Ionophores, Channels, and Pumps

7See the Appendix for a discussion of the different meanings of the term ligand in relation to ion channels and in coordination chemistry. 

Tetracycline and several related antibiotics (aureomycin = chlorotetracycline, terramycin — oxytetracycline dihydrate) are also obtained from various species of Streptomyces, but are of very different structure, being based on the fused polycyclic arene naphthacene. The presence of five hydroxy substituents (six for terramycin) and an NMe2 group, and possibilities for various conformations and tautomers, complicate the study of their 

Stability constants, measured in methanol solution, for alkaline earth complexes of a number of ionophores are given in Table XVI (280,289,571-577).8 The values for the complexes of valinomycin and enniatin B lie between the values for the crown ethers 15C5 and 18C6 (cf. Section II.C.5 above), for the middle four entries the values are slightly higher. Stabilities of enniatin B complexes show a modest maximum for Ca2+, and of valinomycin complexes show stabilities increasing up to Ba2+ (281). LogAi values for the Ca2+ complexes of acetate, benzoate, and salicylate are between 4.5 and 4.7 in methanol (578) - the 

Stability constants for a series of M2+ cations, including Ca2+, with A23187 and with ionomycin have been determined in methanol-water mixtures (Taylor, R. W. Pfeiffer, D. R. Chapman, C. J. Craig, M. E. Thomas, T. P. Pure Appl. Chem. 1993, 65, 579-584) the estimation of stability constants for complexes of the anti-fungal antibiotic pradimicin (BMY-28864) with alkaline earth cations is complicated by the tendency of this antibiotic to aggregate (Hu, M. Ishizuka, Y. Igarashi, Y. Oki, T. Nakanishi, H. Spectrochim. Acta A 1999, 56, 181-191). 

9However, the stability of the Ca2+ complex of the antibiotic squalestatin-1 is significantly greater than that of its citrate component (Bal, W. Drake, A. F. Jezowska-Bojczuk, M. Kozlowski, H. Pettit, L. D. Sadler, P. J. J. Chem. Soc. Chem. Commun. 1994, 555-556). 

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