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Green Solid-Phase Peptide Synthesis-2.1 2-Methytetrahydrofuran and ethyl acetate for solid-phase peptide synthesis under green conditions Yahya E Jad, Gerardo A. Acosta, Thavendran Govender, Hendrik G. Kruger, Ayman El-Faham, Beatriz G. de la Torre, and Fernando Albericio ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01765 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 24, 2016

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ACS Sustainable Chemistry & Engineering

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Green Solid-Phase Peptide Synthesis-2.† 2-

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Methyltetrahydrofuran and ethyl acetate for solid-

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phase peptide synthesis under green conditions

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Yahya E. Jad,a Gerardo A. Acosta,b,c Thavendran Govender,a Hendrik G. Kruger,a Ayman El-

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Faham,d,e Beatriz G. de la Torre,a,* and Fernando Albericioa,b,c,e,f,*

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a

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University Road, Westville, Durban 4001, South Africa. Email, [email protected] or

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[email protected].

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b

Catalysis and Peptide Research Unit, School of Health Sciences, University of KwaZulu-Natal,

CIBER-BBN, Networking Centre on Bioengineering, Biomaterials and Nanomedicine,

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Barcelona Science Park, Baldiri Reixac 10, 08028-Barcelona, Spain

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c

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Barcelona, Spain. Email: [email protected]

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d

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Alexandria 21321, Egypt.

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e

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11451, Saudi Arabia.

Department of Organic Chemistry, University of Barcelona, Martí i Franqués 1–11, 08028-

Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426, Ibrahimia,

Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh

ACS Paragon Plus Environment

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f

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Durban 4001, South Africa

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KEYWORDS: Green chemistry, solid-phase peptide synthesis, 2-MeTHF, solid-phase

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synthesis, polyethylenglycol resins, peptide aggregation.

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Abstract

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DMF, NMP, and DCM are the most widely used solvents for Fmoc solid-phase peptide synthesis

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(SPPS). These solvents are considered hazardous chemicals and are normally used in large

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amounts for washing, deprotection, and coupling steps. Therefore, the use of these reagents is

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indeed in question. Our group recently reported employing 2-MeTHF, which is a green solvent,

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for coupling, but using DMF for the Fmoc removal and washing steps. Herein, total full green

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solvent protocols are reported where DMF and DCM are completely eliminated. Several green

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solvents, such as 2-MeTHF, EtOAc and IPA; temperature; solid-supports; and peptide models

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were evaluated in this study. The best green protocol established is the use of 2-MeTHF for

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Fmoc removal, washing and coupling steps with some more washing with EtOAc at room

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temperature for a short and challenging peptide (Aib-enkephalin pentapeptide). In the case of a

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longer and more difficult peptide (Aib-ACP decapeptide), the best protocol established was

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similar, except for the Fmoc removal and coupling steps that were conducted at 40 °C and in

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combination with the use of a polyethylene glycol resin (ChemMatrix resin).

School of Chemistry and Physics, University of KwaZulu-Natal, University Road, Westville,

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Introduction

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Solid-phase peptide synthesis (SPPS) was first reported by the Nobel Prize Laureate Bruce

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Merrifield in 19631 and it is considered the method of choice for synthesis of peptides, not only


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in research laboratories, but even also in the pharmaceutical industry.2-3 The most common

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scheme for SPPS is that the C-terminal amino acid is anchored to the solid support followed by

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removal of the temporary Nα protecting group. This is followed by addition of the next N-

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protected amino acid with the assistance of a coupling reagent.4 Furthermore, SPPS can be

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classified into two widely used strategies with respect to the Nα protecting group, namely Fmoc

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or Boc approaches. The great advantage of this method is that it allows for the use of an excess

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of reagents in order to facilitate the completion of reactions. The excess reagents or byproducts

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can easily be removed by filtration followed by and several washings steps of the resin. This

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implies the use of a large amount of solvents. The most commonly employed one is DMF, while

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NMP and DCM are also occasionally used. In fact, solvents are the major component of the

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reaction mixture representing 80-90% of the non-aqueous masses; as concluded in a survey by

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GSK at in 2007 about the materials used for the manufacturing of APIs.5 However, the above

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mentioned chemicals were classified as hazardous materials by several selection guides for

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greener chemistry.6-10 Moreover, the use of DMF by the pharmaceutical industry is likely to be

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restricted by REACH.11 Therefore, the search for greener alternatives for DMF in peptide

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synthesis is necessary.

