Ligands discrimination

Ligand substitution kinetics on water-soluble porphyrins have been measured.18,105 106 The Co111 ion of the porphyrin exchanges axial ligands several orders of magnitude more readily than do simple amine or aqua complexes. Relatively low ligand discrimination ratios and positive activation entropies support a dissociative mechanism (equation 18, Table 7). 

NeuAc [221]. The complex of sialoadhesin with NeuAco 2-3Gal l-4Glc (O Fig. 15) shows seven hydrogen bonds and two hydrophobic interactions with sialic acid, while Gal and Glc make fewer contacts [221], consistent with the proposal that lactose is a scaffold upon which ligand discrimination centers on NeuAc. 

Germain, R.N. (2001) The T cell receptor for antigen signaling and ligand discrimination. J.Biol.Chem., 276, 35223-35226. 

Yang, Y., Kochoyan, M., Burgstaller, P., Westhof, E. and Famulok, M. (1996). Structural basis of ligand discrimination by two related RNA aptamers resolved by NMR spectroscopy. Science 272, 1343-1347. 

J. Vojtechovsky, J. Berendzen, K. Chu, L. Schlichting, and R.M. Sweet, Implications for the Mechanism of Ligand Discrimination and Identification of Substrates Derived from Crystal Structures of Myoglobin-Ligand Complexes at Atomic Resolution. Protein Data Bank, Structure Files, 1A6N and 1A6M. 

Using ultrasonic absorption, Tamura " has measured the complexation kinetics of Cd(II) with acetate, propionate, and n-butyrate in aqueous solution at pH 6. There is very little ligand discrimination, with K s in the range 0.91 to 1.14with AFqs= 1.7 to 2.4cm mol and the rate constants for complex formation, kf, 4.1 to 5.4xl0 s , with AFf = 4.6 to 6.7cm mol . Data for complexation of Ag(I) by C22C2 is included in Table 9.2. 

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