are formed with the trivalent lanthanide cations. However, there is no clear relationship between the thermodynamic properties of complexation and the size or charge of the cation. Xylitol and D-glucitol appear to be extremely sensitive to the inner-sphere hydration number change occurring within the lanthanide series. Furthermore, both ligands show a selectivity towards the lanthanide cations that is comparable to that observed with much stronger ligands such as acetate, glycolate or murexide.

Interactions between cations and sugars. Part 7.—Gibbs energies, enthalpies and entropies of association of the trivalent lanthanide cations with ribose in water at 298.15 K

Standard Gibbs energies, enthalpies and entropies of association of the trivalent lanthanide cations with the complexing sites of ribose have been determined in water using a calorimetric method and following a procedure that allows the non-specific interactions to be neglected. For the cations going from La3+ to Tb3+ only 1 : 1 complexes were observed; the stability constants, the values of which range between 3 and 11, show a maximum for Sm3+. No significant complexation of the heavier lanthanide cations was detected. The results have been compared with those obtained for stronger ligands also bearing oxygen atoms as coordinating sites, showing that ribose is as selective towards the lanthanide cations as are these stronger ligands. On the other hand, comparison of the entropies of the complexes with those of the free cations has revealed a behaviour change starting with Eu3+, thus supporting the controversial assumption of a coordination number change along the lanthanide series.

Behavior of cesium and thallium cations inside a calixarene cavity as probed by nuclear spin relaxation. Evidence of cation-pi interactions in water.

We have studied the complexes formed between the p-sulfonatocalix[4]arene and cesium or thallium metal cation, first by carbon-13 longitudinal relaxation of the calixarene molecule at two values of the magnetic field B(0). From the longitudinal relaxation times of an aromatic carbon directly bonded to a proton, thus subjected essentially to the dipolar interaction with that proton, we could obtain the correlation time describing the reorientation of the CH bond. The rest of this study has demonstrated that it is also the correlation time describing the tumbling of the whole calixarene assembly. From three non-proton-bearing carbons of the aromatic cycles (thus subjected to the chemical shift anisotropy and dipolar mechanisms), we have been able to determine the variation of the chemical shift anisotropy when going from the free to the complex form of the calixarene. These variations not only provide the location of the cation inside the calixarene cavity but also constitute a direct experimental proof of the cation-pi interactions. These results are complemented by cesium and thallium relaxation measurements performed again at two values of the magnetic field B(0). An estimation of the mean distance between the cation and the calixarene protons could be obtained. These measurements have also revealed an important chemical shift anisotropy of thallium upon complexation.

Thermodynamics of the Complexation of the p-Sulfonatocalix[4]arene with Simple Model Guests in Water: a Microcalorimetric Study

The formation of supramolecular complexes in water involves interactions of various types which are not always easy to identify, especially when complicated species are involved. A complete thermodynamic characterization of the binding process, which includes the enthalpies and entropies of complexation, is obviously one of the key elements in identifying the stabilizing factors and in understanding how the host and guest assemble. In order to thermodynamically characterize typical interactions of various types, we have thus undertaken a microcalorimetric study of the complexation of p-sulfonatocalixarenes with simple guests bearing different functional groups. Association constants, free energies, enthalpies and entropies for the complexation of the p-sulfonatocalix[4]arene with normal alcohols, alkylammonium, carboxylate and guanidinium ions in water, at pH 7.5 and 298.15 K, are reported. The properties for the binding of lysine and arginine, which bear similar functional groups, are also given. The comparison of the thermodynamic behaviour of these different guests allows the driving factors to be identified. This may constitute a starting point for the understanding of the recognition of more complicated guests.

A comparative study of the determination of the stability constants of inclusion complexes of p-sulfonatocalix[4]arene with amino acids by RP-HPLC and 1H NMR

Reversed-phase high-performance liquid chromatography (Separon SGX C 18, UV detection at 254 nm and acetonitrile–water–trifluoroacetic acid (70∶30∶0.5, v/v), and methanol–water–trifluoroacetic acid (3∶97∶0.5, v/v) as mobile phases) was applied to the study of the host–guest complexation of p-sulfonatocalix[4]arene (SC[4]A) with amino acids as guests in the mobile phase. It was established that the formation of the inclusion complexes results in changes in the retention times of the amino acids. Stability constants of the complexes were determined. The variations in stability constants may be explained in terms of the various interactions (ion-pairing, hydrophobic, aromatic–aromatic and electrostatic interactions) that may occur between a given amino acid and SC[4]A. For the amino acids aspartic acid and proline the association constants were also derived from 1H NMR experiments.