This work is supported by the Natural Science Foundation of China (21708031), China Postdoctoral Science Foundation (BX20180264, 2018M643519), and the Fundamental Research Funds for the Central Universities (2682021ZTPY075).

1. Equipment preparation
NOTE: Carry out all the procedures in an operating fume hood with suitable personal protective equipment.
<ol>
	<li>Assemble the manual peptide synthesis apparatus in the fume hood (Figure 1). Place the three-way stopcocks (see Table of Materials) onto the vacuum manifold (see Table of Materials) and connect to the nitrogen (N2). Make sure to cap the unused inlets.</li>
	<li>A...

<ol>
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J., 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=Cell-penetrating+peptide+conjugates+of+peptide+nucleic+acids+(PNA)+as+inhibitors+of+HIV-1+Tat-dependent+trans-activation+in+cells.">Cell-penetrating peptide conjugates of peptide nucleic acids (PNA) as inhibitors of HIV-1 Tat-dependent trans-activation in cells.</a> Nucleic Acids Research. 33 (21), 6837-6849 (2005).</li><li>Tunnemann, G., 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=Live-cell+analysis+of+cell+penetration+ability+and+toxicity+of+oligo-arginines.">Live-cell analysis of cell penetration ability and toxicity of oligo-arginines.</a> Journal of Peptide Science. 14 (4), 469-476 (2008).</li><li>Shi, 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=Stapling+of+short+cell-penetrating+peptides+for+enhanced+tumor+cell-and-tissue+dual-penetration.">Stapling of short cell-penetrating peptides for enhanced tumor cell-and-tissue dual-penetration.</a> Chemical Communications. 58 (14), 2299-2302 (2022).</li><li>Komin, A., 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=A+peptide+for+transcellular+cargo+delivery:+Structure-function+relationship+and+mechanism+of+action.">A peptide for transcellular cargo delivery: Structure-function relationship and mechanism of action.</a> Journal of Controlled Release. 324, 633-643 (2020).</li><li>Dietrich, L., 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=Cell+permeable+stapled+peptide+inhibitor+of+Wnt+signaling+that+targets+&#946;-catenin+protein-protein+interactions.">Cell permeable stapled peptide inhibitor of Wnt signaling that targets &#946;-catenin protein-protein interactions.</a> Cell Chemical Biology. 24 (8), 958-968 (2017).</li><li>Tian, Y., 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=Stapling+of+unprotected+helical+peptides+via+photo-induced+intramolecular+thiol-yne+hydrothiolation.">Stapling of unprotected helical peptides via photo-induced intramolecular thiol-yne hydrothiolation.</a> Chemical Science. 7 (5), 3325-3330 (2016).</li><li>De Araujo, A. D., 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=Comparative+&#945;-helicity+of+cyclic+pentapeptides+in+water.">Comparative &#945;-helicity of cyclic pentapeptides in water.</a> Angewandte Chemie International Edition. 53 (27), 6965-6969 (2014).</li><li>Chu, Q., 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=Towards+understanding+cell+penetration+by+stapled+peptides.">Towards understanding cell penetration by stapled peptides.</a> Medicinal Chemistry Communications. 6 (1), 111-119 (2015).</li><li>Bock, J. E., Gavenonis, J., Kritzer, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Getting+in+shape:+Controlling+peptide+bioactivity+and+bioavailability+using+conformational+constraints.">Getting in shape: Controlling peptide bioactivity and bioavailability using conformational constraints.</a> ACS Chemical Biology. 8 (3), 488-499 (2013).</li><li>Tian, Y., 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=Effect+of+stapling+architecture+on+physiochemical+properties+and+cell+permeability+of+stapled+&#945;-helical+peptides:+A+comparative+study.">Effect of stapling architecture on physiochemical properties and cell permeability of stapled &#945;-helical peptides: A comparative study.</a> ChemBioChem. 18 (21), 2087-2093 (2017).</li><li>White, C. J., Yudin, A. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Contemporary+strategies+for+peptide+macrocyclization.">Contemporary strategies for peptide macrocyclization.</a> Nature Chemistry. 3 (7), 509-524 (2011).</li><li>Jain, R. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Normalization+of+tumor+vasculature:+An+emerging+concept+in+antiangiogenic+therapy.">Normalization of tumor vasculature: An emerging concept in antiangiogenic therapy.</a> Science. 307 (5706), 58-62 (2005).</li><li>Patgiri, A., Menzenski, M. Z., Mahon, A. B., Arora, P. