Further increase in pH resulted in its complete disappearance. This is most likely due to the fact that at a higher pH, tyrosine deprotonates much faster, and as a consequence, the deprotonated form more easily quenches Cu(III) ions. For scan rates of 50 and 100 V/s the reduction peak is visible also at pH 7.4. So, for short timescales it is possible to reduce the Cu(III) before quenching by tyrosine could take place. (Figure ). These results suggest that the investigated process does not follow a simple EC mechanism.