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Helix-dependent Spin Filtering through the DNA Duplex
Theodore J. Zwang
1
,
Sylvia Hurlimann
1
,
Michael G. Hill
2
, and
Jacque-line K. Barton
1,*
1
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
CA 91125
2
Division of Chemistry, Occidental College, Los Angeles, CA, 90041
Abstract
Recent work suggests that electrons can travel through DNA and other chiral molecules in a spin-
selective manner, but little is known about the origin of this spin selectivity. Here we describe
experiments on magnetized DNA-modified electrodes to explore spin-selective electron transport
through hydrated duplex DNA. Our results show that the two spins migrate through duplex DNA
with different yield, and that spin selectivity requires charge transport
through
the DNA duplex.
Significantly, shifting the same duplex DNA between right-handed B- and left-handed Z-forms
leads to a diode-like switch in spin-selectivity; which spin moves more efficiently through the
duplex depends upon the DNA helicity. With DNA, the supramolecular organization of chiral
moieties, rather than the chirality of the individual monomers, determines the selectivity in spin,
and thus a conformational change can switch the spin selectivity.
Graphical Abstract
Authors are required to submit a graphic entry for the Table of Contents (TOC) that, in
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*
Corresponding Author: jkbarton@caltech.edu.
Notes
The authors declare no competing financial interests.
Supporting Information includes materials and methods, supplementary text, Figs. S1 to S5, Table S1. This material is available free of
charge via the Internet at
http://pubs.acs.org
.
HHS Public Access
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. Author manuscript; available in PMC 2016 December 21.
Published in final edited form as:
J Am Chem Soc
. 2016 December 07; 138(48): 15551–15554. doi:10.1021/jacs.6b10538.
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DNA-mediated charge transport (DNA CT) is well established in both ground and excited
state systems (
1
). Although theoretical models are still being developed, it is clear that the
integrity of the extended
π
-stack of the aromatic heterocycles, the nucleic acid bases, plays a
critical role (
2
4
): electron donors and acceptors must be electronically well coupled into the
π
-stack, typically
via
intercalation, and perturbations that distort the
π
-stack, such as single-
base mismatches, abasic sites, base lesions, protein-binding that kinks the double helix,
attenuate DNA CT dramatically. This latter characteristic has found practical use in
electronic devices and biosensors (
5
7
).
Recent experimental work in the field of spintronics has raised the intriguing possibility that
DNA CT is affected by the inherent spin of the electrons passing through it. The first
experiments to show that double stranded DNA (dsDNA) could function as a spin filter were
conducted under vacuum, where photoelectrons ejected from a gold surface became spin-
polarized after passing through an adsorbed dsDNA monolayer (
8
). Subsequent conductive
AFM measurements showed that the resistance of spin-polarized currents traveling through a
thin film of air-dry dsDNA depended on the ratio of spin up versus spin down electrons
injected into the film (
9
). These observations mirror similar experiments that feature other
chiral organic molecules within a thin film (
10
). Because organic molecules display small
spin-orbit coupling that would otherwise preclude them from exhibiting spin-selective
transport properties, this work has spawned much interest in chirality-induced spin
selectivity (CISS) (
11
13
). Several theories have been offered to account for this effect (
14
17
). One question of particular interest is whether CISS depends more on the isolated
molecular chiral centers or the large-scale macromolecular structures within the films (
15
).
Owing to its ability to undergo macromolecular conformational changes that affect the
helical structure but not the local chirality of the sugar backbone, dsDNA in its native,
hydrated state presents a unique opportunity to differentiate between the monomeric and
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macromolecular parameters of CISS. Of particular interest is the conformational switching
between right-handed B-DNA and left-handed Z-DNA. At high salt concentrations, CG-
repeat sequences in the right-handed B-form can flip into a left-handed zigzag Z-form helix
(
18
). Notably, both B-DNA and Z-DNA support efficient DNA CT (
19
).