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solid-phase+synthesis+of+short+&#945;-helices+stabilized+by+the+hydrogen+bond+surrogate+approach.">Solid-phase synthesis of short &#945;-helices stabilized by the hydrogen bond surrogate approach.</a> Nature Protocols. 5 (11), 1857-1865 (2010).</li><li>Baek, 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=Structure+of+the+stapled+p53+peptide+bound+to+Mdm2.">Structure of the stapled p53 peptide bound to Mdm2.</a> Journal of the American Chemical Society. 134 (1), 103-106 (2012).</li><li>Traboulsi, H., 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=Macrocyclic+cell+penetrating+peptides:+A+study+of+structure-penetration+properties.">Macrocyclic cell penetrating peptides: A study of structure-penetration properties.</a> Bioconjugate Chemistry. 26 (3), 405-411 (2015).</li><li>Tian, Y., 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=A+proline-derived+transannular+N-cap+for+nucleation+of+short+&#945;-helical+peptides.">A proline-derived transannular N-cap for nucleation of short &#945;-helical peptides.</a> Chemical Communications. 52 (59), 9275-9278 (2016).</li><li>Muppidi, A., 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=Rational+design+of+proteolytically+stable,+cell-permeable+peptide-based+selective+Mcl-1+inhibitors.">Rational design of proteolytically stable, cell-permeable peptide-based selective Mcl-1 inhibitors.</a> Journal of the American Chemical Society. 134 (36), 14734-14737 (2012).</li><li>Wiradharma, N., 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=Synthetic+cationic+amphiphilic+&#945;-helical+peptides+as+antimicrobial+agents.">Synthetic cationic amphiphilic &#945;-helical peptides as antimicrobial agents.</a> Biomaterials. 32 (8), 2204-2212 (2011).</li><li>Jones, A. 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The chemical stabilization of peptides by incorporating conformational constraints has proven to be an effective strategy for improving the stability and cell permeability of the peptide26. In this protocol, a step-by-step procedure is described for the synthesis of cyclic CPPs with aromatic cross-links and the evaluation of their permeability across biological barriers. Compared to the hydrophilic lactam or triazole cross-links22,27, the ...

The past few decades have witnessed rapid advances in the development of cell-penetrating peptides (CPPs) for drug delivery. CPPs have been widely used as molecular transporters for the treatment of a range of life-threatening diseases, including neurological disorders1,2, heart diseases3, diabetes4, dermatosis5, and cancer6,7. Cancer remains a global health burden accompanied by a high rate of morbidity and mortality despite widespread research efforts8. A serious obstacle to treating cancer is the limited access of therapeutics to deeper tumor cells due to physiological barriers such as compact extracellular matrix (ECM), abnormal tumor vasculature, multiple membrane barriers, and high interstitial fluid pressure (IFP)9. Thus, developing novel CPPs with superior ability to deliver cargoes across biological barriers is considered an essential strategy for cancer treatment10,11.
CPPs can be categorized into cationic, amphipathic, and hydrophobic CPPs in terms of their physicochemical properties12. Among these, the positively charged HIV-TAT peptide and the synthetic polyarginine are of considerable importance in biomedical research and have been extensively studied to facilitate intracellular drug delivery13. Tunnemann et al. reported that a minimum length of eight arginines is essential for efficient cell penetration of the synthetic polyarginine peptides, based on a cell permeability study conducted using R3 to R12 peptides14. However, these CPPs generally have short plasma half-lives due to their rapid hydrolysis in vivo. In addition, little is known regarding the optimization of the chemical structure of CPPs to increase their trans-barrier ability as it is challenging to penetrate multiple cell membranes15. Thus, the development of novel molecular transporters capable of penetrating biological barriers is strongly desired to enhance drug delivery efficiency. In 2020, Komin et al.16 discovered a CPP called CL peptide, which contains a helix motif (RLLRLLR) and a polyarginine tail (R7) for crossing the epithelial monolayer. A set of CL peptide variants were also synthesized by altering the helical pattern. This exploration could be a significant guide for the development of novel CPPs for the delivery of cargoes across biological barriers. Moreover, Dietrich et al. optimized the cell permeability of the StAX peptide, inhibiting the Wnt/&#946;-catenin signaling pathway by increasing the overall hydrophobicity of the peptides17.
