Noncovalent interactions flexibility

Each class of molecules has a similar structural hierarchy subunits of fixed structure are connected by bonds of limited flexibility to form macromolecules with three-dimensional structures determined by noncovalent interactions. These macromolecules then interact to form the supramolecular structures and organelles that allow a cell to carry out its many metabolic functions. Together, the molecules described in Part I are the stuff of life. We begin with water. 

One remarkable feature of all biological membranes is their flexibility—their ability to change shape without losing their integrity and becoming leaky. The basis for this property is the noncovalent interactions among lipids in the bilayer and the motions allowed to individual lipids because they are not covalently anchored to one another. We turn now to the dynamics of membranes ... 

Above, we showed how a finely modulated balancing act between stabilizing and destabilizing forces yields marginally stable protein structures. We now examine in more detail how the same types of noncovalent interactions are used in differently thermally adapted proteins to achieve appropriate levels of structural stability. The adaptive changes discussed below are relevant to both global protein stability and to the establishment of the appropriate balance between stability and flexibility required for... 

Some anion receptors base themselves on one coordination-type interaction coupled with additional noncovalent interactions. Receptor 68, for example, is prepared by modification of a cyclodextrin with flexible caps (170). This receptor was shown to bind hydrophobic carboxylate anions, such as 69, 330 times more strongly than unfunctionalized /3-cyclodextrin. Both coordination and hydrophobic interactions were of importance in the binding process, their relative contributions being AG = 3.4 kcal moF1 and —4.0 kcal mol-1 respectively (171). 

As stated in Section II.B, macrolide antibiotics interact with peptidyltransferase by forming hydrogen bonds usually but occasionally hydrophobic bonds present in a 508 ribosomal subunit. That is why noncovalent interactions appear to be the key to the antibiotic flexibility and specificity observed between different macrolides and their derivatives and also probably to the solubility of the drugs in water. 

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