DNA Inspired Molecular Wire
Cofacially-Arrayed Polybenzenoid Nano Structures
MUHS SMART Team: Judson Bro, Fernando Buchanan-Nogueron, Payton Gill, Patrick Jordan, Qateeb Khan, Jacob Klusman, Keinan Knight-Boehm,
Hector Lopez, Jed Sekaran, Jose Rosas, Caleb Vogt, Zeeshan Yacoob, and Nicholas Zausch
Teachers: Keith Klestinski and David Vogt
Mentor: Rajendra Rathore, PhD., Department of Chemistry, Marquette University
Abstract
Can DNA Function as a Molecular Wire?
Based on Moore’s Law, every 18 months the number of transistors
that can be placed per unit area on a microchip will double.
However, by the year 2017 this trend will cease to exist due to the
size restrictions imposed by silicon, the main component of
microchips; this has led to the birth of molecular electronics. An
organic molecule that has been extensively tested for its potential in
molecular electronics is DNA. However, recent research has shown
that DNA cannot function as a molecular wire due to its fragile
nature. This is why Raj Rathore's group is developing organic
molecular wires based on robust macromolecular structures. The
specific assembly designed to study the wire behavior consists of a
triad which is made up of polyfluorenes as an electron donor site, a
spacer unit (or wire) made of polyphenylenes, and an electron
donor-acceptor complexation site composed of hexamethylbenzene
(HMB). A chloranil molecule—an electron acceptor—complexes
with the HMB of the triad, and when a laser is shined on HMB/CA
complex, a hole is introduced in the molecular wire which will travel
30 Å to the polyfluorene donor, via an electron hopping mechanism.
The Marquette University High School SMART Team (Students
Modeling A Research Topic) created a physical model of this
molecular wire using 3D printing technology. Supported by a grant
from NIH-NCRR-SEPA.
Space Fill Model of DNA
Graphics Credit: Frederick D. Lewis, et al. 1997
DNA’s unique double helix structure allows it to act as a
molecular wire for the photon – induced transfer of
electrons. The reason for this comes from DNA's co-facial
arrangement–the nucleic acid base pairs are stacked on
top of one another–of the bases in DNA’s structure, which
allows electrons to pass through the center of the double
helix. DNA’s electron transport mechanism relies on the
breaking and reforming of pi-bonds–the second covalent
bond created between two molecules–due to the
decreasing oxidation potential of the parts that make up
DNA.
Through experimentation scientists have found that the
guanine-cytosine base pair has a lower oxidation potential
than the thyamine-adenine base pair. When a photon with
a wavelength of 334 or 260 nm shines on the thyamineadenine base pair, an electron will leave the base pair
causing a hole to form; this will pull an electron from the
guanine-cytosine base pair. The three main reasons why
DNA is not a feasible choice for commercial electron
transfer is due to its relatively low melting point (100° C), its
tendency to relax into varied uncontrolled conformations,
and lastly, DNA’s effectiveness, as a molecular wire
weakens over large distances [figure on right].
Electron Flow in Molecular Wire
e-
+
In the complex shown, a
photon absorbed by an
electron in a pi-bond orbital
of the hexamethylbenzene
(HMB) group [purple]
liberates that electron to
migrate to the chloranil
molecule [green/red],
leaving HMB with a
positively charged hole.
Next, the chloranil
molecule is repelled as
one electron at a time
hops from one
polyphenylene ring to the
other, which takes an
electron away from the
polyflourene [orange/red].
“Silicon technology will reach fundamental limits by 2017. The
party driven by Moore's law will be over…”
Gordon Moore suggested in the 1960's that the
amount of transistors on an integrated circuit
will double every eighteen months. This has
been shown to be true in this field over the past
decades [figure on left], but there are physical
limits to silicon. Several problems, including
electrical isolation and the difficulties of
manipulating ever-smaller pieces of silicon,
make finding a new technology necessary. To
extend Moore's Law, molecules such as Dr.
Rathores’ will allow for the continuation of the
doubling of memory and size of microchips and
beyond...
eee-
Finally, if in solution,
once the chloranil
molecule loses its
electron, it will recomplex with HMB
+
Then, an electron from
the chloranil or,
presumably, another
circuit component,
would move into the
polyflourene.
Graphics Credit: Frederick D. Lewis, et al. 1997
Moore’s Law
Second, an electron
moves to the HMB group
from one of the co-facial
e- polyphenylene aromatic
rings [light
blue/gray/red].
e-
DNA’s increasing ineffectiveness
over larger distances
Graphic Credit: Frederick D. Lewis, et al. 1997
e-
Conclusion
+
Photo Credit: Microchip Technology, Inc. 2009
By allowing for the continuation of Moore's Law, these molecular
wires will play an important role in creating ―molecular‖ microchips
of larger memory and smaller size than the current silicone based
chips [compared in scale to a penny on left]. The robust nature of
these ―molecular‖ wires allows for many new technological
advances that are intended to increase our scientific knowledge
and make our lives easier.
Primary Citation: Frederick D. Lewis, et al.. Distance-Dependent Electron Transfer in DNA Hairpins (1997) Science 277, 673-676. PDB File: Artificial_Assembly_for_ET.pdb, ©Rajendra
Rathore, PhD., Marquette University
A SMART Team project supported by the National Institutes of Health (NIH)-National Center for Research Resources Science Education Partnership Award (NCCR-SEPA).