We have developed an electrochemical assay to investigate dsDNA-promoted CISS under
fluid conditions. Following work by others, (
20
) our study employs a nickel working
electrode capped with a thin (~ 10 nm) layer of gold (Figure 1) (
21
). Thiol-modified DNA
duplexes are then self assembled onto these electrodes, and DNA-binding redox-active
probes are added to the electrolyte solution. Magnetizing the nickel surface with a
permanent neodymium magnet (0.66 T) generates a spin-polarized current when the
potential is poised negative of the formal reduction potential of the DNA-bound probe
molecules. The sign of the polarization can be switched by changing the direction of the
magnetic field, but its magnitude remains the same. Integrating the Faradaic response of
probe-molecule reduction using cyclic voltammetry gives the total number of redox probes
reduced, which can be used to quantify the yield of DNA CT under different experimental
conditions. Importantly, the redox potentials of all of the probes lie well negative of the
potential of zero charge of the working electrode (
22
). As a result, duplexes within the DNA
film line up approximately normal to the gold surface with the magnetic field lines
essentially collinear with the helical axes.
Figure 1 shows the results obtained at a densely packed dsDNA film (16 bp duplexes, ~ 40
pmol/cm
2
) using methylene blue (MB) as the redox probe. We have previously shown that
MB binds reversibly to DNA monolayers and undergoes a proton-coupled, DNA-mediated
2e
reduction to leucomethylene blue (LB) at −220 mV versus AgCl/Ag (
23
). As can be
seen in Figure 1, the yield of MB undergoing electrochemical reduction varies regularly with
the orientation of the underlying magnetic field, “up” versus “down”. The change in yield
measured by cyclic voltammetry is not large but it is highly reproducible. This effect is fully
reversible and can be switched repeatedly by simply flipping over the permanent magnet
beneath the nickel surface. The ratio of the integrated reduction peaks of MB varies by
10.9% ± 0.6% upon switching the magnetic field direction (up/down). Increasing the length
of the individual DNA helices in these films to 30 bp consistently results in a larger ratio,
15±1%. Importantly, the difference in reduction yield is observed regardless of which
direction the nickel is magnetized initially, and the difference persists even when taking
multiple scans. There is also no discernable change in the magnetic field effect upon varying
the scan rate between 10 mV/s and 20 V/s (
21
).
The magnetic field dependence of DNA CT was also examined using Nile blue (NB) as a
redox probe. NB is covalently bound to DNA, conjugated through a DNA base, and has been
used extensively as a covalent redox reporter (Figure 2) (
24
26
). Self-assembled monolayers
of 17 bp thiolated dsDNA with tethered NB (~ 40 pmol/cm
2
) show a change in the
integrated reduction peaks of 7±1% upon switching the magnetic field direction. The
magnitude of this effect increases with increasing length of dsDNA to 12±2% for 29bp,
16±4% for 43bp, and 29±6% for 60bp oligomers (Fig. S1). There is no measurable effect on
the charge-transfer rates with a change in magnetic field direction (
27
). These data with NB,
however, reveal a clear dependence of the yield of DNA CT on magnetic field orientation.
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Given the range of possible etiologies for the observed magnetic field effect on the
electrochemistry of MB and NB, we carried out a series of control experiments (Figure 2).
Monolayers in which MB is adsorbed directly onto the gold-capped nickel electrodes in the
absence of DNA show no differences in the reduction yield of MB upon switching the
orientation of the magnetic field. Similarly, there is no magnetic field effect on the reduction
of MB bound electrostatically to surfaces coated with single stranded DNA. Moreover,
capping the nickel electrodes with a thicker (35-nm) gold layer eliminates the magnetic field
effects, even for electrodes modified with dsDNA.
Non-intercalative redox probes were also examined for comparison. Ru(NH
3
)
6
3+
binds
electrostatically to the phosphate backbone of DNA and undergoes rapid electrochemical
reduction to Ru(NH
3
)
6
2+
at dsDNA-modified electrodes (
28
). Significantly, we find no
magnetic field dependence of the Ru(NH
3
)
6
3+/2+
couple, despite its proximity to the chiral
macromolecule and likely helical path (Figure 1). We also prepared dsDNA with a
covalently bound diazobenzene probe (dabcyl) tethered to the 3
-phosphate near the
electrode surface. This arrangement allowed us to monitor simultaneously the direct
electrode reduction of dabcyl, which contacts the electrode surface, and the DNA-mediated
reduction of MB. There is a significant difference in the up/down yield of MB reduction, but
no measurable difference for the dabcyl signal (Fig. S3).
