Page - 33 - in Charge Transport in DNA - Insights from Simulations

2.6ChargeTransfer inDNA Figure2.3: Energetic profiles of donor andacceptor system in theMarcuspicture. The solid lines show the (diabatic) ground state of the system. Dashed lines show the adiabatic stateswhich occurwith the avoided crossing of the two potentials. Denoted are the three fundamentalCTparameters. Thefirstpart is the inner-sphereRE (λi)whichaccounts for the structural changes in the chargedmolecule. The second contribution is the the outer-sphere RE (λs) which is the energy needed to reorient the environment of the chargedmolecule. The inner-sphere RE has to be computed with quantum-chemical methods[92], while the outer-sphere RE of the environment (e.g. the solvent) can be calculated usingMMmethods. It is extremely important toachievehighaccuracy in thesepa- rameters as theyoccur in theexponential termof theMarcus’ equation. Especially, the computationof theouter-sphereRE isprone to errors as it is highly force-field dependent andoverestimated ingeneral bynon-polarizable force-fields [80]. Figure 2.3 shows the general situation in theMarcus’ picture. The energy of both chargedstates isapproximatedbyaparabola. Themeaningof thethreeparameters canbe illustratedby thisqualitativepicture. Thegeneral issuewithMarcus’ theory is theseveral introducedassumptions. First, the electronic structure of the donor and acceptor system has to be known well for the charged and for the neutral state. This includes knowledge about the (de-)localizationof the charge in the system. Second, theCThas tobe significantly slower than the structural dynamics of the system. Andfinally, themechanismof transporthas tobeknown (adiabatic, non-adiabatic, solvent-controlled, etc.) Toovercome these limitations,moreadvancedcharge transfer calculationmethods were developed and are used in our laboratory. These methods are free of the assumptionsmentioned in connectionwithMarcus’ theory. 33