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Since water is the greenest solvent, several attempts have been carried out to achieve SPPS in

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this media.12-15 However, the most commonly used protecting groups (such as Fmoc, Boc or Z)16

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are hydrophobic compounds and therefore result in solubility issues either during dissolving or

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washing of excess reagents from the resin. Thus, the search for green organic solvent(s) that

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readily dissolve Fmoc amino acid derivatives and coupling reagents as well as having good resin

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swelling properties, is indeed critical. To the best of our knowledge, the first attempt to replace

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DMF with an environmentally friendlier solvent for SPPS was reported by our group in 2009


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when MeCN replaced DMF for coupling, deprotection and washing steps, in combination with

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ChemMatrix, a full PEG resin. PEG resins swell better than PS in solvents such as MeCN.17-18

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Later, we have reported that THF or MeCN are good and friendlier alternatives for DMF in

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SPPS in combination with the ChemMatrix resin, and DIC/OxymaPure as coupling reagents.

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This solvent rendered the product in twice the yield when compared to DMF during SPPS of

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difficult sequences such as Aib-enkephalin pentapeptide or Aib-ACP decapeptide.19 More

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recently, we reported on the use of 2-MeTHF and CPME as green solvents20 for peptide

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synthesis.21 Various types of coupling reagents, and solid supports were employed in that study

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and the best protocol identified involved the use of 2-MeTHF, DIC/OxymaPure as a coupling

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reagent, and PS resin for synthesis of Aib-enkephalin pentapeptide in 97 % purity.21 However,

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DMF was used for removal of Fmoc groups and the washing procedures.19, 21

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2-MeTHF is definitely an eco-friendly solvent22 derived from renewable resources. Biomass

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solvents have recently gained more interest because of the continuous increase of the petroleum

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price and they also offer a promising approach to diminish waste disposal cost.23-24 It is found

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both industrial and academic applications in several synthetic schemes such as organometallics,25

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organocatalysis,26 and biotransformations.27-28 Although, 2-MeTHF is a good solvent for various

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coupling reagents and Fmoc amino acids, it is very costly in comparison to EtOAc or IPA.

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Furthermore, all of these solvents are green29 and are recommended by several selection guides.6,

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8-9, 30-32

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using 2-MeTHF, EtOAc and IPA for coupling, Fmoc removal, and the washing steps.

In this study, a total green solid-phase peptide synthesis (GSPPS) strategy is reported

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Results and discussion

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Since the solubility of Fmoc amino acid derivatives can limit the use of 2-MeTHF for GSPPS,

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their solubilities were previously

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investigated. Here, we expanded our solubility study for to


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all Fmoc L-AA-OHs as well as Fmoc-Aib-OH. All Fmoc derivatives generally showed good

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solubilities in 2-MeTHF where when 0.2 M solutions were prepared for all derivatives. The only

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exception was except for Fmoc-Asn(Trt)-OH which presented a solubility of (0.1M). This is still

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acceptable since the concentration ranges 0.1 or 0.2 M are the most commonly utilized for the

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preparation of peptides on peptide synthesizers.

