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1.
Sci Adv ; 8(10): eabl3522, 2022 03 11.
Article in English | MEDLINE | ID: mdl-35275726

ABSTRACT

Taq DNA polymerase functions at elevated temperatures with fast conformational dynamics-regimes previously inaccessible to mechanistic, single-molecule studies. Here, single-walled carbon nanotube transistors recorded the motions of Taq molecules processing matched or mismatched template-deoxynucleotide triphosphate pairs from 22° to 85°C. By using four enzyme orientations, the whole-enzyme closures of nucleotide incorporations were distinguished from more rapid, 20-µs closures of Taq's fingers domain testing complementarity and orientation. On average, one transient closure was observed for every nucleotide binding event; even complementary substrate pairs averaged five transient closures between each catalytic incorporation at 72°C. The rate and duration of the transient closures and the catalytic events had almost no temperature dependence, leaving all of Taq's temperature sensitivity to its rate-determining open state.


Subject(s)
DNA Replication , Nucleotides , Catalysis , Kinetics , Nucleotides/metabolism , Taq Polymerase/chemistry , Taq Polymerase/genetics , Taq Polymerase/metabolism
2.
J Phys Chem B ; 125(22): 5750-5756, 2021 06 10.
Article in English | MEDLINE | ID: mdl-34038124

ABSTRACT

Single-molecule measurements of protein dynamics help unveil the complex conformational changes and transitions that occur during ligand binding and catalytic processes. Using high-resolution single-molecule nanocircuit techniques, we have investigated differences in the conformational dynamics and transitions of lysozyme interacting with three ligands: peptidoglycan substrate, substrate-based chitin analogue, and indole derivative inhibitors. While processing peptidoglycan, lysozyme followed one of the two mechanistic pathways for the hydrolysis of the glycosidic bonds: a concerted mechanism inducing direct conformational changes from open to fully closed conformations or a nonconcerted mechanism involving transient pauses in intermediate conformations between the open and closed conformations. In the presence of either chitin or an indole inhibitor, lysozyme was unable to access the fully closed conformation where catalysis occurs. Instead, lysozymes' conformational closures terminated at slightly closed, "excited" conformations that were approximately one-quarter of the full hinge-bending range. With the indole inhibitor, lysozyme reached this excited conformation in a single step without any evidence of rate-liming intermediates, but the same conformational motions with chitin involved three hidden, intermediate processes and features similar to the nonconcerted peptidoglycan mechanism. The similarities suggest that these hidden processes involve attempts to accommodate imperfectly aligned polysaccharides in the active site. The results provide a detailed glimpse of the enzyme-ligand interplay at the crux of molecular recognition, enzyme specificity, and catalysis.


Subject(s)
Muramidase , Binding Sites , Catalysis , Catalytic Domain , Ligands , Muramidase/metabolism , Protein Binding , Protein Conformation
3.
Nat Biomed Eng ; 5(7): 713-725, 2021 07.
Article in English | MEDLINE | ID: mdl-33820980

ABSTRACT

Simple and fast methods for the detection of target genes with single-nucleotide specificity could open up genetic research and diagnostics beyond laboratory settings. We recently reported a biosensor for the electronic detection of unamplified target genes using liquid-gated graphene field-effect transistors employing an RNA-guided catalytically deactivated CRISPR-associated protein 9 (Cas9) anchored to a graphene monolayer. Here, using unamplified genomic samples from patients and by measuring multiple types of electrical response, we show that the biosensors can discriminate within one hour between wild-type and homozygous mutant alleles differing by a single nucleotide. We also show that biosensors using a guide RNA-Cas9 orthologue complex targeting genes within the protospacer-adjacent motif discriminated between homozygous and heterozygous DNA samples from patients with sickle cell disease, and that the biosensors can also be used to rapidly screen for guide RNA-Cas9 complexes that maximize gene-targeting efficiency.


