Water noncovalent bonding

The examples provided in the previous sections show that one of the means to obtain new pseudo-polymorphs is via solvent uptake. Since the co-crystallizing solvent may be present in the vapour phase (say water), one may regard the process that leads from an unsolvated to a solvated species (and reverse) as a supramolecular reaction whereby a given set of noncovalent bonds (those between molecules in the nonsolvated form, for example) are broken and a new set of noncovalent bonds (those between host and guest molecules) are formed as shown at the beginning of this chapter in Scheme 2. 

Some ligands (X and Y) in ternary inner-sphere ([X-M-Y]) and onter-sphere ([M - W- -Y]) or ([M - X- -Y]) complexes, where M is the metal ion and W is a water molecule, may interact directly with each other, forming noncovalent bonds (e.g. H-bonds and ion pairs) or undergo steric or electrostatic repulsion, depending on the ligands and the geometry of the complex. 

Noncovalent Bonds. Noncovalent bonds are weaker than covalent bonds but arc crucial for biochemical processes such as the formation of a double helix, hour lundamental noncovalent bond types are electrostatic interactions, hydrot en bonds, van der Waals interactions, and hydrophobic inlerac-turns. T hey differ in geometry, strength, and specificity. Furthermore, these bunds are allected in vastly different ways by the presence of water. Let us consider the characteristics of each ... 

Noncovalent bonds (i.e., hydrogen bonds, ionic interactions, van der Waals forces, and hydrophobic interactions) play important roles in determining the physical and chemical properties of water. They also have a significant effect on the structure and function of biomolecules. 

Water may influence the conformation of macromolecules if it has an effect on any of the non-covalent bonds that stabilize the conformation of the large molecule. These noncovalent bonds may be hydrogen bonds, ionic bonds or apolar bonds. 

Quaternary (4 ) structure is the arrangement of individual polypeptide chains into a noncovalently bonded aggregate. A major factor stabilizing quaternary structure is hydrophobic interaction created when separate polypeptide chains fold into compact three-dimensional shapes that expose their polar side chains to the aqueous environment and shield their nonpolar side chains from the aqueous environment. Any remaining exposed hydro-phobic patches can be shielded from water if two or more polypeptide chains assemble so that their hydrophobic patches are in contact. 

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