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Since the aim of this study is to replace the hazardous chemicals, DMF and DCM, with green

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ones, the new protocols adopting green solvents were compared to other methods. Specifically

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we tested a matrix of methods such as: The standard SPPS method (Table 1, method A), which

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uses DMF and DCM, and the method previously developed by our group that uses 2-MeTHF for

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the couplings and DMF and DCM for the rest remaining of steps (Table 1, method B).21 In

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addition protocol C which uses IPA for the deprotection step and in combination with 2-MeTHF

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for the washing steps was tested. Protocol D is similar to protocol C, but with EtOAc instead of

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replacing IPA. Protocol E uses 2-MeTHF for deprotection and EtOAc for the washings. All new

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protocols (D-E) employed DIC/OxymaPure in 2-MeTHF for the coupling steps, because we have

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previously demonstrated its excellent coupling efficiency.21 DIC/OxymaPure was also used in

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protocols A and B.

17 18 19 20 21 22 23


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Table 1. Different protocols were used on this study. a 2-MeTHF Standard (A)

for coupling only (B)

IPA

EtOAc

2-MeTHF

2-MeTHF

(C)

(D)

1 × IPA

1 × EtOAc

2-MeTHF (E)

1 × DMF Washing

1

1 × 2-MeTHF

1 × DCM

3 × EtOAc

1 × DMF

1 × 2-MeTHF

20 % pip/DMF

20 % pip/IPA

7 min

7 min

2 Deprotection

20 % pip/EtOAc

20 % pip/2MeTHF

7 min

7 min

1 × DMF Washing

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1 × 2-MeTHF 1 × IPA

1 × EtOAc

1 × 2-MeTHF

1 × 2-MeTHF

1 × DCM

3 × EtOAc

1 × DMF

1 × 2-MeTHF

Fmoc-AA-OH/DIC/ OxymaPure (3: 3: 3 equiv) 1 h Coupling

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DMF

2-MeTHF

2

a

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Baseline study

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The peptide model Aib-enkephalin pentapeptide (H-Tyr-Aib-Aib-Phe-Leu-NH2)19 was

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manually assembled with Fmoc-SPPS on RinkAmide-AM-PS and H-RinkAmide-AM-

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ChemMatrix resins. Although it is a short sequence with only five amino acids, the

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presence of two α,α-disubstituted amino acid residues (Aib) increases the possibilities for

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the formation of the des-Aib33 tetrapeptide as a major side-product. The results obtained

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from this synthesis will be were considered as a the baseline before applying selecting the

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best more appropriate protocol to synthesize a more difficult and longer peptide. The best

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result was obtained by protocol E with employing the ChemMatrix resin (purity 95 %).

Red represents hazardous solvents while green represents green solvents


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This is extremely exiting in light of the 97% yield purity obtained (protocol B, PS resin)

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with the conventional environmentally unfriendly solvent (DMF) (protocol B, PS resin).

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Furthermore, protocol D gave also an excellent HPLC purity (91.9 %) with the

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ChemMatix resin. On the other hand, protocol C had a low yield (30.8 %) with

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ChemMatrix resin and also failed for the PS resin. In this regards and with respect of to

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the PS resin, protocol D and E gave far less pure product than protocol A and B.

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Furthermore, a considerable amount of des-Leu (39.8%) in case of protocol D and des-

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Phe (31.0%) in case of protocol E were formed. Therefore, we can conclude that protocol

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C underperformed, while protocols D and E can be considered as a good alternative green

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solvent protocol for SPPS in combination with the ChemMatrix resin. It is important to

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highlight that protocols D and E (ChemMatrix resin) render produced better results than

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the standard protocol A, which uses hazardous solvents (Table 2).