Subject(s)
Biosensing Techniques/methods , CRISPR-Associated Protein 9/metabolism , DNA/genetics , Polymorphism, Single Nucleotide , Anemia, Sickle Cell/genetics , Anemia, Sickle Cell/pathology , Biosensing Techniques/instrumentation , CRISPR-Associated Protein 9/chemistry , DNA/metabolism , Genome, Human , Graphite/chemistry , Heterozygote , Homozygote , Humans , Immobilized Proteins/chemistry , Immobilized Proteins/metabolism , RNA, Guide, Kinetoplastida/metabolism , Superoxide Dismutase-1/genetics , Transistors, Electronic
4.
Bioconjug Chem ; 31(5): 1449-1462, 2020 05 20.
Article in English | MEDLINE | ID: mdl-32302483

ABSTRACT

Advances in bioconjugation, the ability to link biomolecules to each other, small molecules, surfaces, and more, can spur the development of advanced materials and therapeutics. We have discovered that pyrocinchonimide, the dimethylated analogue of maleimide, undergoes a surprising transformation with biomolecules. The reaction targets amines and involves an imide transfer, which has not been previously reported for bioconjugation purposes. Despite their similarity to maleimides, pyrocinchonimides do not react with free thiols. Though both lysine residues and the N-termini of proteins can receive the transferred imide, the reaction also exhibits a marked preference for certain amines that cannot solely be ascribed to solvent accessibility. This property is peculiar among amine-targeting reactions and can reduce combinatorial diversity when many available reactive amines are available, such as in the formation of antibody-drug conjugates. Unlike amides, the modification undergoes very slow reversion under high pH conditions. The reaction offers a thermodynamically controlled route to single or multiple modifications of proteins for a wide range of applications.


Subject(s)
Amines/chemistry , Imides/chemistry , Proteins/chemistry , Hydrogen-Ion Concentration , Kinetics , Lysine/chemistry , Solvents/chemistry , Sulfhydryl Compounds/chemistry , Thermodynamics
5.
Biosensors (Basel) ; 6(3)2016 Jun 24.
Article in English | MEDLINE | ID: mdl-27348011

ABSTRACT

As biosensing devices shrink smaller and smaller, they approach a scale in which single molecule electronic sensing becomes possible. Here, we review the operation of single-enzyme transistors made using single-walled carbon nanotubes. These novel hybrid devices transduce the motions and catalytic activity of a single protein into an electronic signal for real-time monitoring of the protein's activity. Analysis of these electronic signals reveals new insights into enzyme function and proves the electronic technique to be complementary to other single-molecule methods based on fluorescence. As one example of the nanocircuit technique, we have studied the Klenow Fragment (KF) of DNA polymerase I as it catalytically processes single-stranded DNA templates. The fidelity of DNA polymerases makes them a key component in many DNA sequencing techniques, and here we demonstrate that KF nanocircuits readily resolve DNA polymerization with single-base sensitivity. Consequently, template lengths can be directly counted from electronic recordings of KF's base-by-base activity. After measuring as few as 20 copies, the template length can be determined with <1 base pair resolution, and different template lengths can be identified and enumerated in solutions containing template mixtures.


Subject(s)
Biosensing Techniques , DNA , Nanotechnology , DNA-Directed DNA Polymerase , Nanotubes, Carbon , Templates, Genetic , Transistors, Electronic
6.
J Am Chem Soc ; 137(30): 9587-94, 2015 Aug 05.
Article in English | MEDLINE | ID: mdl-26147714

ABSTRACT

DNA polymerases exhibit a surprising tolerance for analogs of deoxyribonucleoside triphosphates (dNTPs), despite the enzymes' highly evolved mechanisms for the specific recognition and discrimination of native dNTPs. Here, individual DNA polymerase I Klenow fragment (KF) molecules were tethered to a single-walled carbon nanotube field-effect transistor (SWCNT-FET) to investigate accommodation of dNTP analogs with single-molecule resolution. Each base incorporation accompanied a change in current with its duration defined by τclosed. Under Vmax conditions, the average time of τclosed was similar for all analog and native dNTPs (0.2 to 0.4 ms), indicating no kinetic impact on this step due to analog structure. Accordingly, the average rates of dNTP analog incorporation were largely determined by durations with no change in current defined by τopen, which includes molecular recognition of the incoming dNTP. All α-thio-dNTPs were incorporated more slowly, at 40 to 65% of the rate for the corresponding native dNTPs. During polymerization with 6-Cl-2APTP, 2-thio-dTTP, or 2-thio-dCTP, the nanocircuit uncovered an alternative conformation represented by positive current excursions that does not occur with native dNTPs. A model consistent with these results invokes rotations by the enzyme's O-helix; this motion can test the stability of nascent base pairs using nonhydrophilic interactions and is allosterically coupled to charged residues near the site of SWCNT attachment. This model with two opposing O-helix motions differs from the previous report in which all current excursions were solely attributed to global enzyme closure and covalent-bond formation. The results suggest the enzyme applies a dynamic stability-checking mechanism for each nascent base pair.


