1 / 19

Electrons on the Double Helix: Charge Transport in DNA?

A detour right off the start: :. Electrons on the Double Helix: Charge Transport in DNA?. D eoxyribo n ucleic A cid. Structure proposed by Watson and Crick – 50 years ago Double Helix-Ladder structure Bicomplementary strands.

tucker
Download Presentation

Electrons on the Double Helix: Charge Transport in DNA?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. A detour right off the start:: Electrons on the Double Helix: Charge Transport in DNA?

  2. Deoxyribonucleic Acid • Structure proposed by Watson and Crick – 50 years ago • Double Helix-Ladder structure • Bicomplementary strands “This structure has novel features which are of considerable biological interest” -Watson and Crick,NatureApril 25, 1953.

  3. DNA in our body • Almost 2 meters per strand of DNA wound up at many different levels of structure in a single chromosome. • Encodes the • human genome

  4. DNA Structure • Unique H-bonding base-base coupling • Guanine-Cytosine • Adenine-Thymine • Auto-recognition, self-assembly • Strong p-p bonding between bases • Eley and Spivey (1962): noted that DNA shows resemblance to high mobility aromatic crystals TTF-TCNQ; suggested it as efficient structure for electron transfer. • Charge Transfer • Charge transport (diffusion?) • Electrical conductivity • along stack? from Di Felice et al.

  5. Why it could be important: - DNA damage repair - Bottom up electronics fabrication

  6. The proposed conduction mechanism: charge transport across the bases

  7. Charge Transfer along DNA stack? • Initial experiments (Barton group ‘93) showed possibility of long range charge transfer • Fluorescent group quenched by electron acceptor 20-40 A away. • Transfer efficiency e-br with b ~ 0.2A-1 • Counter to prevailing paradigm of transfer efficiency b ~ 1.5 A-1 from Marcus theory. • Perturbations to the base stack show that charge transfer is through base pairs • “Chemistry-at-a-distance” in other exp. • Long range mobile electrons makes possible interesting electronic effects on double helix.  ‘p-way’ – called ‘wire-like’ “Ask not what physics can do for biology, ask what biology can do for physics”- Stan Ulam

  8. Charge transfer at long distances is independent of the distance. Possibility of electrical conduction?

  9. Physicists Get Involved • Many attempts to measure DC transport across few DNA strands • Unlike charge transfer experiments, no consensus has emerged. • Extreme sensitivity to details Fink and Schönenberger 1999 Metal 1MW/10m Porath et al. 1999 Semiconductor 1MW/10m Cai et al. 2000 Semiconductor >1010W/100 Kasomov et al. 2000 Metal/Super. 300kW/1m Yoo et al. 2001 Semiconductor (polarons) De Pablo et al. 2000 Insulator >1012W/10m Zhang et al. 2002 Insulator > 106(W-cm) • Contact Resistances? • Strong length effects? • Substrate interaction? • Large parameter space? • Residual salt • Weak links? • Damage from probe?

  10. Ac Conductivity Any conducting wire acts like an antenna at ac fields For randomly oriented DNA strands placed in a uniform electric field the loss W due to the motion of electric charges along the strands is, to a good approximation,given by where V is the volume of the conducting medium (see below),E0 is the time averaged applied ac field at the position of the sample, the factor of 1/3 results from a geometrical average of random orientations of the DNA segments with respect to the direction of the applied uniform electric field, and s refers to the real part of the complex conductivity. P. Tran, B. Alavi and G.Gruner: Phys. Rev Lett (2000)

  11. DNA vs linear chain organic conductors (TMTSF)2PF6 dsDNA ssDNA should behave differently

  12. Optical conductivity of DNA and doped Silicon • Phenomenological similarity between doped semiconductor and DNA. • High AC conductivity difficult to rationalize with low DC conductivity Dipole Relaxation Losses in DNA M. Briman, N. P. Armitage,E. Helgren, and G. Gruner NANO LETTERS 2004Vol. 4, No. 4733-736

  13. Phys Rev Lett Nov 2000 The model: fluctuating bases lead to time dependent transfer rate for electrons. This leads to a rate limiting factor for the charge diffusion.

  14. The role of water • Water per nucleotide can be correlated to humidity by Brauer-Emmett-Teller (1938) equation: “Adsorption of Gases in Multimolecular Layers” • Water molecules 2 (3) types • 1st layer characterized by binding energy e1 • 2nd layer characterized by binding energy by eL • Also permanent 0th layer

  15. Indistinguishability of dsDNA and ssDNA AC conductivity is evidence for no conduction between bases. • Effects of hydration in dsDNA. • Hydration itself; well described by BET equation. • The conformational state of dsDNA also changes  At high humidity some conduction might be due to an increase in base-base electron transfer. • Evidence for water dipole absorption being a major contribution to the AC conductivity. Briman, NPA, Helgren, Gruner (2004) NPA, Briman, Gruner (2004)

  16. TeraHertz Absorption in DNA e- e- single strand DNA No possibility for base-base tunneling Double strand DNA possibility for tunneling between base pairs

  17. Digression: Biexponential Debye Relaxation in H2O • Electromagnetic absorption in water can be characterized by two separate dipole relaxation process. • Single molecule rotation  tF=170 fs ~ 5 THz • Collective motion of transient tetrahedrally coordinated water clusters tD=8.5 ps ~ 0.15 THz • ps timescales makes THz crucial Single Molecule Rotation Tetrahedral Cluster Rotation

  18. Dipole Relaxation Effects in DNA • Conductivity normalized to the volume occupied by water is well-described biexponential Debye model. • Low humidities is dominated by single molecule rotation. • High humidities collective effects play a larger role. • Consistent with low(no) base-base conduction

  19. “Researchers from the University of California, Los Angeles, have hammered the final nail in the coffin.” -New Scientist, 2003

More Related