Low pH myoglobin photoproducts.

Recently, there has been interest in determining the conditions under which the iron-histidine bond ruptures in myoglobin at low pH, so that the effect of proximal heme ligation can be studied. A 220-cm-1 Raman mode, assigned to iron-histidine stretching, is clearly visible after photolysis of aqueous MbCO samples below pH4 at room temperature (Sage et al. Biochemistry. 30:1237-1247). In contrast, Iben et al. (Biophys. J. 59:908-919) do not observe this mode upon photolysis of a pH3 MbCO sample in a glycerol/water glass at low temperature. In order to account for both the low temperature and the room temperature experiments, Iben et al. suggest a scheme involving an unusual protonation state of the proximal histidine. Here, we discuss some inconsistencies in their explanation of the room temperature results and offer instead a simple modification of an earlier model. In addition, circular dichroism data are presented that indicate partial unfolding of MbCO in aqueous solution below pH4, and raise questions about the claim of Iben et al. that MbCO remains folded in 75% glycerol at pH3.