Spectral properties unique to the myoglobins lacking the usual distal histidine residue

Myoglobins can be divided into two groups. One group contains the usual myoglobins that have histidine at the distal (E7) position, and the other contains a few, but interesting myoglobins that lack the usual distal histidine residue. Spectroscopic examinations have shown that there is a remarkable difference in the Soret band between the two types of myoglobin, and an absorbance ratio of the Soret peak of the acidic met-form to that of the oxy-form seems to be very useful as a simple criterion for predicting whether or not a myoglobin has the usual distal histidine residue.