Conformational restriction of unstructured linear peptides by cyclization is an effective way to enhance their proteolytic stability and permeablity18,19,20. The structural reinforcement increases the protease resistance of cyclic peptides, making them more stable in vivo compared to their linear counterparts. In addition, the cyclization of peptides can potentially mask the polar peptide backbone by promoting intramolecular hydrogen bonding, thus increasing the membrane permeability of the peptides21. In the past two decades, chemoselective cyclization methods have become effective strategies for the construction of cyclic peptides with different architectures, such as all-hydrocarbon, lactam, triazole, m-xylene, perfluoroaryl, and other cross-links22,23. The biological barrier imposed by the sophisticated tumor microenvironment could reduce the penetration of drugs in solid tumors24. We have previously found that the cyclic CPPs displayed superior resistance to enzymatic digestion over their linear counterparts20. Furthermore, the overall hydrophobicity of the peptides is critical for their enhanced cell permeability22. Based on the studies discussed above, the combination of a positively charged pattern, elevated overall hydrophobicity, and enhanced proteolysis stability can be hypothesized to increase the permeability of CPPs across biological barriers. In a recent study, we identified two cyclic CPPs with aromatic crosslinks at positions i and i+7 that exhibit improved permeability in tumor cells and tissues compared to their linear counterparts15. Here, a concise synthetic protocol for the synthesis of fluorescently labeled cyclic CPPs and the key steps to investigate their permeability are presented.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>1,2-ethanedithiol</td>
			<td>Aladdin</td>
			<td>K1722093</td>
			<td>stench</td>
		</tr>
		<tr>
			<td>2-(7-Azobenzotriazole)-N,N,N&#39;,N&#39;-tetramethyluronium hexafluorophosphate (HATU)</td>
			<td>HEOWNS</td>
			<td>A-0443697</td>
			<td></td>
		</tr>
		<tr>
			<td>4,4&#39;-bis(bromomethyl)biphenyl</td>
			<td>TCI</td>
			<td>B1921</td>
			<td></td>
		</tr>
		<tr>
			<td>4T1 cells</td>
			<td>ATCC</td>
			<td></td>
			<td>4T1 cells were cultured in DMEM medium supplemented with 10% FBS (Hyclone) in a 37 &#176;C humidified incubator containing 5% CO2.</td>
		</tr>
		<tr>
			<td>Acetonitrile&#160;</td>
			<td>Adamas</td>
			<td>1484971</td>
			<td>toxicity</td>
		</tr>
		<tr>
			<td>Dichloromethane</td>
			<td>Energy</td>
			<td>W330229</td>
			<td>skin harmful</td>
		</tr>
		<tr>
			<td>Diethyl ether</td>
			<td>Aldrich</td>
			<td>673811</td>
			<td>flammable</td>
		</tr>
		<tr>
			<td>Dimethyl sulfoxide</td>
			<td>Beyotime</td>
			<td>ST038</td>
			<td>skin harmful</td>
		</tr>
		<tr>
			<td>Dulbecco&#8217;s Modified Eagle Medium (DMEM)</td>
			<td>Gibco</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Electrospray Ionization Mass Spectrometer</td>
			<td>Waters</td>
			<td>G2-S Tof</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethylene Diamine Tetraacetic Acid (EDTA)</td>
			<td>BioFroxx</td>
			<td>1340</td>
			<td></td>
		</tr>
		<tr>
			<td>Fetal bovine serum (FBS)</td>
			<td>HyClone</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Flow cytometer</td>
			<td>Beckman Coulter</td>
			<td>CytoFLEX</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorescein isothiocyanate isomer (FITC)</td>
			<td>Energy</td>
			<td>E0801812500</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorescent microscope</td>
			<td>Carl Zeiss</td>
			<td>Axio Observer 7</td>
			<td></td>
		</tr>
		<tr>
			<td>Fmoc-Arg(Pbf)-OH</td>
			<td>HEOWNS</td>
			<td>F-81070</td>
			<td></td>
		</tr>
