Determination of stability constants of Cu(I), Cd(II) & Zn(II) complexes with thiols using fluorescent probes

• Fluorometric indicators were used in titrations to resolve metal binding constants.
• Binding constants were affirmed for biogenic thiol complexes of Cd and Zn.
• New binding constants for biogenic thiols with Cu(I) were determined.
• Least squares minimization was used to fit stability constants to titration data.
• Thiols are unlikely to buffer extracellular reduced copper in seawater.

Fluorometric competing-ligand titrations were used to measure stability constants of Zn(II), Cd(II) and Cu(I) complexes of cysteine and glutathione (GSH). Cu(I)-stability constants were also determined for the dipeptides Arg-Cys and Gln-Cys which are produced by a marine alga under copper stress. The fluorescent ion indicators FluoZin-1 and BTC (Invitrogen) were used as competing ligands in titrations involving Zn(II) and Cd(II). Phen Green SK (Invitrogen) was likewise used in Cu(I) titrations. Conditional and cumulative general stability constants were determined using a least squares fit of the titration data to speciation models. The measured stability constants of Cd(II) and Zn(II) complexes were consistent with previous work, validating our method and assumptions. Our results also include the first general stability constants for Cu(I)-cysteine complexes and an alternative set for Cu(I)-GSH complexes. While these stability constants indicate that Cu(I) forms strong complexes with thiols, they are not strong enough to effectively buffer Cu(I) in seawater.

A copper-specific fluorometric ion indicator was used to measure conditional and general stability constants for copper(I)-thiol complexes. Speciation models incorporating these constants predict that in surface seawater thiol ligands are much less effective at buffering Cu(I) concentrations than Cu(II) ligands with equivalent ion affinities.Figure optionsDownload as PowerPoint slide