Additional evidence for heme release in myoglobin-DDAB films on pyrolitic graphite

In a recent paper [1] we claimed that heme release can occur in myoglobin-DDAB films under certain conditions. Our conclusion was based on the similar voltammetric response and electrocatalytic behavior of myoglobin-DDAB and heme-DDAB films. Our findings were challenged by Guto and Rusling [P. M. Guto and J. F. Rusling, Electrochem. Commun., 8 (2006) 455–459.], who claim that heme release does not occur in the pH 5–7 range. In this paper we present additional evidence for our heme release hypothesis and comment on the points put forward by Guto and Rusling. Firstly, we show that the UV/Vis spectrum of a myoglobin-DDAB solution is virtually identical to that of a heme-DDAB solution. Secondly, we show that the presence of apomyoglobin can explain the small differences in voltammetric response between a heme-DDAB and a Mb-DDAB film. Thirdly, we show that the electron transfer rate constants for Mb-DDAB and heme-DDAB films are similar. The thin layer behavior observed in these experiments cannot be rationalized for intact myoglobin. All these results are in concordance with our heme release hypothesis.