		<tr>
			<td>Fmoc-Cys(Trt)-OH</td>
			<td>GL Biochem</td>
			<td>GLS201115-35202</td>
			<td></td>
		</tr>
		<tr>
			<td>Fmoc-&#946;Ala-OH</td>
			<td>Adamas</td>
			<td>51341C</td>
			<td></td>
		</tr>
		<tr>
			<td>HeLa cells</td>
			<td>ATCC</td>
			<td></td>
			<td>HeLa cells were cultured in DMEM supplemented with 10% FBS (Hyclone) in a 37 &#176;C humidified incubator containing 5% CO2.</td>
		</tr>
		<tr>
			<td>High-Performance Liquid Chromatography</td>
			<td>Agilent</td>
			<td>Agilent 1260</td>
			<td></td>
		</tr>
		<tr>
			<td>High-Performance Liquid Chromatography column</td>
			<td>Agilent</td>
			<td>Poroshell EC-C18 120, 4.6 &#215; 150 mm (pore size 120 &#197;, particle size 4 &#956;m)</td>
			<td></td>
		</tr>
		<tr>
			<td>Lyophilizer</td>
			<td>SP Scientific</td>
			<td>Vir Tis</td>
			<td></td>
		</tr>
		<tr>
			<td>Methanol</td>
			<td>Aldrich</td>
			<td>9758</td>
			<td>toxicity</td>
		</tr>
		<tr>
			<td>Microtiter plate</td>
			<td>Thermo &#956;drop plate</td>
			<td>N12391</td>
			<td></td>
		</tr>
		<tr>
			<td>Morpholine</td>
			<td>HEOWNS</td>
			<td>M99040</td>
			<td>irritant</td>
		</tr>
		<tr>
			<td>Multi-technology microplate reader</td>
			<td>Thermo</td>
			<td>VARIOSKAN LUX</td>
			<td></td>
		</tr>
		<tr>
			<td>N,N-Diisopropylethylamine</td>
			<td>HEOWNS</td>
			<td>E-81416</td>
			<td>irritant</td>
		</tr>
		<tr>
			<td>N,N-Dimethyl formamide</td>
			<td>Energy</td>
			<td>B020051</td>
			<td>harmful to skin</td>
		</tr>
		<tr>
			<td>Poly-Prep column</td>
			<td>Bio-Rad</td>
			<td>7321010</td>
			<td>polypropylene chromatography columns</td>
		</tr>
		<tr>
			<td>Rink Amide MBHA resin (0.572 mmol/g)</td>
			<td>GL Biochem</td>
			<td>GLS180301-49101</td>
			<td></td>
		</tr>
		<tr>
			<td>Three-way stopcocks</td>
			<td>Bio-Rad</td>
			<td>7328107</td>
			<td></td>
		</tr>
		<tr>
			<td>Tissue culture plate insert</td>
			<td>LABSELECT</td>
			<td>14211</td>
			<td></td>
		</tr>
		<tr>
			<td>Trifluoroacetic acid</td>
			<td>HEOWNS</td>
			<td>T63278</td>
			<td>corrosive</td>
		</tr>
		<tr>
			<td>Triisopropylsilane</td>
			<td>HEOWNS</td>
			<td>T-0284475</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypsin</td>
			<td>BioFroxx</td>
			<td>1004</td>
			<td></td>
		</tr>
		<tr>
			<td>Vacuum manifold</td>
			<td>Promega</td>
			<td>A7231</td>
			<td></td>
		</tr>
	</tbody>
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construction of cyclic cell-penetrating peptides for enhanced penetration of biological barriers

Cancer has been a grand challenge in global health. However, the complex tumor microenvironment generally limits the access of therapeutics to deeper tumor cells, leading to tumor recurrence. To conquer the limited penetration of biological barriers, cell-penetrating peptides (CPPs) have been discovered with excellent membrane translocation ability and have emerged as useful molecular transporters for delivering various cargoes into cells. However, conventional linear CPPs generally show compromised proteolytic stability, which limits their permeability across biological barriers. Thus, the development of novel molecular transporters that can penetrate biological barriers and exhibit enhanced proteolytic stability is highly desired to promote drug delivery efficiency in biomedical applications. We have previously synthesized a panel of short cyclic CPPs with aromatic crosslinks, which exhibited superior permeability in cancer cells and tissues compared to their linear counterparts. Here, a concise protocol is described for the synthesis of the fluorescently labeled cyclic polyarginine R8 peptide and its linear counterpart, as well as key steps for investigating their cell permeability.

