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Quantum‐mechanical tunneling of water in heme proteins
Author(s) -
Lamb Don C.,
Kriegl Jan,
Kastens Klaus,
Nienhaus G. Ulrich
Publication year - 2000
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/1099-1395(200010)13:10<659::aid-poc287>3.0.co;2-5
Subject(s) - chemistry , flash photolysis , arrhenius equation , heme , quantum tunnelling , myoglobin , ligand (biochemistry) , kinetics , kinetic isotope effect , photodissociation , carbon monoxide , photochemistry , molecule , chemical physics , reaction rate constant , activation energy , atomic physics , deuterium , organic chemistry , biochemistry , physics , receptor , optoelectronics , quantum mechanics , enzyme , catalysis
Heme proteins are nanolaboratories that can be used to investigate chemical and biochemical phenomena. For example, the rebinding of carbon monoxide to the Fe 2+ –protoporphyrin‐IX prosthetic group in heme proteins after photodissociation at low temperature (<50 K) proceeds predominantly via quantum‐mechanical tunneling, as implied by the presence of an isotope effect on the recombination rate coefficients. Heme proteins are among the few systems where the tunneling of entire molecules has been studied in depth. Recently, we have shown that aquometmyoglobin, which has an Fe 3+ heme with a water molecule at the sixth coordination, can be photoreduced at 80 K, yielding a myoglobin with a photodissociable water ligand bound to the ferrous heme iron (Fe 2+ ). We have investigated the rebinding of the water ligand after laser flash excitation using transient absorption spectroscopy with monitoring in the Soret band from 12 to 150 K and 10 ns to 10 s. The kinetics are non‐exponential, owing to the heterogeneous nature of the protein ensemble investigated, and are described with a kinetic model involving an enthalpy barrier distribution. Above 50 K, the temperature dependence of the kinetics follows the Arrhenius law, implying that ligand rebinding occurs by classical barrier crossing. Below 50 K, the rebinding occurs markedly faster than predicted from the Arrhenius law, suggestive of molecular tunneling through the barrier. We have used a simple model to describe the kinetics, and fitting of this model to the experimental data allowed us to determine the model parameters governing the tunneling rate coefficient. Copyright © 2000 John Wiley & Sons, Ltd.