Lecture 5 Bioelectronics

Nature’s transistors, rectifiers, capacitors ………..

[O2]

Time

ADP ADP ADP

Slope current

Current through your mitochondria

The respiratory chain

0

+600

-400

Sugar

O2+ 4H+ 2H2ODrop in E across gaps is conserved as proton gradientfor ATP synthesis

The mitochondrial battery

E (mV)

Mitochondrialmembrane

electrons

O2 + 4e- + 4H+ 2 H2O

H+

Cytochrome c Oxidase

An electron transfer-driven proton pump

5 metals ions3 -redox centres

CuA (Bi-nuclear Cu)Haem a

Haem a3 - Cub

HQNO

Protein based conducting pathways Formate Dehydrogenase

Multielectron catalysts - molecular wires?

Nitrite reductase

NO2- + 10H+ + 8e- NH4

+ + 2H2O

Hydroxylamine oxidase

NH2OH HNO2 + 4H+ + 4e-

Inspiration from Nature - molecular wires conducting in water

12nm

Marcus Theory

For non-adiabatic electron transfer between donor and acceptorseparated by distance R.

D-|A+ D|A

kET is a function of: Distance between D and ADriving force

ket =(4π2/ h)TDA

2(FC)

TDA

2=TDA

0 2exp(−β(R−Ro))

FC=(4πλkT)−1/ 2 exp[−(−ΔG0 −λ)2 / 4λkT]

Nature knows Marcus Theory

Distance

~ 1.4 Å-1

Driving force

~ 0.7 eV

Page et al Nature (1998)

A physicist’s current is a biochemists rate

Distance

If ~ 1.4 Å-1 then rate drops 10 fold every increase of 1.6Å between donor and acceptor

1013s-1 = 1.6 µA

109s-1 = 0.16 nA

103s-1 = 0.16 fA

Protein based conducting pathways - mobile carriers

Interprotein electron transfer - the cytochrome c/cytochrome b5 paradigm

_

_

++

• Stopped-flow kineticsOne of fastest known interprotein ET reactionsDiffusion limited at low I Still 108 M-1 s-1 at physiological I

• Affinity measurements (by Spectrometry and potentiometry)Weak complex - KD 100µM at physiological I

• Potential measurements at bulk equilibrium and by direct electrochemistry at surfacesCyt b5 redox potential goes up 40-80mV when bound to a positively charged surface

Multihaem cytochromes - nature’s electrical contacts

• React with solid metal oxides

• Mobilisation of FeII from solid iron oxides

• Reduction of soluble UVI to insoluble UIV oxides

• Shewanella - 39 multihaem cytochome genes

heterogeneous ETcontact resistance

Surface attachment/localisation

2D packing and interprotein ET

--

Source

++

Drain

Bias application?Gating?

A protein based transistor for nanotechnology?

A biochemically gated transistor?

Analyte

-- ++

Haem - a cofactor of choice

A conductor - cytochromes A catalyst - P450’s A carrier of dioxygen - globins A sensor - for O2, CO, NO, oxidation state - globins, CooA, PAS etc

NN

NN

Fe

-O O O O-

MN

Ligand1

Ligand2

NN

N

1.5nm

How do we connect electronically to proteins?

Protein electrochemistry

Needs functionalised surfaces - e.g. SAMs on gold, ‘Special’ Graphite

N

S

N

S

N

S

N

S

H3N+

S

S

COO -

COO -

Thiopyridine

Small peptides

Cytochrome c electrochemistry

Electrochemically driven conformational change.

N-stateHis-Fe-Met

+270mV

short timescale<100ms

long timescale>1000s

NR

NOAO

AR

i

VOxRed

A-stateHis-Fe-Lys

-220mV

Electrochemistry and nanotechnology

• AFM on DNA aligned proteins

• Electrochemical AFM

• Electrochemical STM

• Test conductance of assembliese.g. two tip STM or patterned electrodes and conducting AFM tips

110nm500bp – 170nm