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Table 2. Baseline study of full green solvent protocol by SPPS of Aib-enkephalin pentapeptide Entry Protocol Purity (%) b a

ChemMatrix resin

14 15 16 17 18

PS resin

Penta

des-Aib

Penta

des-Aib

1

Ac

53.0

47.0

71.8

28.2

2

Bd

81.9

18.1

97.0

3.0

3

C

30.8

16.2

NA

NA

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D

91.9

5.8

38.6

1.6

5

E

95.0

5.0

41.6

1.6

a

Same conditions/solvents used for the synthesis as in Table 1, b Determined by HPLC using the following condition: A linear gradient of 5–95% 0.1% TFA in CH3CN/0.1% TFA in H2O over 30 min was applied, with a flow rate of 0.3 mL/min−1 and detection at 220 nm using an YMC-Triart C18 (3 µm, 3.0 × 150 mm) column c Data extracted from ref. 34 in case of PS resin and ref. 21 in case of ChemMatrix resin, d Data extracted from ref. 21.


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Solid-phase assembling on ChemMatrix resin of a peptide with tendency to aggregate

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Evaluation of these two green protocols (D and E with the ChemMatrix resin) for SPPS

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of peptides with a tendency to aggregate is important because aggregation is believed to

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be more determinant for the final purity than steric hindrance.35 For this purpose, a

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previously studied model Aib-ACP decapeptide (H-Val-Gln-Aib-Aib-Ile-Asp-Tyr-Ile-

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Asn-Gly-NH2)19 was chosen. This is the modified version of the model peptide, ACP(64-

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74),36 which is probably the most used model for determining the efficiency of new SPPS

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protocols; in regards to coupling reagents, resins; heating conditions, or, as for this case,

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solvent systems. Although this can be considered a short peptide, it contains a sequence

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very prone to aggregate.35 In addition, the two alanine residues in position 66 and 67 in

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the parent sequence were substituted by two Aib residues with the idea intent of

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introducing steric hindrance. Similar to Aib-enkephalin pentapeptide, des-Aib 9-mer is

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the most common side product observed because of the high sterically hindered nature of

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the Aib residue.

15 16 17 18 19 20 21 22


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Table 3. HPLC purity for Aib-ACP decapeptide assembled on ChemMatrix resin under different

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conditions. a Entry Protocol b

3 4

Deca (%)

des-Aib(%)

1

Ac

37.8

34.0

2

B

70.0

24.1

3

D

30.3

41.4

4

E

37.3

25.1

5

E+ deprotection at 40 °C

42.8

49.9

6

E+ deprotection/coupling at 40 °C

87.1

8.9

a

Determined by HPLC using the same condition reported in ref. 19. b Same conditions/solvents used for the synthesis as in Table 1. c Data extracted from ref. 19.

5

6 7

Figure 1. HPLC chromatograms of Aib-ACP decapeptide assembled on ChemMatrix resin.

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HPLC conditions are the same as reported in ref. 19.


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Protocol E gave better HPLC purity than protocol D and similar comparable to the purity

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obtained to by protocol A. However, it gave lower yields purity than protocol B, where only the

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coupling reactions were carried out in 2-MeTHF. A carefully analysis of the HPLC

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chromatogram obtained from protocol E shows a considerable amount of peptides impurities

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potentially derived from incomplete Fmoc deprotection, identified as N-truncated peptides. From

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this observation, the Fmoc removal conditions were modified from treating the resin with 20%

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piperidine in 2-MeTHF for 7 min at room temperature to 15 min at 40 °C37 in the same solvent

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mixture. A negligible amount of Fmoc N-protected peptides were observed by using this

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condition. However, the yield purity of the decapeptide was still less lower than with protocol B

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(entry 5 vs 2, in Table 3). Next, when both deprotection and coupling steps were carried out at 40

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°C, this protocol was found to be superior (87.1 % product - entry 6, Table 3) to the conventional

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(with hazardous solvents) and green methods. At this point, it can be concluded that the use of

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the green solvent protocol E for synthesis of a challenging peptide sequence (tendency to

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aggregation and steric hindrance) should occur be conducted at a slightly higher temperature

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(Fig. 1).