In this protocol, a synthetic procedure to constrain the linear polyarginine R8 into its cyclic form was presented. The SPPS was conducted manually using a simple apparatus (Figure 1). The detailed synthetic process of SPPS is shown in Figure 2. Briefly, the resin was sufficiently swelled, followed by deprotection of the N&#945;-Fmoc protecting group. Then, the N&#945;-Fmoc-protected amino acid was anchored on the resin unt...

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Construction of Cyclic Cell-Penetrating Peptides for Enhanced Penetration of Biological Barriers

This protocol describes the synthesis of cyclic cell-penetrating peptides with aromatic cross-links and the evaluation of their permeability across biological barriers.

Cancer has been a grand challenge in global health. However, the complex tumor microenvironment generally limits the access of therapeutics to deeper ...

A serious obstacle for cancer treatment is the limited access of therapeutics to deeper tumor cells due to physiological barriers. Thus, the development of novel molecular transporters capable of treating biological barriers is strongly desired to enhance the drug delivery efficiency. The purpose of this procedure is to synthesize the cyclic cell-penetrating peptides for enhanced cell-to-cell penetration.The advantage of this method is that the peptide cyclization can be easily achieved through substitution reactions with cysteines and resin without requiring any metal catalysts. The incorporation of aromatic cross-links improves the overall hydrophobicity of the peptides compared to the hydrophilic lactam or triazole cross-links, thereby enhancing their permeability. To begin, assemble the manual peptide synthesis apparatus in the fume hood.Place the three-way stopcocks onto the vacuum manifold and connect them to nitrogen. Make sure to cap the unused inlets. Attach a 10-milliliter polypropylene column onto the three-way stopcocks and drain the reaction mixture or solvents from the polypropylene column using a rubber pipette bulb or a vacuum via a waste trap.To prepare the resin for peptide synthesis, add 4 to 5 milliliters of DMF to the required amount of resin and transfer it to the polypropylene column with gentle nitrogen bubbling for 30 minutes to swell the resin adequately. Drain the DMF and add 4 to 5 milliliters of 50%morpholine/DMF to the resin. Gently bubble nitrogen for 30 minutes to remove the N-terminal Fmoc group, and then drain the mixture.Wash the resin thoroughly three times by adding 4 to 5 milliliters of DMF to the column and bubbling with nitrogen for at least 1 minute each time. In the same way, wash the resin with dichloromethane three times and DMF three times. Next, dissolve 648.8 milligrams of Fmoc and PBF-protected arginine and 372.6 milligrams of HATU in 5 milliliters of DMF in a centrifuge tube.Add 348.4 microliters of DIPEA to activate the coupling reaction, and transfer the reaction mixture to the polypropylene column with resin. Gently agitate the mixture with nitrogen bubbling for 1 to 2 hours, and then repeat the coupling reaction once. Drain the reaction mixture and wash the resin sequentially with DMF, dichloromethane, and again DMF three times each for at least 1 minute each time.Remove the Fmoc-protecting group and wash the resin following the procedure shown earlier before proceeding to couple the next amino acid. Next, couple the beta-alanine as a spacer for FITC labeling using the same process shown for amino acid coupling, and then add a mixture of FITC, DIPEA, and DMF to the polypropylene column in the dark to perform FITC labeling of peptides on the resin. The reaction would take almost 8 hours.To perform the cyclization of the linear peptide, add a mixture of trifluoroacetic acid, triisopropylsilane, and dichloromethane to the polypropylene column for 2 minutes to selectively remove the trityl-protecting group of cysteine. Drain the mixture and repeat the procedure until the yellowish solution becomes colorless to completely remove the trityl-protecting group. Perform sequential washes of the resin with DMF and dichloromethane at least three times and dissolve 4, 4'bis-bromomethyl-biphenyl in DMF with DIPEA.Add the solution to the column and react for 4 hours. Wash the resin with 4 to 5 milliliters of methanol twice for five minutes each, and dry it with a continuous flow of nitrogen. For peptides containing cysteines, treat the resin with