Subject(s)
DNA Polymerase I/chemistry , DNA Polymerase I/metabolism , Deoxyribonucleotides/chemistry , Deoxyribonucleotides/metabolism , Nanotubes, Carbon/chemistry , Polyphosphates/metabolism , Molecular Structure , Polyphosphates/chemistry
7.
Nano Lett ; 15(8): 5248-53, 2015 Aug 12.
Article in English | MEDLINE | ID: mdl-26189911

ABSTRACT

A single point defect surrounded on either side by quasi-ballistic, semimetallic carbon nanotube is a nearly ideal system for investigating disorder in one-dimensional (1D) conductors and comparing experiment to theory. Here, individual single-walled nanotubes (SWNTs) are investigated before and after the incorporation of single point defects. Transport and local Kelvin Probe force microscopy independently demonstrate high-resistance depletion regions over 1.0 µm wide surrounding one point defect in semimetallic SWNTs. Transport measurements show that conductance through such wide depletion regions occurs via a modified, 1D version of Poole-Frenkel field-assisted emission. Given the breadth of theory dedicated to the possible effects of disorder in 1D systems, it is surprising that a Poole-Frenkel mechanism appears to describe defect scattering and resistance in this semimetallic system.

8.
ACS Chem Biol ; 10(6): 1495-501, 2015 Jun 19.
Article in English | MEDLINE | ID: mdl-25763461

ABSTRACT

Single-molecule techniques can monitor the kinetics of transitions between enzyme open and closed conformations, but such methods usually lack the resolution to observe the underlying transition pathway or intermediate conformational dynamics. We have used a 1 MHz bandwidth carbon nanotube transistor to electronically monitor single molecules of the enzyme T4 lysozyme as it processes substrate. An experimental resolution of 2 µs allowed the direct recording of lysozyme's opening and closing transitions. Unexpectedly, both motions required 37 µs, on average. The distribution of transition durations was also independent of the enzyme's state: either catalytic or nonproductive. The observation of smooth, continuous transitions suggests a concerted mechanism for glycoside hydrolysis with lysozyme's two domains closing upon the polysaccharide substrate in its active site. We distinguish these smooth motions from a nonconcerted mechanism, observed in approximately 10% of lysozyme openings and closings, in which the enzyme pauses for an additional 40-140 µs in an intermediate, partially closed conformation. During intermediate forming events, the number of rate-limiting steps observed increases to four, consistent with four steps required in the stepwise, arrow-pushing mechanism. The formation of such intermediate conformations was again independent of the enzyme's state. Taken together, the results suggest lysozyme operates as a Brownian motor. In this model, the enzyme traces a single pathway for closing and the reverse pathway for enzyme opening, regardless of its instantaneous catalytic productivity. The observed symmetry in enzyme opening and closing thus suggests that substrate translocation occurs while the enzyme is closed.


Subject(s)
Molecular Dynamics Simulation , Muramidase/chemistry , Viral Proteins/chemistry , Acetylglucosamine/chemistry , Amino Acid Substitution , Bacteriophage T4/chemistry , Bacteriophage T4/enzymology , Escherichia coli/genetics , Escherichia coli/metabolism , Gene Expression , Hydrolysis , Kinetics , Motion , Muramic Acids/chemistry , Muramidase/genetics , Mutation , Protein Structure, Secondary , Protein Structure, Tertiary , Recombinant Proteins/chemistry , Recombinant Proteins/genetics , Thermodynamics , Viral Proteins/genetics
9.
Nano Lett ; 14(3): 1329-36, 2014 Mar 12.
Article in English | MEDLINE | ID: mdl-24527984

ABSTRACT

Using a model system of single, isolated carbon nanotubes loaded with high-capacitance metal-oxide films, we have quantitatively investigated electrochemical composites on the single-nanotube scale. Electrochemical charging and discharging of a model MnO2 storage material was used to probe interfacial charge transfer and surface impedances at the nanotube interface. We found that one single-walled carbon nanotube has an apparent surface resistivity of 30 mΩ cm(2), approximately 4 times smaller than for a multiwalled carbon nanotube and 50 times smaller than the 1.5 Ω cm(2) resistivity of Pt or graphite films. The improvement originates in the electrochemical-transport properties of microelectrodes shrunk to a nanotube's dimensions rather than any unique nanotube property like curvature, bandstructure, or surface chemistry. In explaining the enhanced performance of certain nanotube-containing composites, the results overturn widely held assumptions about nanotubes' roles while also providing guidelines for optimizing effective composites.

