Charge transfer in polypeptides

 

Sheh-Yi Sheu

Department of Life Science,

National Yang-Ming University, Taipei, Taiwan

 

The charge and electron transfer in polypeptides are here integrated in a bifunctional model involving electronic charge transfer coupled to special internal rotation. Present molecular dynamics simulations that describe these motions in the chain result in the mean first passage times for the hopping process of an individual step. This “rest and fire” mechanism is formulated in detail, i.e., individual amino acids are weakly coupled and must first undergo alignment to reach the special strong coupling. This bifunctional model contains the essential features demanded by our prior experiments. The molecular dynamics results yield a mean first passage time distribution peaked at about 140 fs, in close agreement with our direct femtosecond measurements. In logic gate language this is a strongly conducting ON state resulting from small firing energies, the system otherwise being a quiescent OFF state. The observed time scale of about 200 fs provides confirmation of our simulations of a transport, a model of extreme transduction efficiency. It explains the high efficiency of charge transport observed in polypeptides. We contend that moderate speed of weak coupling is required in our model by the bifunctionality of peptides. This bifunctional mechanism agrees with our data and contains valuable feature for a general model of long-range conductivity, final reactivity, and binding at a long distance.

 

Fig. 1.   Charge transport in a polypeptide. (a) Polypeptide chain. Charge is first created on the donor and then hops through the amino acid chain until reaching the acceptor. (b) On each amino acid, the motion of the rotors is mapped into a Ramachandran plot. Here a simple two-dimensional (2D) area in phase space represents the hinge, i.e. the junction of two amino acids. The exit or gate part (orange) is the charge ratchet position. After the motion of rotors reaches the gate part, charge jumps to the next amino acid. The iteration of the previous procedure makes the charge hops to the final site. (Ref. from Proc. Natl. Acad. Sci. U.S.A., 97, 1068(2000)).

Reference:

1.        E.W. Schlag, Sheh-Yi Sheu, Dah-Yen Yang, H.L. Selzle and S.H. Lin, "Charge conductivity in peptides: Dynamic simulations of a bifunctional model supporting experimental data", Proc. Natl. Acad. Sci. USA, 97, 1068(2000).

2.        E.W. Schlag, Sheh-Yi Sheu, Dah-Yen Yang, H.L. Selzle and S.H. Lin, "Theory of charge transport in polypeptides", J. Phys. Chem. B 104, 7790(2000).

3.        Sheh-Yi Sheu, E.W. Schlag, Dah-Yen Yang, and H.L. Selzle, "Efficiency of charge transport in a polypeptide chain: the isolated system", J. Phys. Chem. B, 0000(2001).