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Troubleshooting of the use of green solvent protocols with the PS resin

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After optimizing a green protocol for the ChemMatrix resin, we evaluated its the use of for a

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PS resin. The deprotection conditions were modified from 20 % piperidine solution for 7 min at

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room temperature to 15 min at 40 °C. Application of these changes to protocols D and E

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translated into an increased yield of the pentapeptide in comparison to room temperature

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conditions. Although, protocol D with only the deprotection step carried out at 40 °C rendered

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less product in comparison to protocols A and B, where DMF and DCM were used, protocol E

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rendered produced the higher yield purity when compared to protocol A and slightly lower yield


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purity than protocol B. In any case, it can be considered an excellent result (88.6%, entry 6,

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Table 4) when the difficulty of this sequence is taken into consideration. These results were

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encouraging enough to test the same protocol for the synthesis of the most demanding Aib-ACP

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decapeptide.

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When protocol E with deprotection at 40 °C was applied, the Aib-ACP decapeptide was

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obtained in a very low yield (7.1%). Therefore, protocol E with coupling and deprotection steps

7

performed at 40 °C was attempted; executed similar to reproduce the best protocol established

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with for the ChemMatrix resin. Although, an increased crude yield was obtained with the PS

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resin (25.4 %), it was clearly less than the purity obtained when ChemMatrix was used (87.1%)

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under the same conditions (Fig.2).

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Table 4. HPLC purity for Aib-enkephalin pentapeptide solid phase assembled on PS.a Entry Protocol b

12 13

Penta

des-Aib

des-Leu

des-Phe

1

Ac

71.8

28.2

NA

NA

2

Bd

97.0

3.0

NA

NA

3

D

38.6

1.6

39.8

NA

4

D+ deprotection at 40 °C

54.6

29.4

0.5

NA

5

E

41.6

1.6

1.4

31.0

6

E+ deprotection at 40 °C

88.6

1.1

1.0

NA

a

See footnote a in Table 2. b For conditions used through the synthesis, see Table 1, extracted from ref. 34, d Data extracted from ref. 21.

c

Data


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Figure 2. HPLC chromatograms of Aib-ACP decapepitde assembled on PS resin. HPLC

3

conditions are the same as reported in ref. 19.

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Conclusions

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A total GSPPS has been developed based on the use of the a full polyethylenglycol resin

6

(ChemMatrix) resin, DIC/OxymaPure and 2-MeTHF as a solvent. For the synthesis of peptides

7

with tendency to aggregate, it is recommended to perform both the Fmoc removal and the

8

coupling steps at slightly higher temperature (40 °C). It is envisaged that this strategy should be

9

easily translated to a microwave synthesis. It was also demonstrated that not only the coupling

10

step is key for a successful synthesis, but that the removal of the Fmoc is also demanding and it

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should be sufficiently addressed. It is important to highlight that in terms of the coupling step,

12

the green solvent 2-MeTHF is offering better performance than DMF. This is another example

13

where the use of a green solvent is not detrimental at all for conventional chemical reactions.

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Regarding the use of the PS resin, other green solvents should be evaluated for improved

15

synthesis of demanding peptides.

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ASSOCIATED CONTENT


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Electronic Supplementary Information (ESI) available: [Experimental details and HPLC

2

chromatograms] and these materials are available free of charge via the Internet at

3

http://pubs.acs.org.”

4

AUTHOR INFORMATION

5

Corresponding Author

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B. G. de la Torre. E-mail: [email protected].

7

F. Albericio. E-mail: [email protected] or [email protected].

8

Author Contributions

9

The experimental part of the work was carried out by YEJ and GAA. The manuscript was

10

written through contributions of all authors. All authors have given approval to the final version

11

of the manuscript.

12

Notes

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†

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B. G.; Govender, T.; Kruger, H. G.; El-Faham, A.; Albericio, F., 2-Methyltetrahydrofuran and

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cyclopentyl methyl ether for green solid-phase peptide synthesis, Amino Acids, 2016, 48, 419–

16

426.