We examined the effect of an intervening single base mismatch in the film (Fig. S4). A
mismatch incorporated into dsDNA between the surface and the redox probe decreases the
yield of CT to either MB or NB, which shows that the DNA duplex mediates the CT (
24
);
charge migrates
through
the DNA base pair stack. Interestingly, the spin selectivity
measured through a mismatch mirrors the magnitude of the effect seen in well-matched
duplexes of similar length. This result suggests that when charge is successfully transported
through dsDNA with a mismatch, it travels through the entire dsDNA to the probe; the
attenuation in CT yield with a mismatch leads to an interruption of some of that CT, but has
no effect on spin selectivity.
Combined, these results indicate that: (i) spin polarized currents induced by the underlying
magnetic field are needed for spin selectivity in the DNA electrochemistry; (ii) spin
selectivity requires double stranded DNA; and (iii) the magnetic field effects are observed
only with probes that undergo CT reactions mediated by the DNA duplex.
If the helical structure of dsDNA is responsible for the apparent CISS behavior in these
films, it follows that reversing the chirality of the helices would switch the sense of the
magnetic field effect. Indeed, this is precisely what we find. Both methylated and
unmethylated monolayers of 16bp duplexes featuring d(CG)
8
repeats were self-assembled
onto gold-capped nickel. Circular dichroism confirms that DNA oligomers containing 5-
methylcytosine, d(
m
CG)
8
undergo a B-to-Z transition in the presence of 10 mM MgCl
2
,
while the unmethylated analog, d(CG)
8
remains B-form (Fig. S5); methylated Z-DNA
reverts back to B-DNA upon rinsing away the MgCl
2
(
18
,
29
,
30
). Previous work has shown
that MB intercalates into both B- and Z-DNA and undergoes DNA-mediated reduction in the
presence of 10 mM MgCl
2
(
19
).
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We carried out the electrochemistry to examine B- and Z-form helices on a multiplexed chip
(
24
) consisting of 16 separate gold-capped nickel regions that allowed for the simultaneous
comparison of four distinct monolayers under the identical magnetic field (Figure 3). In the
absence of MgCl
2,
both methylated and unmethylated DNA films show the same favored
magnetization direction for a higher yield of MB reduction (up/down ratio = 18±3%). Upon
addition of 10 mM MgCl
2
, the unmethylated films show no change in behavior, but the
methylated films switch which magnetic field direction promotes the higher yield of MB
reduction (up/down ratio = −9±2%). Replacing the buffer with one that lacks MgCl
2
reverts
the structure from Z- to B-form and restores the original characteristics, yielding again an
up/down ratio of 18±2% for both films.
In addition to functioning as a magnetic field diode, switching between B- and Z-form
dsDNA gives a difference in the magnitude of DNA CISS; normalized to the yield of
electrochemically active MB and with the assumption that 10 mM MgCl
2
results in
complete conversion of surface-bound DNA to Z-form, B-DNA appears to have an
approximately 50% larger spin selectivity than Z-DNA. This change in magnitude of spin
selectivity correlates well with the change in pitch between B-DNA and Z-DNA (3.32 nm
and 4.56 nm respectively) but may result from other differences between the two forms
(such as the greater
π
-stacking in the B- versus Z-form) (
18
,
29
,
30
). These data suggest that
the charge is moving through the duplex along a helical path, because a charge moving in a
fully delocalized
π
-stacked column would not be able to interact with the handedness of the
macromolecule; helical transport among delocalized domains of a few base pairs is possible.
The CISS measured in these experiments is significantly larger than expected for molecules
that lack large spin-orbit coupling. Calculating the energy difference between the two
electron spins at the surface of fully magnetized nickel (~0.6 T) yields a gap (
μ
B
gB
1
cm
−1
) far lower than k
b
T at ambient temperature. Several theoretical models have been
proffered to rationalize the large CISS exhibited by chiral organic films (
16
,
17
,
31
35
).
