NNNS Chemistry blog
DNA electron transfer in 30 seconds
27 September 2009 - basics
DNA is a macromolecule that stores genetic information inside a cell. This molecule resembles a spiral staircase with rungs composed of hydrogen bond paired nucleotides. These nucleotides are aromatic and as all rungs are stacked parallel to each other pi stacking occurs as in a "pi way".
A charge transfer complex can form between a donor molecular unit and an acceptor unit along the polymer chain. Electron transfer (ET) between donor and acceptor takes place when an electron is injected into the chain (reductive ET, formation of radical anions) or when an electron is removed (oxidative hole transport, formation of radical cations). The phenomenon is not limited to DNA, proteins also show ET
One way to inject an electron is by photoexcitation of a suitable chromophore. Electron transfer takes place by any of three reaction mechanisms: akin a molecular wire (a delocalized electron moves along the DNA bridge), via a superexchange mechanism (single jump, no localization on the bridge) or via a hopping mechanism (localized electrons move along the bridge in a series of steps).
The rate of electron transfer can be fitted with a modified Marcus equation (k = Ae(-beta.R)) that contains an important constant simply called the beta constant describing the exact nature of distance (R) dependence.
ET in DNA is studied by attaching synthetic light sensitive acceptor and donor chromophores / photo oxidants to it. One way to do this is by sticking a metal to an intercalating ligand (a metallointercalator). Chromophores can also be a part of a hairpin loop and often the chemistry involved is made possible by phosphoramidites. Time-resolved spectroscopy enables the measurement of the time lapsed between exciting the donor molecule and onset of acceptor light emission. Reductive ET processes can also be studied by radiolysis.
The main characteristics of ET are: distances are covered of between 10 and 200 angstrom, the process is extremely fast. ET is also found to depend on many variables such as nucleotide distribution, local DNA conformations and interference from simultaneous proton migration.
ET can also lead to cleavage of the DNA strand, serving not as just the bridge but also as a reactant and guanine is especially vulnerable having the lowest ionization potential of the 4 DNA bases. The guanine radical cation can be studied with ESR. Chemical hole traps stabilize radical cations for example in cyclopropane modified guanine units. Electron transfer thus triggers DNA damage which is biologically relevant. UV radiation and oxidative stress are known to induce cancer. Interestingly electron transfer is also found to trigger certain DNA repair mechanisms.
Nanotech applications: sensitive electrochemical readout for DNA chips. Because of its conductive properties DNA is a building material in molecular electronics.
For the only open-access review on this topic see Wagenknecht, 2006 here.