Green Solid-Phase Peptide Synthesis-1: Jad, Y. E.; Acosta, G. A.; Khattab, S. N.; de la Torre,

17

Acknowledgment

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This work was funded in part by the National Research Foundation (NRF), the University of

19

KwaZulu-Natal (South Africa), and the Generalitat de Catalunya (2014 SGR 137) and MEC

20

(CTQ2015-67870-P). Additionally, the authors thank the Deanship of Scientific Research at

21

King Saud University for funding this work through the Prolific Research Group Program (PRG-


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Page 14 of 17

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1437-33; Saudi Arabia). Finally, the authors thank Y. Luxembourg (Luxembourg Bio

2

Technologies Ltd) for his continuous support of this study.

3

ABBREVIATIONS

4

ACP, acyl carrier protein; Aib, 2-aminoisobutyric acid; AM, aminomethyl; Boc, tert-

5

butyloxycarbonyl; DIC, N,N′-diisopropylcarbodiimide; Fmoc, fluorenylmethyloxycarbonyl;

6

GSK, GlaxoSmithKline; REACH, Registration, Evaluation, Authorization and Restriction of

7

Chemicals; OxymaPure, ethyl cyano(hydroxyimino)acetate; PEG, polyethylene glycol; PS,

8

polystyrene; SPPS, solid-phase peptide synthesis; Z, carboxybenzyl

9

REFERENCES

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1. Merrifield, R. B., Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. J. Am. Chem. Soc. 1963, 85 (14), 2149-2154. 2. Zompra, A. A.; Galanis, A. S.; Werbitzky, O.; Albericio, F., Manufacturing peptides as active pharmaceutical ingredients. Future Med. Chem. 2009, 1 (2), 361-77. 3. Bruckdorfer, T.; Marder, O.; Albericio, F., From production of peptides in milligram amounts for research to multi-tons quantities for drugs of the future. Curr. Pharm. Biotechnol. 2004, 5 (1), 29-43. 4. Lloyd-Williams, P.; Albericio, F.; Giralt, E., Chemical Approaches to the Synthesis of Peptide and Proteins. 1997. 5. Constable, D. J. C.; Jimenez-Gonzalez, C.; Henderson, R. K., Perspective on Solvent Use in the Pharmaceutical Industry. Org. Process Res. Dev. 2007, 11 (1), 133-137. 6. Prat, D.; Hayler, J.; Wells, A., A survey of solvent selection guides. Green Chem. 2014, 16 (10), 4546-4551. 7. Alfonsi, K.; Colberg, J.; Dunn, P. J.; Fevig, T.; Jennings, S.; Johnson, T. A.; Kleine, H. P.; Knight, C.; Nagy, M. A.; Perry, D. A.; Stefaniak, M., Green chemistry tools to influence a medicinal chemistry and research chemistry based organisation. Green Chem. 2008, 10 (1), 3136. 8. Henderson, R. K.; Jimenez-Gonzalez, C.; Constable, D. J. C.; Alston, S. R.; Inglis, G. G. A.; Fisher, G.; Sherwood, J.; Binks, S. P.; Curzons, A. D., Expanding GSK's solvent selection guide - embedding sustainability into solvent selection starting at medicinal chemistry. Green Chem. 2011, 13 (4), 854-862. 9. Prat, D.; Pardigon, O.; Flemming, H.-W.; Letestu, S.; Ducandas, V.; Isnard, P.; Guntrum, E.; Senac, T.; Ruisseau, S.; Cruciani, P.; Hosek, P., Sanofi’s Solvent Selection Guide: A Step Toward More Sustainable Processes. Org. Process Res. Dev. 2013, 17 (12), 1517-1525. 10. Alder, C. M.; Hayler, J. D.; Henderson, R. K.; Redman, A. M.; Shukla, L.; Shuster, L. E.; Sneddon, H. F., Updating and further expanding GSK's solvent sustainability guide. Green Chem. 2016, 18, 3879-3890.


