From DNA to Protein

Transcription

Translation

The Genetic Code

Sickle Cell Anemia

OH

SH

SH

+

SH

Au

SHSH

SH

pinhole16

SHSH

SH

MB+

Barton and co-workers showed that the electron transfer was not sufficiently sensitive to the dynamic motions of a G-A mismatch to perturb the electroniccoupling through the bases; hence, the mismatch was not detected.

To solve this problem, they used an electrocatalytic system that coupled ferricyanide as an oxidant to recycle the reduced form of MB (leucomethylene blue) back to MB. The electrocatalysis amplified the sensitivity to the base motions and allowed the G-A mismatch to be detected.

17

Gooding et al. have used 2,6-Disulfonic Acid Anthraquinone (AQDS) as intercalator. AQDS is a anionic intercalator :

Negative Intercalators

The greater sensitivity of the AQDS to electronic perturbations could infer that the AQDS is less well electronically coupled with the base stack than the MB, as inferred by the low rate of electron transfer.

Anal. Chem. 2003, 75, 3845.18

SHSH

SH

SHSH

SH

SH

OH

SH

OH

SH

OH

SH

OH

+

SH

SH

pinhole pinhole

19

SHSH

SH

SH

OH

SH

OH

SHSH

SHSH

OH

SH

OH

SHSH

SH

SH

OH

SH

OH

AQDS

20

Anal. Chem. 2003, 75, 3845.21

ASH

SHSH

SH

OH

SH

OH

SHSH

SHSH

OH

SH

OHB

Ea

Ec

22

Sensors and Actuators B, 111–112, 2005, 515.

If there was a mismatch in the duplex, the electron transfer

was either completely diminished or greatly reduced.

23

Charge Migration Through theDNA Double Helix

Introduction

• The charge-transport in DNA have intrigued:• chemists• Physicists• biologists

• The striking similarity of the π-stacked array of DNA bases to π-stacked solid-state conductors has prompted the suggestion that DNA might efficiently facilitate charge transport.

Charge transfer in DNA

Charge mobility in DNA has consequences for:

DNA damage, which dictates biological damage from: Radiation UV, Light Chemicals

Nanoscale electronic devices

Page 17

Charge Transfer through DNA

Less than a decade after discovering of structural features of DNA

double helix, the first experiments to answer how charge can be

transported through DNA chains were carried out.

ButThis question is still debated-Does DNA act as: An insulator,

A semiconductor

Or a molecular wire???

Insulators, semiconductors and conductors

Energy

Insulator

Conductionband

Valenceband

SemiconductorConductor

Electron

Electron

Hole

The early years

Eley and Spivey (1962)*:Conduction in DNA arises from thermally excited electrons on the paired bases, which traverse along the -stacks of the DNA bases

Gregoli, Olast and Bertinchamps (1982)**:

Charge migration occurs via the stacked bases, but may be hindered by interfering factors **

Radiat. Res. 89*Trans. Farad. Soc. 58

Page 20

Photoinduced Methods

h

DonorAcceptor e-

F. D. Lewis, et al., Acc. Chem. Res. 34, 159, 2001.H-A. Wagenknecht Angew. Chem. Int. Ed. 42, 2454, 2003G. B. Schuster, Acc. Chem. Res. 33, 253, 2000.

Page 22

Key Steps

1. Covalent labeling of the DNA with redox-active

probes.

2. Photochemical initiation of the charge transfer

process.

3. Spectroscopic or electrochemical detection of the

charge transfer processes or analysis of irreversible

DNA products yielded by the charge transfer reaction.

Page 23

• Charge transport can occur through DNA over at least short distance

• Positive charge (hole) transport:Nucleobase guanine is the carrier of positive charge.

• Electron transport:

Thymine and cytosine are the charge carriers?

Hole or electron??????

Page 24

Photoinduced transfer (1)

Donor[Ru(phen’)2dppz] 2+

Acceptor[Rh(phi)2phen’] 3+

Murphy et al. Science 262 (1993)

15 base pairs

Hybridization of DNA strands, each intercalated with metal complex

Ru-complex luminesces, but not when connected to Rh-complex via DNA

DNA is a conductor

Means of transfer

Tunneling(independent of temperature)

Boon and Barton (illustrations)Curr. Opin. Struct. Biol. 12 (2002)

Hopping(dependent of temperature)

kct=k0e-R

kct=k0(T)N-

Grozema et al. (theory)J. Am. Chem. Soc. 122 (2000)