Structure making and breaking

Interactions related to the structure-making and breaking of solvents (< 5%)... 

Interactions Related to the Structure-Making and -Breaking of Solvent... 

Information accumulated thus far on the hydration of mineral ions has been critically analyzed in the recent review by Marcus [163] entitled Effect of ions on the structure of water Structure making and breaking. It is important that definite changes have been noted in the water structure and in the structure of diffuse hydration shells with electrolyte concentrations. Neutron diffraction of CaCl2 and Ca(N03)2 solutions in D2O has shown [164] a decrease of Ca hydration number from 10 to 6 when the salt concentration increased from 1 to 4.5 M. 

The water structure-making and -breaking effects of salts, as obtainable from the internal pressure of their aqueous solutions, was discussed by Dack (1976). The internal pressure, Pi, is defined by the first equality ... 

The range of the coefficient of P2c is due to that of the number of water molecules in the second hydration sphere of the cations. The opposite signs of Pd and P2c and their partial compensation then produce the observed signs of 5i, in agreement with the water structure-making and -breaking properties of the cations. For the anions the correlation is ... 

Results from measurements of the spin-lattice relaxation times of fructose in aqueous solutions containing alkali metal halides have been interpreted in terms of structure-making and -breaking properties of the alkali metal ions. ... 

Yet there is little consensus - and sometimes plain contradiction - about what structure-making and -breaking actually entails. Does structure-making render water denser, or does it, in making water more ice-like, actually reduce the density More to the point, this concept lacks any real evidence from experimental studies of the structure of electrolyte solutions that significant changes to the bulk hydrogen-bonded network of water really do occur in the presence of salts. 

Marcus Y (2009) Effect of ions on the structure of water structure making and breaking. Chem Rev 109 1346-1370... 

Compare the geometry of maleic anhydride+propene, the ene transition state, to those of the reactants (maleic anhydride and propene). Is bond making and breaking occurring at once In particular, is the migrating hydrogen partially bonded to two carbons (rather than being fully bonded to one carbon ) Draw a Lewis structure to represent the transition state. Use dashed lines (.. and to represent partial bonds. 

Aqueous solutions of non-electrolytes, especially of non-polar solutes, may show the reverse effect and increase the proportions of ice-like components. The non-polar part of organic electrolytes such as soaps and wetting agents may predominate in increasing the ice component. Thus solutes can be divided into two classes structure making and structure breaking, and in some metal-finishing process solutions both types of solute may be added. 

There is rather a small number of functional groups or structural motifs that have the ability to carry out efficient modification of cellular DNA. ° These molecules are of chemical and biological interest because they successfully execute a difficult balancing act—they possess sufficient reactivity to make and break covalent bonds... 

A somewhat curious effect arises when additional water is dosed on top of this synthetic inner layer, in that the work function is observed to exhibit substantial further decreases. This implies that the water molecules in the multilayers above the inner layer assume some measure of preferential orientation, induced by the presence of the adsorbed bromide in the first layer. This result is probably connected to structure making and structure breaking, or hydrophobic and hydrophilic, properties of soild surfaces, but will not be discussed in detail here. 

This section considers recent attempts to extrapolate from observed structures to the experimentally unattainable transition state structures for bond making and breaking reactions. (Transition state structures for conformational changes are experimentally observable in the crystal in favourable cases, as described in Section 3, pages 135-136.) The approach builds on the regular correlations between structure and reactivity, particularly the linear correlations with bond length described in the previous section. 

Remember, an enzyme is just a tool for making and breaking chemical bonds to bring about structural changes. The chemical mechanism of those changes does not affect the overall energy change involved in the reaction. 

O Neil and Truesdell (1991) have introduced the concept of structure-making and structure-breaking solutes structure makers yield more positive isotope fractionations relative to pure water whereas structure breakers produce negative isotope fractionations. Any solute that results in a positive isotope fractionation is one that causes the solution to be more structured as is the case for ice structure, when compared to solutes that lead to less structured forms, in which cation - H2O bonds are weaker than H2O - H2O bonds. 

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