Name: a high-throughput-compatible fret-based platform for identification and characterization of botulinum neurotoxin light chain modulators
Uploaded: 2013-12-27T13:00:00
Description: Watch this Scientific Journal Video about A High-throughput-compatible FRET-based Platform for Identification and Characterization of Botulinum Neurotoxin Light Chain Modulators at JoVE.com

This work was supported by a grant from the National Institutes of Health (AI082190 to T.J.D.) and the California Institute for Regenerative Medicine (TB1-01186 and CL1-00502).

Manual Screening or IC50 Determination
This protocol can be used to determine the relative potency of a compound (IC50 value) by preparing a dilution series of the compound, or to manually screen for small-molecule inhibitors at a single concentration.
1. Preparation of Buffers, Reagents,&#160;and Required Instrumentation
<ol>
	<li>Prepare 50 ml&#160;of assay buffer (40 mM HEPES, pH 7.4 and 0.01% Tween-20) and filter sterilize....

<ol>
	<li>Schantz, E. J., Johnson, E. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Properties+and+use+of+botulinum+toxin+and+other+microbial+neurotoxins+in+medicine.">Properties and use of botulinum toxin and other microbial neurotoxins in medicine.</a> Microbiol. Rev. 56, 80-99 (1992).</li><li>Sloop, R. R., Cole, B. A., Escutin, R. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Human+response+to+botulinum+toxin+injection:+type+B+compared+with+type+A.">Human response to botulinum toxin injection: type B compared with type A.</a> Neurology. 49, 189-194 (1997).</li><li>Willis, B., Eubanks, L. M., Dickerson, T. J., Janda, K. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+strange+case+of+the+botulinum+neurotoxin:+using+chemistry+and+biology+to+modulate+the+most+deadly+poison.">The strange case of the botulinum neurotoxin: using chemistry and biology to modulate the most deadly poison.</a> Angew. Chem. Int. Ed. Engl. 47, 8360-8379 (2008).</li><li>Tacket, C. O., Shandera, W. X., Mann, J. M., Hargrett, N. T., Blake, P. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Equine+antitoxin+use+and+other+factors+that+predict+outcome+in+type+A+foodborne+botulism.">Equine antitoxin use and other factors that predict outcome in type A foodborne botulism.</a> Am. J. Med. 76, 794-798 (1984).</li><li>Capek, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Enhancing+the+Pharmacokinetic+Properties+of+Botulinum+Neurotoxin+Serotype+A+Protease+Inhibitors+Through+Rational+Design.">Enhancing the Pharmacokinetic Properties of Botulinum Neurotoxin Serotype A Protease Inhibitors Through Rational Design.</a> ACS Chem. Neurosci. 2, 288-293 (2011).</li><li>Schmidt, J. J., Bostian, K. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Proteolysis+of+synthetic+peptides+by+type+A+botulinum+neurotoxin.">Proteolysis of synthetic peptides by type A botulinum neurotoxin.</a> J. Protein Chem. 14, 703-708 (1995).</li><li>Schmidt, J. J., Bostian, K. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endoproteinase+activity+of+type+A+botulinum+neurotoxin:+substrate+requirements+and+activation+by+serum+albumin.">Endoproteinase activity of type A botulinum neurotoxin: substrate requirements and activation by serum albumin.</a> J. Protein Chem. 16, 19-26 (1997).</li><li>Schmidt, J. J., Stafford, R. G., Bostian, K. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Type+A+botulinum+neurotoxin+proteolytic+activity:+development+of+competitive+inhibitors+and+implications+for+substrate+specificity+at+the+S1'+binding+subsite.">Type A botulinum neurotoxin proteolytic activity: development of competitive inhibitors and implications for substrate specificity at the S1' binding subsite.</a> FEBS Lett. 435, 61-64 (1998).</li><li>Schmidt, J. J., Stafford, R. G., Millard, C. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-throughput+assays+for+botulinum+neurotoxin+proteolytic+activity:+serotypes+A,+B,+D,+and+F.">High-throughput assays for botulinum neurotoxin proteolytic activity: serotypes A, B, D, and F.</a> Anal. Biochem.. 296, 130-137 (2001).</li><li>Schmidt, J. J., Stafford, R. