Antibiotics selective binding

Anthraquinone, 1-hydroxy-calcium aluminum chelate compound, 1,2 metal complexes dyes, 6,86 Antiarthritis drugs labelled gold compounds, 6, 969 metal complexes, 6,758 Antibiotic M139163,2, 974 Antibiotics ionophoric, 6, 553 metal complexes selective binding, 6, 552... 

Of the fonr possible optical isomers of chloramphenicol, only the o-threo form is active. This antibiotic selectively inhibits protein synthesis in bacterial ribosomes by binding to the 50S subunit in the region of the A site involving the 23 S rRNA. The normal binding of the aminoacyl-tRNA in the A site is affected by chloramphenicol in such a... 

Although rum ammonia levels are not routinely measured, it is a useful indicator of Reye s syndrome and should be monitored in newborns at risk of developing hyperammonemia Ammonia is produced in many analytically useful enzyme reactions and the ammonium ISE has been used as the base sensor in several enzyme electrodes (see next section). In addition to valinomycin, other antibiotics such as the nonactin homalogs and gramicidins also behave as ionophores. The nonactin homolo were originally studied for their ability to selectively bind potassiiun ions It was then discovered that ammonium ions were preferred over potassium ions, and the selectivity coefficient Knh+ = 0.12 was reported. Since ammonia is present at fairly low levels in serum, this selectivity is not sufficient to to accurately measure NH4 in the presence of K. An extra measure of selectivity can be gained by using a gas permeable membrane to separate the ammonia gas from the sample matrix... 

For examfde, cydic dodecadepsipeptide antibiotic valinomycin (18) is refure-sented as Cydo-(D-Val-Lac-Val-D-Ifyv)3 where Lac and Hyv re esent lactic add and cr-oxyisobutyric add, re ctively. ValinonQrcin selectively binds K, when it acts as an antibiotic. As shown in Fig. 31, valinomycin takes a bracelet structure and has a cavity in the middle with a diameter of 6—7A. Sizes of hydrated cations are... 

Amphoteridn is a complex amphoteric polyene antibiotic that binds to cell membranes and forms a pore through which ions can pass, with consequences that include loss of potassium ions from within the cell. Since the antibiotic binds more readily to fungal cell membranes than mammalian, its action is relatively selective. It can potentiate the action of certain other antifungals. and it may be used with flucytosine. Also, it confers antifungal activity on rifampicin (normally antibacterial). As it has an appreciable renal toxicity, it needs to be used with caution in some patients. Nystatin is a polyene antibiotic similar in structure to amphotericin, often used for local treatment. 

For examfde, cyclic dodecadepsipeptide antibiotic valinomycin (18) is represented as Cyclo D-Val-Lac-Val-D-ifyv)3 where Lac and Hyv re esent lactic acid and a-oxyisobutyric add, re ctively. ValinoiiQrcin selectively binds K, when it acts as an antibiotic. As shown in Fig. 31, valinomycin takes a bracelet structure and has a cavity in the middle with a diameter of 6—7A. Sizes of hydrated cations are 4.5—5.0 A for K, Rb, and Cs and 5.5—7.4 A for Na and Li . It is understandable that the cavity fits in with K. Carbonyl groups are distributed along the inside wall of the cavity which is necessarily polar. Alifdiatic side chains form the outside wall of the bracelet whidi is necessarily nonpolar. Valinomycin binds in the hydro-0ulic interior of the cavity and transports the ion across the lipid bilayer of the cell... 

Figure 15-6b shows how the electrode works. The key in this example is the ligand, L (called an ionopkore), which is soluble inside the membrane and selectively binds analyte ion. In a potassium ion-selective electrode, for example, L could be valinomycin, a natural antibiotic secreted by certain microorganisms to carry ion across cell membranes. The ligand L is chosen to have a high affinity for analyte cation and low affinity for other ions. 

Box 6-1 describes a marine ecosystem in which NH3 is oxidized to NO2 (nitrite) and then to NOJ (nitrate). Ammonium ion in the top millimeter of sediment was measured with a microelectrode made by drawing a glass capillary tube to a fine point with an opening diameter of 1 p,m. Liquid ion exchanger drawn into the tip of the capillary serves as the ion-selective membrane in Figure lS-6. The natural antibiotic nonactin is used as the ligand L in Figure lS-6. It selectively binds... 

---