Preexponential-limited solid state chemistry: Ultrafast rebinding of a heme–ligand complex in a glass or protein matrix

Abstract

Ultrafast spectroscopy is used to investigate the temperature dependence of a bimolecular chemical reaction occurring at reaction centers embedded in a glycerol:water glass. The reaction centers consist of carbon monoxide bound to protoheme (PH–CO), or to myoglobin at pH=3 (Mb3–CO), a protein containing PH–CO with a broken proximal histidine–Fe bond. These systems have in common a small energetic barrier for rebinding of the photodissociated ligand. In the glass, the ligand is caged, so that only geminate rebinding is possible. Rebinding is not exponential in time. For t≳20 ps, the survival fraction of deligated heme N(t)∝t−n(n≥0). Below 100 K, rebinding is dominated by an inhomogeneous distribution of activation enthalpy P(ΔH‡) and n is temperature dependent. Inhomogeneous means that every site has a unique barrier. Above 150 K, n becomes independent of temperature. In this high temperature limit, the distribution of preexponential factors, attributed to a distribution of activation entropy P(ΔS‡), dominates rebinding. A picosecond two-pulse experiment demonstrates that the entropy distribution is also inhomogeneous. This work is the first study of heme–ligand rebinding in both low and high temperature limits, which allows a direct investigation of the nature of the activation entropy distribution in a glass. Because ligand rebinding in Mb3–CO and PH–CO is similar, despite the existence of a protein in Mb3–CO which provides a larger free volume for the ligand than does PH–CO, it is concluded that the low energetic barrier encourages immediate ligand rebinding and that the ligand does not diffuse far from the rebinding site at low temperature.