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11. Regulation (EC) no 1907/2006 of the European Parliament and of the Council of 18 December 2006 12. Hojo, K.; Ichikawa, H.; Maeda, M.; Kida, S.; Fukumori, Y.; Kawasaki, K., Solid-phase peptide synthesis using nanoparticulate amino acids in water. J. Pept. Sci. 2007, 13 (7), 493-497. 13. Hojo, K.; Hara, A.; Kitai, H.; Onishi, M.; Ichikawa, H.; Fukumori, Y.; Kawasaki, K., Development of a method for environmentally friendly chemical peptide synthesis in water using water-dispersible amino acid nanoparticles. Chem. Cent. J. 2011, 5, 49. 14. Hojo, K.; Shinozaki, N.; Nozawa, Y.; Fukumori, Y.; Ichikawa, H., Aqueous MicrowaveAssisted Solid-Phase Synthesis Using Boc-Amino Acid Nanoparticles. Appl. Sci. 2013, 3 (3), 614-623. 15. De Marco, R.; Tolomelli, A.; Greco, A.; Gentilucci, L., Controlled Solid Phase Peptide Bond Formation Using N-Carboxyanhydrides and PEG Resins in Water. ACS Sustainable Chem. Eng. 2013, 1 (6), 566-569. 16. Isidro-Llobet, A.; Alvarez, M.; Albericio, F., Amino acid-protecting groups. Chem. Rev. 2009, 109 (6), 2455-504. 17. Zalipsky, S.; Chang, J. L.; Albericio, F.; Barany, G., Polymer-supported syntheses and separations of peptides and proteins Preparation and applications of polyethylene glycolpolystyrene graft resin supports for solid-phase peptide synthesis. Reactive Polymers 1994, 22 (3), 243-258. 18. Garcia-Martin, F.; Quintanar-Audelo, M.; Garcia-Ramos, Y.; Cruz, L. J.; Gravel, C.; Furic, R.; Cote, S.; Tulla-Puche, J.; Albericio, F., ChemMatrix, a poly(ethylene glycol)-based support for the solid-phase synthesis of complex peptides. J. Comb. Chem. 2006, 8 (2), 213-20. 19. Jad, Y. E.; Acosta, G. A.; Khattab, S. N.; de la Torre, B. G.; Govender, T.; Kruger, H. G.; El-Faham, A.; Albericio, F., Peptide synthesis beyond DMF: THF and ACN as excellent and friendlier alternatives. Org. Biomol. Chem. 2015, 13 (8), 2393-2398. 20. Antonucci, V.; Coleman, J.; Ferry, J. B.; Johnson, N.; Mathe, M.; Scott, J. P.; Xu, J., Toxicological Assessment of 2-Methyltetrahydrofuran and Cyclopentyl Methyl Ether in Support of Their Use in Pharmaceutical Chemical Process Development. Org. Process Res. Dev. 2011, 15 (4), 939-941. 21. Jad, Y. E.; Acosta, G. A.; Khattab, S. N.; Torre, B. G.; Govender, T.; Kruger, H. G.; ElFaham, A.; Albericio, F., 2-Methyltetrahydrofuran and cyclopentyl methyl ether for green solidphase peptide synthesis. Amino Acids 2016, 48 (2), 419-426. 22. Pace, V.; Hoyos, P.; Castoldi, L.; Domínguez de María, P.; Alcántara, A. R., 2Methyltetrahydrofuran (2-MeTHF): A Biomass-Derived Solvent with Broad Application in Organic Chemistry. ChemSusChem 2012, 5 (8), 1369-1379. 23. Gu, Y.; Jerome, F., Bio-based solvents: an emerging generation of fluids for the design of eco-efficient processes in catalysis and organic chemistry. Chem. Soc. Rev. 2013, 42 (24), 95509570. 24. Clark, J.; Farmer, T.; Hunt, A.; Sherwood, J., Opportunities for Bio-Based Solvents Created as Petrochemical and Fuel Products Transition towards Renewable Resources. Int. J. Mol. Sci. 2015, 16 (8), 17101-17159. 25. Krishnan, S.; Schreiber, S. L., Syntheses of Stereochemically Diverse Nine-Membered Ring-Containing Biaryls. Org. Lett. 2004, 6 (22), 4021-4024. 26. Shanmuganathan, S.; Greiner, L.; Domínguez de María, P., Silica-immobilized piperazine: A sustainable organocatalyst for aldol and Knoevenagel reactions. Tetrahedron Lett. 2010, 51 (50), 6670-6672.


