Long lived intermediate metal site structure upon binding of cadmium to plastocyanin

Perturbed angular correlation of γ-rays (PAC) spectroscopy of cadmium substituted plastocyanin shows one dominant metal site configuration at pH 7.5. Lowering the pH to 4.8 a fraction of the molecules undergoes structural change and loses the bound cadmium ion. At pH 4.4 all plastocyanin is in the apo-form. Increasing the pH back to neutral pH values two distinct metal site coordination geometries were observed. One of the two signals is the same as that found initially at pH 7.5; the other form is stable for hours at 1 °C, indicating the existence of a long lived intermediate metal site structure. The cadmium ion is surrounded by the same ligands (His37, Cys84, His87 and Met92) in both forms, however the metal center in the long lived intermediate metal site structure can be best described by a larger His–metal–His angle.