Abstract
The role of the protonation of the transferrin amino acid ligands involved in complex formation with iron in the presence of nitrilotriacetate has been elucidated. The C-terminal site of transferrin binds to Fe(nta) to produce FeH3Tc; second-order rate constant k1=(7.00 ± 0.05)× 103 dm3 mol–1 s–1, stability constant K1(1.00 ± 0.10)× 10–5 mol dm–3. This lowers the deprotonation pKa of probably the phenolic side-chain of one tyrosine which loses a proton and, thereby, leads to FeH2Tc; dissociation constant K1a=(4.50 ± 0.50)× 10–7 mol dm–3 and a possible complex stability constant K1′≈ 2.3 × 10–9 mol dm–3. As for the N-terminal site, it binds to Fe(nta) by a process controlled by a slow proton transfer; second-order rate constant k2a=(4.50 ± 0.30)× 106 dm3 mol–1 s–1, a reverse rate constant k–2= 0.40 ± 0.05 s–1, proton dissociation constant K2a=(8.5 ± 1.1)× 10–8 mol dm–3. It remains to be shown whether this slow proton transfer controls a change of the conformation of the binding site or if it occurs because of the particular conformations of the binding sites in neutral media.
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More From: Journal of the Chemical Society, Dalton Transactions
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