an effective cleavage cocktail of trifluoroacetic acid, triisopropylsilane, and water or trifluoroacetic acid, triisopropylsilane, 1, 2-ethanedithiol, and water.Treat the peptide-bound resin for 2 to 3 hours to cleave the peptide, and then remove the trifluoroacetic acid carefully with a stream of nitrogen. To obtain the crude peptides, add 4 to 5 milliliters of diethyl ether to the cleaved peptide preparation to precipitate the crude peptides, and centrifuge at 10, 000 x g for 4 minutes. Carefully discard the supernatant and air-dry the peptide for 3 minutes in an effective fume hood.Dissolve the small-scale crude peptide in 800 microliters of acetonitrile, and then analyze it using reverse-phase high-performance liquid chromatography and liquid chromatography-mass spectrometry. Inoculate 100, 000 HeLa cells in 2 milliliters of DMEM in a 12-well chamber with a tissue culture plate insert, and incubate for 24 hours in a 37 degrees Celsius humidified incubator containing 5%carbon dioxide. Remove the medium and incubate the cells in the chambers with 1 milliliter of 10 micromolar FITC-R8 or FITC-sR8-4 in FBS-free DMEM for 1 hour.After that, remove the medium containing the peptides and wash the cells three times with 1 milliliter of PBS. Add 1 milliliter of fresh FBS-free DMEM to the chambers, and then co-incubate the HeLa cells in the chamber with the tissue culture plate insert, with the HeLa cells on the round coverslips at the bottom for 2 hours. Fix the HeLa cells on the round coverslips with 2.5%glutaraldehyde for 15 minutes, and then stain the cells with DAPI for 15 minutes.Finally, observe the HeLa cells on the coverslips under a fluorescence microscope. A schematic diagram of the synthesis of FITC-labeled linear R8 peptide and FITC-labeled stapled R8 peptide is presented here. The HPLC and MS spectra of the linear R8 peptide and the stapled R8 peptide are shown in this figure.The retention time of the stapled peptide was substantially longer than that of the linear analog, indicating an enhanced overall hydrophobicity of the peptide after cyclization with the hydrophobic cross-link. This graphical image represents the stability of the linear peptide and the stapled peptide in the presence of 25%FBS. The cyclic R8 remains 77.3%intact after incubation with FBS for 4 hours, while its linear counterpart was mostly degraded, suggesting enhanced proteolytic stability of the cyclic R8 peptide.Live-cell fluorescence microscopy images of HeLa cells and 4T1 cells after 1 hour incubation with 3 micromolar FITC-labeled linear R8 peptide and FITC-labeled stapled R8 peptide are shown here. It can be seen that the cells treated with cyclic R8 with aromatic cross-link exhibited higher intracellular fluorescence than those treated with their linear counterpart. Similar results were obtained with flow cytometry analysis.To further investigate whether cyclic R8 confers enhanced cell-to-cell penetration, transwell models were used to simulate the barrier permeability of the peptides from one cell layer to another. The cyclic R8 clearly exhibited higher trans-barrier penetration than the linear R8 peptide, as indicated by a significant increase in intracellular fluorescence. Complete deprotection of the trityl group of cysteine and resin is important for the following cyclization step.Besides, the cyclization efficiency also depends on the specific sequences and the lengths of the peptides due to steric effects. In such a case, using a resin with lower loading capacity or cyclization the peptides under dilute concentrations in solution phase would be helpful. These cyclic peptides showed an enhanced cell-to-cell permeability compared to their linear counterparts.We believe they hold great promise for conquering biological barriers and will be used for a molecular for further applications in the field of drug delivery.

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1.8K vistas.  Southwest Jiaotong University. Un obstáculo serio para el tratamiento del cáncer es el acceso limitado de la terapéutica a las células tumorales más profundas debido a las barreras fisiológicas. Por lo tanto, el desarrollo de nuevos transportadores moleculares capaces de tratar las barreras biológicas es muy deseado para mejorar la eficiencia de la administración de fármacos. El propósito de este procedimiento es sintetizar los péptidos cíclicos que penetran en las células para mejorar la penetración de célu...