Charge transport properties of ideal and natural DNA segments, as mutation detectors

DNA sequences of ideal and natural geometries are examined, studying their charge transport properties as mutation detectors. Ideal means textbook geometry. Natural means naturally distorted sequences; geometry taken from available databases. A tight-binding (TB) wire model at the base-pair level is recruited, together with a transfer matrix technique. The relevant TB parameters are obtained using a linear combination of all valence orbitals of all atoms, using geometry, either ideal or natural, as the only input. The investigated DNA sequences contain: (i) point substitution mutations – specifically, the transitions guanine (G) ↔ adenine (A) – and (ii) sequences extracted from human chromosomes, modified by expanding the cytosine–adenine–guanine triplet [(CAG)n repeats] to mimic the following diseases: (a) Huntington's disease, (b) Kennedy's disease, (c) Spinocerebellar ataxia 6, (d) Spinocerebellar ataxia 7. Quantities such as eigenspectra, density of states, transmission coefficients, and the – more experimentally relevant – current–voltage (I–V) curves are studied, intending to find adequate features to recognize mutations. To this end, the normalised deviation of the I–V curve from the origin (NDIV) is also defined. The features of the NDIV seem to provide a clearer picture, being sensitive to the number of point mutations and allowing to characterise the degree of danger of developing the aforementioned diseases.