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fluorigenic+substrates+for+the+protease+activities+of+botulinum+neurotoxins+serotypes+A,+B,+and+F..">Fluorigenic substrates for the protease activities of botulinum neurotoxins serotypes A, B, and F..</a> Appl. Environ. Microbiol. 69, 297-303 (2003).</li><li>Pellett, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Progress+in+cell+based+assays+for+botulinum+neurotoxin+detection.">Progress in cell based assays for botulinum neurotoxin detection.</a> Curr. Top. Microbiol. Immunol. 364, 257-285 (2013).</li><li>Boldt, G. E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis,+characterization+and+development+of+a+high-throughput+methodology+for+the+discovery+of+botulinum+neurotoxin+a+inhibitors.">Synthesis, characterization and development of a high-throughput methodology for the discovery of botulinum neurotoxin a inhibitors.</a> J. Comb. Chem. 8, 513-521 (2006).</li><li>Henkel, J. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Catalytic+properties+of+botulinum+neurotoxin+subtypes+A3+and+A4.">Catalytic properties of botulinum neurotoxin subtypes A3 and A4.</a> Biochemistry. 48, 2522-2528 (2009).</li><li>Joshi, S. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Detection+of+biologically+active+botulinum+neurotoxin--A+in+serum+using+high-throughput+FRET-assay.">Detection of biologically active botulinum neurotoxin--A in serum using high-throughput FRET-assay.</a> J. Pharmacol. Toxicol. Methods. 65, 8-12 (2012).</li><li>Smith, G. R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reexamining+hydroxamate+inhibitors+of+botulinum+neurotoxin+serotype+A:+extending+towards+the+beta-exosite.">Reexamining hydroxamate inhibitors of botulinum neurotoxin serotype A: extending towards the beta-exosite.</a> Bioorg. Med. Chem. Lett. 22, 3754-3757 (2012).</li><li>Eubanks, L. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+in+vitro+and+in+vivo+disconnect+uncovered+through+high-throughput+identification+of+botulinum+neurotoxin+A+antagonists.">An in vitro and in vivo disconnect uncovered through high-throughput identification of botulinum neurotoxin A antagonists.</a> Proc. Natl. Acad. Sci. U.S.A. 104, 2602-2607 (2007).</li><li>Baldwin, M. R., Bradshaw, M., Johnson, E. A., Barbieri, J. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+C-terminus+of+botulinum+neurotoxin+type+A+light+chain+contributes+to+solubility,+catalysis,+and+stability..">The C-terminus of botulinum neurotoxin type A light chain contributes to solubility, catalysis, and stability..</a> Protein Expr. Purif. 37, 187-195 (2004).</li><li>Zhang, J. H., Chung, T. D., Oldenburg, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Simple+Statistical+Parameter+for+Use+in+Evaluation+and+Validation+of+High+Throughput+Screening+Assays.">A Simple Statistical Parameter for Use in Evaluation and Validation of High Throughput Screening Assays.</a> J. Biomol. Screen. 4, 67-73 (1999).</li><li>Capkova, K., Hixon, M. S., McAllister, L. A., Janda, K. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Toward+the+discovery+of+potent+inhibitors+of+botulinum+neurotoxin+A:+development+of+a+robust+LC+MS+based+assay+operational+from+low+to+subnanomolar+enzyme+concentrations.">Toward the discovery of potent inhibitors of botulinum neurotoxin A: development of a robust LC MS based assay operational from low to subnanomolar enzyme concentrations.</a> Chem. Commun. , 3525-3527 (2008).</li><li>Pires-Alves, M., Ho, M., Aberle, K. K., Janda, K. D., Wilson, B. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tandem+fluorescent+proteins+as+enhanced+FRET-based+substrates+for+botulinum+neurotoxin+activity.">Tandem fluorescent proteins as enhanced FRET-based substrates for botulinum neurotoxin activity.</a> Toxicon. 53, 392-399 (2009).</li></ol>

The FRET-based BoNT/A LC assay described here represents an attractive method for identifying and characterizing small molecules that modulate BoNT/A LC activity. The solution-based nature of the assay makes this protocol amenable to high-throughput screening described in the Automated Operation section of the Protocol Text. The Z-factor is often used to determine the suitability of an assay for HTS campaigns18. Determined as Z = 1 &#8211; 3 (&#963;p + &#963;n)/(&#956;p - &#956...

