Electron Tunneling in the His¹²⁶ Ru-Modified Azurin:  Tunneling Jumps between Protein Strands via Hydrogen Bonds

Using the tunneling current method recently implemented with the ZINDO quantum chemical model, electron tunneling in the His¹²⁶ Ru-modified blue copper protein Pseudomonas aeruginosa azurin is investigated. The protein structure relevant to the electron transfer process is fully accounted for in quantum mechanical calculations carried out in conjunction with the protein pruning procedure. An interesting feature of the system is that the donor and acceptor complexes are coordinated to two parallel β-strands, so that the tunneling electron has to jump from one strand to the other during the reaction. The two strands are connected by several hydrogen bonds. Using the method of atomic tunneling currents, we investigate how this interstrand tunneling occurs. We find that one of the five hydrogen bonds involved with a length of 2.08 Å is responsible for 82% of the interstrand tunneling current. The tunneling electron is initiated at the Cys¹¹² strand and then crosses this hydrogen bond to the Met¹²¹ strand and finally reaches the Ru atom via the His¹²⁶ terminal. The calculations of the absolute value of the tunneling amplitude carried out with the unadjusted ZINDO model are stable but result in a tunneling matrix element which is about an order of magnitude smaller than that obtained from the experimental data. If, however, a correction tunneling factor is introduced, on the basis of matching an independent ab initio calculation, the rate of activationless electron transfer obtained with such a “tuned” ZINDO model agrees well with that found in the experiment.