10.
Phys Chem Chem Phys ; 15(36): 14879-95, 2013 Sep 28.
Article in English | MEDLINE | ID: mdl-23752924

ABSTRACT

Single molecule bioelectronic circuits provide an opportunity to study chemical kinetics and kinetic variability with bond-by-bond resolution. To demonstrate this approach, we examined the catalytic activity of T4 lysozyme processing peptidoglycan substrates. Monitoring a single lysozyme molecule through changes in a circuit's conductance helped elucidate unexplored and previously invisible aspects of lysozyme's catalytic mechanism and demonstrated lysozyme to be a processive enzyme governed by 9 independent time constants. The variation of each time constant with pH or substrate crosslinking provided different insights into catalytic activity and dynamic disorder. Overall, ten lysozyme variants were synthesized and tested in single molecule circuits to dissect the transduction of chemical activity into electronic signals. Measurements show that a single amino acid with the appropriate properties is sufficient for good signal generation, proving that the single molecule circuit technique can be easily extended to other proteins.


Subject(s)
Biocatalysis , Muramidase/metabolism , Kinetics , Models, Molecular , Muramidase/chemistry
11.
J Am Chem Soc ; 135(21): 7861-8, 2013 May 29.
Article in English | MEDLINE | ID: mdl-23631749

ABSTRACT

Single-molecule studies of enzymes open a window into their dynamics and kinetics. A single molecule of the catalytic domain of cAMP-dependent protein kinase A (PKA) was attached to a single-walled carbon nanotube device for long-duration monitoring. The electronic recording clearly resolves substrate binding, ATP binding, and cooperative formation of PKA's catalytically functional, ternary complex. Using recordings of a single PKA molecule extending over 10 min and tens of thousands of binding events, we determine the full transition probability matrix and conversion rates governing formation of the apo, intermediate, and closed enzyme configurations. We also observe kinetic rates varying over 2 orders of magnitude from one second to another. Anti-correlation of the on and off rates for PKA binding to the peptide substrate, but not ATP, demonstrates that regulation of enzyme activity results from altering the stability of the PKA-substrate complex, not its binding to ATP. The results depict a highly dynamic enzyme offering dramatic possibilities for regulated activity, an attribute useful for an enzyme with crucial roles in cell signaling.


Subject(s)
Cyclic AMP-Dependent Protein Kinases/metabolism , Adenosine Triphosphate/metabolism , Catalysis , Kinetics , Nanotubes, Carbon
12.
J Am Chem Soc ; 135(21): 7855-60, 2013 May 29.
Article in English | MEDLINE | ID: mdl-23631761

ABSTRACT

Bioconjugating single molecules of the Klenow fragment of DNA polymerase I into electronic nanocircuits allowed electrical recordings of enzymatic function and dynamic variability with the resolution of individual nucleotide incorporation events. Continuous recordings of DNA polymerase processing multiple homopolymeric DNA templates extended over 600 s and through >10,000 bond-forming events. An enzymatic processivity of 42 nucleotides for a template of the same length was directly observed. Statistical analysis determined key kinetic parameters for the enzyme's open and closed conformations. Consistent with these nanocircuit-based observations, the enzyme's closed complex forms a phosphodiester bond in a highly efficient process >99.8% of the time, with a mean duration of only 0.3 ms for all four dNTPs. The rate-limiting step for catalysis occurs during the enzyme's open state, but with a nearly 2-fold longer duration for dATP or dTTP incorporation than for dCTP or dGTP into complementary, homopolymeric DNA templates. Taken together, the results provide a wealth of new information complementing prior work on the mechanism and dynamics of DNA polymerase I.