Botulinum neurotoxin A (BoNT/A), the most potent toxin currently known (LD50 ~1 ng/kg)1, is a potent neurotoxin that is produced by the bacterium Clostridium botulinum. Within an affected host, BoNT/A disrupts neurotransmission at the neuromuscular junction by binding motor neurons, internalizing into the cytosol, and ultimately cleaving neuronal proteins that are essential for acetylcholine exocytosis. Once inside a neuron, BoNT/A can persist for as long as several months2. Long-term inhibition of acetylcholine release hampers normal muscle contraction and results in flaccid paralysis, which, in severe cases, may result in cardiac and/or respiratory failure. Because of its extreme potency, ease of production, and long-term effects within the host, the CDC has labeled all BoNT serotypes as high-risk bioterrorism agents.
The mechanism of action of the toxin involves numerous steps, including binding to neuronal surface receptors, cellular uptake via receptor-mediated endocytosis, and translocation into the neuron cytosol. BoNT/A is comprised of two chains, a heavy and light chain, and both chains are required for toxicity. The heavy chain (HC) contains binding and translocation domains, while the light chain (LC) is a zinc-dependent metalloprotease that translocates from the endosome to the cytoplasm. Once inside the cytosol, the LC/A metalloprotease localizes to the inner cytoplasmic membrane and cleaves the membrane-bound host protein SNAP-25. SNAP-25 is a member of the SNARE (Soluble N-Ethylmaleimide-Sensitive Factor Attachment Protein Receptor) protein family, which plays a crucial role in acetylcholine exocytosis (reviewed in reference3). LC/A cleavage of SNAP-25 impairs SNARE complex function, which inhibits acetycholine neurotransmitter release and impairs muscle contraction.
Currently, treatment for botulism is limited and often includes administration of an equine neutralizing antibody4; however, because the toxin is rapidly internalized into neurons, the antibody has the antibody has a narrow time window of administration. Thus, many researchers believe that the BoNT/A LC may be a better therapeutic target. Because the LC is a zinc-dependent metalloprotease, one approach to inhibit LC/A activity has been to develop compounds that chelate the active-site zinc ion. For example, hydroxamate compounds chelate the active-site zinc and have excellent in vitro potency (Ki of the best BoNT/A LC small molecule inhibitor to date is 77 nM)5. However, many small molecules fail to advance as therapeutics due to various problems ex vivo or in vivo, including poor aqueous solubility, rapid metabolism, and/or high cytotoxicity. Therefore, new compounds with improved pharmacological and pharmacokinetic properties are needed. Small molecule compound identification often involves high-throughput screening (HTS) to identify novel scaffolds. Initial methods for BoNT/A LC activity screening were based on HPLC detection of short peptide substrate cleavage, which is time-consuming and not amenable to HTS applications6-8. Subsequently, Schmidt and colleagues9 developed a high-throughput BoNT/A LC activity assay that utilizes a fluorescein-labeled peptide substrate covalently attached to a microtiter plate. The BoNT/A LC cleaves the substrate and releases fluorescein, which can be quantified with a fluorometer. The plate format of this assay allows numerous compounds to be screened simultaneously; however, the assay requires labeling synthetic peptides with fluorescein and coating the assay plates with derivatized substrate molecules, which are cumbersome techniques. A much simpler method for detecting BoNT/A LC activity at low concentrations was later described by Schmidt et al., where a series of fluorogenic substrates were utilized to monitor BoNT LC activity in real time10. Additional techniques described in the literature include a depolarization after resonance energy transfer-based assay to detect and quantify BoNT activity in crude extracts; this method can be used for high-throughput applications10,11, although it requires sophisticated equipment to measure fluorescence resonance energy transfer (FRET) and polarization signals. Finally, several cell-based models for BoNT intoxication have been reported (reviewed in reference11) that will enable researchers to study the often limiting properties of compounds previously mentioned, including cytotoxicity, cell permeability, and stability. However, most of the existing cell-based assays are not amenable to HTS, and are labor and time intensive.
Herein, we describe a detailed protocol for a HTS method that utilizes the commercially available FRET-based BoNT/A LC substrate. The substrate is based on the SNAP-25 cleavage sequence and is a synthetic 13-mer peptide that contains a terminal fluorophore and quencher. BoNT/A cleavage separates the fluorophore and quencher, abolishing FRET and increasing measured fluorescence, which can be continually measured in a fluorometer plate reader. The assay is used routinely in our, as well as other laboratories, to identify new classes of BoNT/A LC inhibitors or to determine the relative potency of previously identified compounds5,12-15. This assay is suitable for HTS because of its simplicity, automation potential, low cost of materials, and ability to screen numerous compounds simultaneously (see reference16; Cagli&#269; et al., submitted; Bompiani&#160;et al., in preparation). In addition to HTS, this assay can be used to compare the relative potency of compounds by determining the IC50 value (concentration required to inhibit 50% of BoNT/A LC activity) of a compound. The assay can either be performed manually in a 96-well format (Manual Screening section of the Protocol Text) or can be automated in a 384-well format for HTS (Automated Operation section of the Protocol Text).

