Thepossibility of electrical conductivity in DNA was first proposed by Eley in1962, after the helical structure of DNA was discovered by Watson and Crick. Tillnow there has been a large number of researches on the charge transportmechanism in a DNA and the charge transport falls under three terms.
a. Electrontunneling from donor to acceptor b. Chargehopping between discrete base orbitalsc.
Acombination of the above twoInthe first category the charge transport it is assumed that the DNA behaves as a1-D aromatic crystal with electron conductivity. (22,23) This suggeststhat charge transfer takes place along the 1-D pathway formed by overlapbetween orbitals inneighbouring base pairs. The electrons do not exchange energy with themolecules and are therefore never localized.(22)Inthe second category the charge transport involves the transport of chargesthrough the base pairs.
It takes place in multip steps over a large distanceand electrons do exchange energy with the moleucules. The transport shows weakdependence on the distance between the Donor and Acceptor sites.Manyother methods of charge transport have been proposed such as: There ispossibility of combination of holehopping and continuous orbital mechanism because of structural flexibility ofDNA. (24) Ithas been found that rate of charge transport exhibits an experimentaldependence on distance , between the donor andacceptor sites.kCT(R) = k0wherek0 is the pre exponential factor, is the fall off parameter and it is used todistinguish between different charge transport mechanisms that have beenproposed for DNA.Alarge value of -1tells that the charge transport mechanism is a single step tunneling process(27) and it has a large dependence on the Donor Acceptor distance.
The chargedensity on the bridge is always negligible in single step tunneling process.Asmall value of -1indicates that the electron transfer rate weakly depends upon the distancebetween the Donor and Acceptor sites and there are two types of mechanismswhich have been proposed which depend weakly upon the distance between Donorand Acceptor sites.Oneis the incoherent hopping mechanism (27,31) In this mechanism the charge hopsbetween the donor and acceptor through the base pairs. In this the kCT doesnot decay exponentially and hence is not a properparameter. In this mechanism the logarithmic of charge migration rate (kCT)is directly proportional to the logarithmic of the number of base pairs orhopping steps it takes between the donor and acceptor sites until it reachesthe acceptor site.; is the power parameter.Thevalue of is 2 for unbiaseddiffusive hopping and it is between 1 and 2 for a random walk process (27)Theother mechanism which depends weakly on distance between the Donor and acceptorsites implies that donor and acceptor are coupled to each other stronglythrough a bridge. A substantial charge density is present on the bridge and thecharge travels almost coherently .
Sovarious other values of have been reported and are still furtherresearch is going on it.