Subject(s)
DNA Polymerase I/chemistry , Catalysis , DNA/chemistry , Deoxyadenine Nucleotides/chemistry , Deoxycytosine Nucleotides/chemistry , Deoxyguanine Nucleotides/chemistry , Templates, Genetic
13.
Nano Lett ; 13(2): 625-31, 2013 Feb 13.
Article in English | MEDLINE | ID: mdl-23323846

ABSTRACT

Single-molecule experimental methods have provided new insights into biomolecular function, dynamic disorder, and transient states that are all invisible to conventional measurements. A novel, nonfluorescent single-molecule technique involves attaching single molecules to single-walled carbon nanotube field-effective transistors (SWNT FETs). These ultrasensitive electronic devices provide long-duration, label-free monitoring of biomolecules and their dynamic motions. However, generalization of the SWNT FET technique first requires design rules that can predict the success and applicability of these devices. Here, we report on the transduction mechanism linking enzymatic processivity to electrical signal generation by a SWNT FET. The interaction between SWNT FETs and the enzyme lysozyme was systematically dissected using eight different lysozyme variants synthesized by protein engineering. The data prove that effective signal generation can be accomplished using a single charged amino acid, when appropriately located, providing a foundation to widely apply SWNT FET sensitivity to other biomolecular systems.


Subject(s)
Muramidase/chemistry , Muramidase/metabolism , Nanotubes, Carbon/chemistry , Protein Engineering , Signal Transduction , Models, Molecular , Transistors, Electronic
14.
Science ; 335(6066): 319-24, 2012 Jan 20.
Article in English | MEDLINE | ID: mdl-22267809

ABSTRACT

Tethering a single lysozyme molecule to a carbon nanotube field-effect transistor produced a stable, high-bandwidth transducer for protein motion. Electronic monitoring during 10-minute periods extended well beyond the limitations of fluorescence techniques to uncover dynamic disorder within a single molecule and establish lysozyme as a processive enzyme. On average, 100 chemical bonds are processively hydrolyzed, at 15-hertz rates, before lysozyme returns to its nonproductive, 330-hertz hinge motion. Statistical analysis differentiated single-step hinge closure from enzyme opening, which requires two steps. Seven independent time scales governing lysozyme's activity were observed. The pH dependence of lysozyme activity arises not from changes to its processive kinetics but rather from increasing time spent in either nonproductive rapid motions or an inactive, closed conformation.


Subject(s)
Muramidase/chemistry , Muramidase/metabolism , Bacteriophage T4/enzymology , Biocatalysis , Electric Conductivity , Fluorescence Resonance Energy Transfer , Hydrogen-Ion Concentration , Kinetics , Microscopy, Atomic Force , Nanotubes, Carbon , Peptidoglycan/metabolism , Protein Conformation , Pyrenes , Static Electricity , Thermodynamics , Transistors, Electronic
15.
J Am Chem Soc ; 134(4): 2032-5, 2012 Feb 01.
Article in English | MEDLINE | ID: mdl-22239748

ABSTRACT

The dynamic processivity of individual T4 lysozyme molecules was monitored in the presence of either linear or cross-linked peptidoglycan substrates. Single-molecule monitoring was accomplished using a novel electronic technique in which lysozyme molecules were tethered to single-walled carbon nanotube field-effect transistors through pyrene linker molecules. The substrate-driven hinge-bending motions of lysozyme induced dynamic electronic signals in the underlying transistor, allowing long-term monitoring of the same molecule without the limitations of optical quenching or bleaching. For both substrates, lysozyme exhibited processive low turnover rates of 20-50 s(-1) and rapid (200-400 s(-1)) nonproductive motions. The latter nonproductive binding events occupied 43% of the enzyme's time in the presence of the cross-linked peptidoglycan but only 7% with the linear substrate. Furthermore, lysozyme catalyzed the hydrolysis of glycosidic bonds to the end of the linear substrate but appeared to sidestep the peptide cross-links to zigzag through the wild-type substrate.


Subject(s)
Molecular Dynamics Simulation , Muramidase/metabolism , Peptidoglycan/biosynthesis , Bacteriophage T4/enzymology , Biocatalysis , Hydrolysis , Muramidase/chemistry , Nanotubes, Carbon/chemistry , Peptidoglycan/chemistry , Peptidoglycan/metabolism
16.
Nano Lett ; 11(3): 1055-60, 2011 Mar 09.
Article in English | MEDLINE | ID: mdl-21280660

ABSTRACT

A variation of scanning gate microscopy (SGM) is demonstrated in which this imaging mode is extended into an electrostatic spectroscopy. Continuous variation of the SGM probe's electrostatic potential is used to directly resolve the energy spectrum of localized electronic scattering in functioning, molecular scale devices. The technique is applied to the energy-dependent carrier scattering that occurs at defect sites in carbon nanotube transistors, and fitting energy-resolved experimental data to a simple transmission model determines the electronic character of each defect site. For example, a phenolic type of covalent defect is revealed to produce a tunnel barrier 0.1 eV high and 0.5 nm wide.