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr >
			<td >HEPES</td>
			<td >Teknova</td>
			<td >H1021</td>
			<td ></td>
		</tr>
		<tr >
			<td >Tween-20</td>
			<td >Fisher Scientific</td>
			<td >BP337-100</td>
			<td ></td>
		</tr>
		<tr >
			<td >Methanol (HPLC-grade)</td>
			<td >Sigma-Aldrich</td>
			<td >34860</td>
			<td ></td>
		</tr>
		<tr >
			<td >Isopropanol (HPLC-grade)</td>
			<td >Sigma-Aldrich</td>
			<td >650447</td>
			<td ></td>
		</tr>
		<tr >
			<td >96-well Black assay plate</td>
			<td >Costar</td>
			<td >3915</td>
			<td ></td>
		</tr>
		<tr >
			<td >384-well Low-volume black assay plate</td>
			<td >Greiner</td>
			<td >788076</td>
			<td ></td>
		</tr>
		<tr >
			<td >SNAPtide FITC/Dabcyl substrate</td>
			<td >List Biological Laboratories</td>
			<td >521</td>
			<td >FRET-based BoNT/A LC substrate</td>
		</tr>
		<tr >
			<td >Pin cleaning solution</td>
			<td >V&#38;P Scientific</td>
			<td >VP 110</td>
			<td ></td>
		</tr>
		<tr >
			<td >Lint-free blotting paper</td>
			<td >V&#38;P Scientific</td>
			<td >VP 540DB</td>
			<td ></td>
		</tr>
		<tr >
			<td >Biomek Seal and Sample Aluminum foil lids</td>
			<td >Beckman Coulter</td>
			<td >538619</td>
			<td ></td>
		</tr>
	</tbody>
</table>

a high-throughput-compatible fret-based platform for identification and characterization of botulinum neurotoxin light chain modulators

Botulinumneurotoksin (BoNT) er et potent og potentielt dødbringende bakterietoksin, som binder til vært motoriske neuroner, der er internaliseret i cellen, og spalter intracellulære proteiner, der er afgørende for neurotransmitter release. BoNT består af en tung kæde (HC), der medierer værtscelle binding og internalisering, og en let kæde (LC), der spalter intracellulære værtsproteiner er essentielle for frigivelse af acetylcholin. Mens behandlinger, der hæmmer toksinbinding / internalisering har et lille tidsvindue administration, forbindelser, der er målrettet intracellulær LC aktivitet har en meget større tidsvindue af forvaltninger, særligt relevante i betragtning af den ekstremt lang halveringstid af toksinet. I de seneste år, har små molekyler blevet stærkt analyseret som potentielle LC inhibitorer baseret på deres forøget cellulær permeabilitet i forhold til større terapeutika (peptider, aptamers, osv.). Bly identifikation involverer ofte high-throughput screening (HTS), hvorstore biblioteker af små molekyler screenes baseret på deres evne til at modulere terapeutisk target funktion. Her beskriver vi en FRET-baserede assay med en kommerciel BoNT / A LC substrat og rekombinant LC, der kan automatiseres for HTS af potentielle Bont hæmmere. Desuden beskriver vi en manuel teknik, der kan anvendes til opfølgning sekundær screening eller til sammenligning af styrken af ​​flere kandidat-forbindelser.

