Page - 85 - in Charge Transport in DNA - Insights from Simulations

6.3ChargeTransportCalculations The increase of transmission inDry1 systems canbe explained in termsof theCT parameters. The EC increase for all Dry1 and some of the Dry2 systems, while other characteristics like the fluctuations of the IP remain the same. Starting at the microhydration level Dry2, the structural disorder makes the calculations of transmission impracticable. TheLandauer–Büttiker calculationsasperformed in thisworkwereused toobtain a simple indicator of the molecular conductivity. Basically, the calculated trans- missions represent a measure of structural and dynamical disorder of the linear conducting chain, assuming coherent transport. This assumes thatCToccurs in a single-step fashion, i.e. a hopping picture of transport is excluded. The fact that this approximation does not capture the CT in DNA fully and that the time or- der of conformations (the true time dependence of CT parameters) does play a role, has been shown recently by using the time series of CT parameters as de- scribed above.[128] Still, even this improved description neglects the polarization of environment, its response toCTand the effect onCTenergetics. Therefore, the Landauer transmissions described above aswell as the improved time-dependent treatment in Ref. 128 are both merely indicative. A hopping picture would also have to involve on a more systematic footing the effect of environmental polar- ization,which canbequantifiedbymeansof (outer-sphere) reorganization energy (RE), oneof the crucial parameters inMarcus’ theoryof electron transfer (see also chapter 2.6.1).[71, 85] RE is the energy required to transform the structure of the donorandacceptormoleculesaswell asof theenvironment fromthat correspond- ing to the initial charge state α to that of the final state β (after the transfer has taken place). The RE of the solvent represents themajor contribution to the acti- vation energy for CT and is a decisive factor determining the efficiency of CT in systems immersed inapolarizablemedium;aqueoussolution isa typical example. Detailed information onhow to calculate theREofDNAwithMDsimulations is given in the lastpart of chapter2.6.1 The values of RE for the fully hydrated system are in accordance with previous works.Withthedescribedapproachtheλs forCTinDNAwerefoundtobe1.21eV for aadenine–adenineCTevent and1.41 eV for aguanine–guanineCTevent. [93] Interestingly, nearly identical valueswere obtained for theDry1 system,with the difference smaller than 0.1 eV, see Fig. 6.4. In Dry2, the spread of the obtained values is huge due to structural distortions, and so the larger RE observed for 85