Aspects of each of these models can be used to understand our data. In addition, it is
worthwhile to consider other factors not currently included in these models that are
important in the context of DNA CT, such as the large polarizability of the
π
-stack in
dsDNA (
36
) or the delocalization of domains across multiple adjacent nucleotides (
37
,
38
).
Our experiments thus demonstrate that magnetic fields can affect the flow of electrons
through native, hydrated dsDNA. Significantly, our data show that electrochemically
generated DNA CISS is observed only at films containing duplex DNA and with redox
probes intercalated into the
π
-stack that undergo DNA-mediated CT. Magnetic field effects
are not observed with redox reporters bound electrostatically to the DNA duplex nor with
tethered reporters that contact the surface directly. It is not simply the electrostatic helical
field that is responsible for the spin-selectivity. Nor is it simply the chiral centers on the
DNA; redox reporters bound to single stranded DNA do not show magnetic field effects. As
with DNA CT, the extended
π
-stack appears to play the crucial role: reversing the
handedness of the helix in the films generates a diode-like spin-filtering response. It is
interesting to consider how conformational changes such as that between B- and Z-DNA
might be utilized as a diode in organic spintronics, indeed, how this spin filtering might be
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applied in practical devices. Finally, it is intriguing to consider whether Nature exploits this
helix-dependent spin selectivity of DNA in some context.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
We are grateful to the NIH (GM61077) and the Moore Foundation for their financial support. TJZ is also an NSF
GRFP fellow (DGE-1144469). We thank Dr. Natalie Muren for discussions. We thank John Abendroth, Professor
Paul Weiss, Elizabeth O’Brien, and Philip Bartels for providing gold-capped nickel surfaces.
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27. To test for effects of magnetic field on the CT rate, we varied the scan rate from 50 mV/s to 20 V/s
(Fig. S2); we see no difference in the cathodic/anodic peak splittings when the magnetic field
direction is switched, suggesting that there is no measurable effect on the charge-transfer rates
with a change in magnetic field direction. We stress however that previous work has shown that in
these electrochemical experiments the DNA CT rates are limited by tunneling through the
alkanethiol linker, ( Drummond TG, Hill MG, Barton JK. J Am Chem Soc. 2004; 126:15010.
[PubMed: 15547981] ) not transport through the DNA, so small changes in the inherent tunneling
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Figure 1. Cyclic voltammetry on electrodes modified with 16 bp dsDNA
A.
Illustration of the dsDNA modified electrodes with 1 μM methylene blue (MB) (left) or
10 μM Ru(NH
3
)
6
3+
(right).
B.
For intercalated MB (above) and electrostatically bound
Ru(NH
3
)
6
3+
(below) reduction yield upon switching the magnetic field direction. Data were
normalized to the first scan with the magnetic field pointing up.
C.
Representative cyclic
voltammograms with the magnet up (red, solid) and magnet down (blue, dotted).
D.
Difference plot showing the current when the magnetic field is pointing up minus the current
when the magnetic field is pointing down. The Ru(NH
3
)
6
3+
experiments were typically done
following MB experiments on the same surface.
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Figure 2. Representative cyclic voltammetry data for various assemblies of DNA-modified
electrodes
29 bp dsDNA or ssDNA was tethered to a gold-capped nickel surface with an alkanethiol
linker. Insets display the full cyclic voltammogram, while the larger plot displayed is
centered around the reduction peak of the redox probe.
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Figure 3. Switching of methylated and unmethylated dsDNA measured on a single multiplexed
chip
A.
A multiplexed chip was prepared with 4 distinct monolayers.
B.
Summary of cyclic
voltammetry data for the two magnetizations were collected for all four quadrants with no
MgCl
2
, then with 10 mM MgCl
2
, then after washing away the MgCl
2
. Each bar represents a
minimum of 4 separate electrode surfaces.
C.
Representative data for 30 bp (top) methylated
d(
m
CG)
15
and (bottom) unmethylated d(CG)
15
Data are plotted as the difference in current
for a reductive sweep when the magnetic field is pointing up minus the current when the
magnetic field pointing down.
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