6 Structure organization in solution




Structure organization in solution
IL-benzene mixtures
Enzyme stabilization

It is evident that “pure” 1,3-dialkyimidazolium ionic liquids in the solid, liquid and gas phase are well-organized hydrogen-bonded polymeric supramolecules. Moreover there is now much evidence indicating that some of the supramolecular organization can be maintained when they are mixed with other substances.

Simplified two-dimensional model of the structural featuresThe introduction of other molecules and macromolecules occurs with a disruption of the hydrogen bond network and in some cases generating nano-structures with polar and non-polar regions where inclusion-type compounds can be formed.
These inclusion compounds can involve molecules (such as arenes), ions (such as charged transition-metal complexes), macromolecules (such as enzymes or cellose) and nanoparticles (such as transition-metal nano clusters) and the stabilization of this process is mainly due to the electronic and steric effects provided by the nano-structures of the type [(DAI)x(X)x-n]n+ [(DAI)x-n(X)x)]n-.

When they are infinitely diluted in other molecules they can form solvent separated ion pairs that an increase of the concentration of the imidazolium salt they collapse to form contact ion pairs - through hydrogen bonds involving the cation with the anion – and a further salt concentration increment leads to triple ions, etc, (Figure 9).

Ionic liquids-benzene mixtures

benzene molecule1,3-Dialkylimidazolium ionic liquids-aromatic mixtures form liquid clathrates and in the case of a DMI.PF6-benzene mixture the inclusion compound [(DMI.PF6)2(benzene)]n could be trapped and its X-ray structure determined. This compound maintains a three-dimensional hydrogen bonded network of anions and cations with the formation of channels in which the benzene molecules are included. The benzene molecules are isolated within the channels and they are staggered through π-π sandwich between two imidazolium cations. It is worth to note that again one cation is surrounded by three anions and in its turn each anion is surrounded by three cations.


Enzyme stabilization

The presence of these hydrogen-bonded nano-structures with polar and non-polar regions may be responsible for the stabilization of enzymes supported in ionic liquids that can maintain their functionality under very extreme denaturative conditions. It is well known that the thermal stability of enzymes is enhanced in both aqueous and anhydrous media containing polyols as a consequence of the increase in hydrogen bond interactions. Thus, both the solvophobic interactions essential to maintain the native structure and the water shell around the protein molecule are preserved by the “inclusion” of the aqueous solution of free enzyme into the IL network, resulting in a clear enhancement of the enzyme stability.

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