Page - 72 - in Charge Transport in DNA - Insights from Simulations

ChargeTransport inStretchedDNA Figure5.2: Hole transport efficiency expressed as electric current at high voltage through the studied DNA devices depending on the end-to-end distance of theDNAstrand. Thedistance isgivenrelative to the equilibriumlength of the respectiveDNAoligomer in a freeMDsimulation. erate stretching. There is little change of current through DNA species with all purine nucleobases on one strand (GG, AA and GA) until the elongation of ca. 30%. TheotherDNAoligomers,with ‘mixed’nucleobase sequences, showasteep decreaseof conductivity, startingat the elongationof10 to20%. This canbe com- paredwith thedependenceof themeasured current passing throughDNAon the distanceof electrodes, reported inseveral experiments.[8–10,12] Inall of these, the stretchingof theDNAspecies led to anattenuationof conductivity at somepoint, which occurredwithin a narrow interval of the end-to-enddistance. The authors explained theseobservations in termsof theDNAspeciesdetaching fromtheelec- trodes (possibly consecutively, if severalDNAmoleculeshadbeenboundbetween theelectrodes andweredetaching ina sequentialmanner). Analternative interpretationcanbeproposedonthebasisof thesimulationresults: The point at which the conductivity of DNA of mixed sequence drops steeply maycorrespond to theonsetofoverstretching transition, rather thanadetachment of DNA from an electrode or another crude structural defect. Importantly, this ‘breakingpoint’doesnot requireanoverstretching transition tobecompleted, and occurs atmuch smaller relative elongations – of ca. 20%for the quite shortDNA oligomers studiedhere, andarguably even smaller for longeroligomers. 72