Subject(s)
Microscopy/methods , Nanotubes, Carbon , Spectrum Analysis/methods , Static Electricity
17.
Phys Rev Lett ; 104(6): 066401, 2010 Feb 12.
Article in English | MEDLINE | ID: mdl-20366836

ABSTRACT

Motivated by recent experiments, we investigate how NO3-SWNT interactions become energetically favorable with varying oxidation state of a single-walled carbon nanotube (SWNT) using first-principles calculations. Chemisorption becomes less endothermic with respect to physisorption when the SWNT oxidation state is elevated. Importantly, the dissociative incorporation of an oxygen atom into the SWNT sidewall becomes highly favorable when the SWNT oxidation state is elevated from electron density depletion in the vicinity, as caused experimentally using electrochemical potential. The elevation of the SWNT oxidation state through accumulating local charge transfer from the surrounding molecules does not have the same effect. Our investigation reveals the crucial effects of the SWNT oxidation state in understanding the molecule-SWNT interaction.

18.
Nano Lett ; 10(3): 896-901, 2010 Mar 10.
Article in English | MEDLINE | ID: mdl-20155964

ABSTRACT

Individual single-walled carbon nanotubes (SWCNTs) become sensitive to H(2) gas when their surfaces are decorated with Pd metal, and previous reports measure typical chemoresistive increases to be approximately 2-fold. Here, thousand-fold increases in resistance are demonstrated in the specific case where a Pd cluster decorates a SWCNT sidewall defect site. Measurements on single SWCNTs, performed both before and after defect incorporation, prove that defects have extraordinary consequences on the chemoresistive response, especially in the case of SWCNTs with metallic band structure. Undecorated defects do not contribute to H(2) chemosensitivity, indicating that this amplification is due to a specific but complex interdependence between a defect site's electronic transmission and the chemistry of the defect-Pd-H(2) system. Dosage experiments suggest a primary role is played by spillover of atomic H onto the defect site.


Subject(s)
Conductometry/methods , Crystallization/methods , Hydrogen/analysis , Nanostructures/chemistry , Nanostructures/ultrastructure , Nanotechnology/methods , Palladium/chemistry , Hydrogen/chemistry , Materials Testing , Particle Size
19.
Nano Lett ; 9(10): 3586-91, 2009 Oct.
Article in English | MEDLINE | ID: mdl-19754066

ABSTRACT

We investigate electronic devices consisting of individual, metallic, single-walled carbon nanotubes contacted by Pt electrodes in a field effect transistor configuration, focusing on improvements to the metal-nanotube contact resistance as the devices are annealed in inert environments including ultrahigh vacuum. At moderate temperatures (T < 880 K), thermal processing results in high resistance contacts with thermally activated barriers. Higher temperatures (T > 880 K) achieve nearly transparent contacts. In the latter case, analytical surface measurements reveal the catalytic decomposition of hydrocarbons into graphene layers on the Pt surface, suggesting that improved electronic behavior is primarily due to the formation of an all-carbon nanotube-graphite interface rather than to the improvement of the nanotube-Pt one.

20.
Nano Lett ; 9(8): 2991-5, 2009 Aug.
Article in English | MEDLINE | ID: mdl-19637886

ABSTRACT

Dual color four-wave-mixing (FWM) microscopy is used to spatially resolve the third-order optical response from individual carbon nanotubes. Good signal-to-noise is obtained from single-walled carbon nanotubes (SWNT) sitting on substrates, when the excitation beams are resonant with electronic transitions of the nanotube, by detecting the FWM response at the anti-Stokes frequency. Whereas the coherent anti-Stokes (CAS) signal is sensitive to both electronic and vibrational resonances of the material, it is shown that the signal from individual SWNTs is dominated by the electronic response. The CAS signal is strongly polarization dependent, with the highest signals found parallel with the enhanced electronic polarizibility along the long axis of the SWNT.

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