Tunneling-pathway model

Figure 4 Distance dependence of driving-force-optimized electron tunneling times in Ru-labeled P. aeruginosa azurin. The sohd line is the distance decay predicted by the tunneling-pathway model for ET along an ideal 8-strand ( 8 = 1.0 A ). The dashed line is the best fit to the data fi =. A ) ...

Employing the tunneling-pathway model, Beratan, Betts, and Onuchic... 

The observed rate constants for intramolecular ET over 20-23 A from the asRu center at His60 (k = 3.2 1.2 s ) and His6 (k = 1.6 0.6 s" ) to the Fe =0 heme (Table I) were considered initially to be slow given the large driving force for reaction 9 (-AG = 1.04 eV) (27). However, analysis of the ET pathway between the Ru and heme centers in CCP by the tunneling pathway model (38) predicted ET rates similar to those observed (A. English and... 

The pathway model makes a number of key predictions, including (a) a substantial role for hydrogen bond mediation of tunnelling, (b) a difference in mediation characteristics as a function of secondary and tertiary stmcture, (c) an intrinsically nonexponential decay of rate witlr distance, and (d) patlrway specific Trot and cold spots for electron transfer. These predictions have been tested extensively. The most systematic and critical tests are provided witlr mtlrenium-modified proteins, where a syntlretic ET active group cair be attached to the protein aird tire rate of ET via a specific medium stmcture cair be probed (figure C3.2.5). 

Since the Marcus model was initially a classical or later a semi-classical theory, the introduction of quantum effects was considered to account for these observations. In particular, tunnelling pathways of e.t. would increase the rates of some reactions. A closer analysis has, however, led to the conclusion that this could not be a general explanation [80]. 

Megan, L. J., Kurnikov, 1. V. and. Beratan, D. N, 2002, The nature of tunneling pathway and average packing density models for protein-mediated electron transfer. Journal of Physical Chemistry A. 106, 2002-2006. 

Figure 22 Scheme of tip-monolayer-substrate showing relevant distances in multiple conduction pathway model chain-to-chain distance dec, natural alkanethiol length dn, effective thickness as a function of tilt 0. Tunneling through chain (blue) has decay coefficienty c and through space (red) has coefficient yffg. 

---