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27. Simeo, Y.; Sinisterra, J. V.; Alcantara, A. R., Regioselective enzymatic acylation of pharmacologically interesting nucleosides in 2-methyltetrahydrofuran, a greener substitute for THF. Green Chem. 2009, 11 (6), 855-862. 28. Hoyos, P.; Quezada, M. A.; Sinisterra, J. V.; Alcántara, A. R., Optimised Dynamic Kinetic Resolution of benzoin by a chemoenzymatic approach in 2-MeTHF. J. Mol. Catal. B: Enzym. 2011, 72 (1–2), 20-24. 29. MacMillan, D. S.; Murray, J.; Sneddon, H. F.; Jamieson, C.; Watson, A. J. B., Evaluation of alternative solvents in common amide coupling reactions: replacement of dichloromethane and N,N-dimethylformamide. Green Chem. 2013, 15 (3), 596-600. 30. Jiménez-González, C.; Curzons, A.; Constable, D. C.; Cunningham, V., Expanding GSK’s Solvent Selection Guide—application of life cycle assessment to enhance solvent selections. Clean Techn. Environ. Policy 2004, 7 (1), 42-50. 31. Eastman, H. E.; Jamieson, C.; Watson, A. J. B., Development of Solvent Selection Guides. Aldrichim. Acta 2015, 48 (2), 51-55. 32. Prat, D.; Wells, A.; Hayler, J.; Sneddon, H.; McElroy, C. R.; Abou-Shehada, S.; Dunn, P. J., CHEM21 selection guide of classical- and less classical-solvents. Green Chem. 2016, 18 (1), 288-296. 33. Deletion is abbreviated into “de” however readers will get confused with Dconfiguration, therefore “s” is added to be used as prefix as "des". 34. Jad, Y. E.; de la Torre, B. G.; Govender, T.; Kruger, H. G.; El-Faham, A.; Albericio, F., Oxyma-T, expanding the arsenal of coupling reagents. Tetrahedron Lett. 2016, 57 (31), 35233525. 35. Paradis-Bas, M.; Tulla-Puche, J.; Albericio, F., The road to the synthesis of "difficult peptides". Chem. Soc. Rev. 2016, 45 (3), 631-654. 36. Acosta, G. A.; del Fresno, M.; Paradis-Bas, M.; Rigau-DeLlobet, M.; Cote, S.; Royo, M.; Albericio, F., Solid-phase peptide synthesis using acetonitrile as a solvent in combination with PEG-based resins. J. Pept. Sci. 2009, 15 (10), 629-33. 37. 40 °C is a moderate temperature and even higher temperature using microwave are considered green. Ref 15 is an example for microwave peptide synthesis.

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Green Solid-Phase Peptide Synthesis-2. 2-Methyltetrahydrofuran and ethyl acetate for

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solid-phase peptide synthesis under green conditions

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Yahya E. Jad,a Gerardo A. Acosta,b,c Thavendran Govender,a Hendrik G. Kruger,a Ayman El-

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Faham,d,e Beatriz G. de la Torre,a,* and Fernando Albericioa,b,c,e,f,*


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1 2

Synopsis: The hazardous solvents DMF and DCM were substituted by greener alternative 2-

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MeTHF and EtOAc for GSPPS (green solid phase peptide synthesis).


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Green Solid-Phase Peptide Synthesis 2. 2 ... - ACS Publications

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