Binding of small molecules and ions

Equilibria in the formation of complex ions with metals are treated exactly as is the binding of small molecules and ions to macromolecules.87-89 Stepwise constants are defined for the formation of complexes containing one, two, or more ligands L bound to a central metal ion M. The binding constants K/s are usually referred to as P s as in Eq. 6-84. 

It is appropriate at this point to indicate our personal motivation for carrying out structural studies, the types of compounds we study, and the experimental conditions we employ. In a very general sense we are interested in the bonding of small molecules and ions, e.g., 02, N2, NO, N2 R+, olefins, and acetylenes, to transition-metal complexes. Because of our interest in bonding, we seek the best solutions we can attain. Rapid, qualitative answers to conformational problems are not our interest. Since those transition-metal systems that bind small molecules generally have the metal in a low oxidation state, and since a low oxidation state is usually stabilized by ligands of the type PR3 (R = alkyl or aryl), solution to our problems involves typically the determination of a large number of structural parameters. With only a few exceptions the intensity data are obtained at room temperature on a Picker FACS-1 computer-controlled diffractometer. Usually the ratio of observations to variables is at least 10, and it is often 20 to 30. 

This system can be used on and off line. These cartridges have a high capacity for binding of small molecules and contain different bonding phases, ranging from silica gel, C-18 (octadecylsilane), florisil, phenyl, aminopropyl, ion exchange materials (both anionic and cationic), to affinity materials such as immunoadsor-bents and molecular imprinted polymers (MIPs) [38-49]. 

The binding of small molecules to larger ones is basic to most biological phenomena. Substrates bind to enzymes and hormones bind to receptors. Metal ions bind to ATP, to other small molecules, and to metalloproteins. Hydrogen ions bind to amino acids, peptides, nucleotides, and most macromolecules. In this section we will consider ways of describing mathematically the equilibria involved. 

Rosenthal AE (1967) A graphic method for the determination and presentation of binding parameters in a complex system. Analytical Biochemistry 20 525-532 Sansom LN, Evans AM (1995) What is the True Clinical Significance of Plasma Protein Binding Displacement Interactions Drug Safety 12(4) 227—233 Scatchard G (1949) The attractions of protein for small molecules and ions. New York Academic of Sciences 51 660-692... 

One of the typical chemical characteristics of metal ions and/or their complexes is their ability to bind specifically and reversibly with small molecules, such as alkene, carbon monoxide, molecular nitrogen, and moleeular oxygen [1-3]. For instance, ethylene coordinatively binds with silver or copper ions, and molecular oxygen with metalloporphyrins. When such metal ions or metalloporphyrins are molecularly bound in macromolecules to form metal complexes, the reversible binding of small molecules with the metal complexes in macromolecules offers extensive potential applications, such as separation membranes, absorbents, and sensors. 

Transport function Numerous small molecules and ions are transported effectively through the body only after binding to proteins. For example, fatty acids are carried between fat (adipose) tissue and other tissues or organs by serum albumin, a blood protein. Hemoglobin, a well-known example, carries oxygen from the lungs to other body tissues, and transferrin is a carrier of iron in blood plasma. 

The shift in Eyz value can be used to estimate the ratio of equilibrium constants for the binding of the oxidized and reduced forms of ions to DNA molecule. Similarly, for the detection of small molecules and micelles interactions this value was used [81]. 

The reversible binding of small molecules (substrates. Inhibitors, effectors etc.) to enzyi es or related proteins has been a subject of intense studies by HD method. Selected recent examples can be found in refs. 64-77. Almost an equal number of publications concern the evaluation of binding parameters in higher order assemblies, such as receptors, membranes, protein aggregates etc, (refs. 78-89 see also Chapter 14). Reports of binding of metal ions to proteins (ref. 19,48,81-83, 90-92) and those vhich involve nucleid acids (ref. 49,93-95) should be mentioned. Applications of HD method to three-interacting component systems (such as metal ions and phosphates) are found in refs. 1-3,14,45,46,54-56,96-98 and in Chapter 4. 

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