Spectral hole burning and selection of conformational substates in chromoproteins.

We investigated spectral holes burnt at 1.5 K into the origins of several tautomeric forms of mesoporphyrin IX-substituted horseradish peroxidase at pH 8 under pressures up to 2 MPa. From the pressure-induced lineshift the compressibility of the apoprotein could be determined. We found that the compressibility changed significantly when measured at different tautomer origins. It was concluded that there must be a correlation between the tautomer configurations of the chromophore and the actual structures of the apoprotein. As a consequence, specific conformational substates of the protein can be selected by optical selection of the associated tautomers.