The FRET-based BoNT/A LC assay can be performed manually in a low-throughput manner to characterize known inhibitors or screen small libraries; alternatively, the protocol can be scaled-down and automated for high-throughput screening (HTS) with large libraries with the aim of identifying novel BoNT/A LC inhibitors. Regardless of the approach taken, an increase in fluorescence should be observed over time when BoNT/A LC is incubated with the substrate (Figure 1A shows a representative plot of fluorescenc...

Watch this Scientific Journal Video about A High-throughput-compatible FRET-based Platform for Identification and Characterization of Botulinum Neurotoxin Light Chain Modulators at JoVE.com

A High-throughput-compatible FRET-based Platform for Identification and Characterization of Botulinum Neurotoxin Light Chain Modulators

The botulinum neurotoxin type A light chain (BoNT/A LC) is a metalloprotease that enters motor neurons, cleaves its substrate SNAP-25, and disrupts neurotransmission, thereby resulting in flaccid paralysis. Utilizing a high-throughput-compatible FRET-based assay, large libraries of small molecules can be screened for their impact on BoNT/A LC enzymatic activity.

En High-throughput-kompatible FRET-baseret platform til identifikation og karakterisering af botulinumneurotoksin lyskæde modulatorer

Botulinum neurotoxin (BoNT) is a potent and potentially lethal bacterial toxin that binds to host motor neurons, is internalized into the cell, and ...

Research

JoVE Journal

Chemistry

Isolation and Chemical Characterization of Lipid A from Gram-negative Bacteria

Isolering og kemisk karakterisering af lipid A fra gramnegative bakterier

Lipopolysaccharide (LPS) is the major cell surface molecule of gram-negative bacteria, deposited on the outer leaflet of the outer membrane bilayer. LPS ...

Qualitative Identification of Carboxylic Acids, Boronic Acids, and Amines Using Cruciform Fluorophores

Kvalitativ Identifikation af carboxylsyrer, borsyrer og Aminer Brug korsformede Fluoroforer

Molecular cruciforms are X-shaped systems in which two conjugation axes intersect at a central core. If one axis of these molecules is substituted with ...

Folding and Characterization of a Bio-responsive Robot from DNA Origami

Foldning og karakterisering af en Bio-responsiv Robot fra DNA Origami

The DNA nanorobot is a hollow hexagonal nanometric device, designed to open in response to specific stimuli and present cargo sequestered inside. Both ...

Synthesis and Characterization of Functionalized Metal-organic Frameworks

Syntese og karakterisering af funktionaliserede Metal-organisk Frameworks

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and ...

Preparation and Characterization of Individual and Multi-drug Loaded Physically Entrapped Polymeric Micelles

Forberedelse og karakterisering af individuelle og Multi-drug Loaded Fysisk indesluttet polymermiceller

Amphiphilic block copolymers like polyethyleneglycol-block-polylactic acid (PEG-b-PLA) can self-assemble into micelles above their critical micellar ...

Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization

Ethylen Polymeriseringer Brug Parallel Pressure reaktorer og en Kinetic Analyse af Chain Transfer Polymerisation

We demonstrate a method for high-throughput catalyst screening using a parallel pressure reactor starting from the initial synthesis of a nickel ...

Synthesis and Characterization of Supramolecular Colloids

Syntese og karakterisering af supramolekylære Colloids

Control over colloidal assembly is of utmost importance for the development of functional colloidal materials with tailored structural and mechanical ...

Low-energy Cathodoluminescence for (Oxy)Nitride Phosphors

Low-energy Cathodoluminescence for OxyNitride Phosphors

Lavenergi Cathodoluminescence for (oxy) Nitride Fosforfarver

Nitride and oxynitride (Sialon) phosphors are good candidates for the ultraviolet and visible emission applications. High performance, good stability and ...

Preparation and Characterization of C60/Graphene Hybrid Nanostructures

Preparation and Characterization of C60/Graphene Hybrid Nanostructures

Forberedelse og karakterisering af C60/Graphene Hybrid nanostrukturer

Physical thermal deposition in a high vacuum environment is a clean and controllable method for fabricating novel molecular nanostructures on graphene. We ...

Synthesis of a Thiol Building Block for the Crystallization of a Semiconducting Gyroidal Metal-sulfur Framework

Syntese af en Thiol byggesten for krystallisering af halvledende Gyroidal Metal-svovl rammer

We present a method for preparing thioester molecules as the masked form of the thiol linkers and their utilization for accessing a semiconducting and ...