Structure of Fmoc-Trp-OH
CAS No.: 35737-15-6
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Synthesis of Modular DO3A-Based High-Relaxivity Contrast Agents for MRI of Prostate Cancer
Dana Qiang Murphy Soika ;
Abstract: Contrast agents (CAs) are small molecules used in magnetic resonance imaging (MRI) to help diagnose various forms of cancer. While MRI is advantageous over other clinical imaging techniques, limitations of today’s contrast agents containing gadolinium (Gd) hinder their safety, sensitivity, and specificity. The conventional CAs that MRI relies on are considered low-relaxivity and are not optimally effective at enhancing MR signal. Additionally, they lack cell-specificity and circulate throughout the body. Furthermore, unbound Gd3+ is nephrogenic which prevents its use in patients with impaired renal function. In the clinic, these limitations mean high dosages of these compounds must be administered to patients in order to produce an image that struggles to highlight the exact tumor location. Our aim was to improve conventional CAs by synthesizing a high-relaxivity (HR) targeted contrast agent (HR-TCA). The cell-specific nature of the HR-TCA will allow for its accumulation at tumor sites while the HR will produce a stronger MR signal per molecule of CA. Combined, this means a much lower and therefore safer dose of CA can be used to produce an image of the exact tumor location with superior contrast. Our modular approach allows us to easily combine this HR contrast agent (HR-CA) to any targeting peptide using a linker in a convergent, one-step synthesis. Our synthetic approach for the HR-CA module attaches a macrocyclic chelator, DO3A, to the side chain of an orthogonally protected alanine. This is a modification to the approach published by Boros in which t-butyl groups were utilized to protect DO3A. In our modular approach, Gd is chelated early to protect the acetic acid donor arms of DO3A from participating in unwanted side reactions for the remainder of the synthesis, eliminating any need to expose the final HR-TCA to the harsh acidic conditions of TFA that are necessary to remove t-butyl protecting groups. Upon removal of the N-and C-terminal protecting groups, the HR-CA module is coupled directly to our in-house synthesized targeting module which is comprised of the targeting agent (DCL) and linker (DSS) already attached to afford the final HR-TCA. T1 relaxation measurements of relevant intermediates and the final product were performed to compare their relaxivities with those of commercial CAs used in clinics, labs, and hospitals today. Although the HR-CA and final HR-TCA exhibited only a modest increase in T1 relaxivity compared to commercial CA Gd-DOTA, in a striking discovery it was observed that the presence of both a tryptophan spacer and an Fmoc protecting group boosted the T1 relaxivity significantly.
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CAS No. : | 35737-15-6 |
Formula : | C26H22N2O4 |
M.W : | 426.46 |
SMILES Code : | O=C(O)[C@H](CC1=CNC2=CC=CC=C12)NC(OCC3C4=C(C5=C3C=CC=C5)C=CC=C4)=O |
MDL No. : | MFCD00037126 |
InChI Key : | MGHMWKZOLAAOTD-DEOSSOPVSA-N |
Pubchem ID : | 978343 |
GHS Pictogram: |
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Signal Word: | Warning |
Hazard Statements: | H302-H315-H319-H335 |
Precautionary Statements: | P261-P305+P351+P338 |
* All experimental methods are cited from the reference, please refer to the original source for details. We do not guarantee the accuracy of the content in the reference.
Yield | Reaction Conditions | Operation in experiment |
---|---|---|
1. Peptide synthesis 1.1 General synthetic procedures A general method for the synthesis of the peptidomimetics of the present invention is exemplified in the following. This is to demonstrate the principal concept and does not limit or restrict the present invention in any way. A person skilled in the art is easily able to modify these procedures, especially, but not limited to, choosing a different starting position within the ring system, to still achieve the preparation of the claimed cyclic peptidomimetic compounds of the present invention. Coupling of the first protected amino acid residue to the resin . In a dried flask, 2-chlorotritylchloride resin (polystyrene, 1percent crosslinked; loading: 1.4 mMol/g) was swollen in dry CH2CI2 for 30 min (7 mL CH2CI2 per g resin). A solution of 0.8 eq of the Fmoc-protected amino acid and 6 eq of DIPEA in dry CH2CI2/DMF (4/1) (10 mL per g resin) was added. After shaking for 2-4 h at rt the resin was filtered off and washed successively with CH2CI2, DMF, CH2CI2, DMF and CH2CI2. Then a solution of dry CH2CI2/MeOH/DIPEA (17:2:1) was added (10 mL per g resin). After shaking for 3 x 30 min the resin was filtered off in a pre-weighed sinter funnel and washed successively with CH2CI2, DMF, CH2CI2, MeOH, CH2CI2, MeOH, CH2CI2 (2x) and Et20 (2x). The resin was dried under high vacuum overnight. The final mass of resin was calculated before the qualitative control. Loading was typically 0.6 - 0.7 mMol/g. The following preloaded resins were prepared: Fmoc-Dab(Boc)-2-chlorotrityl resin, Fmoc-DDab(Boc)-2-chlorotrityl resin, Fmoc-Lys(Boc)-2-chlorotrityl resin, Fmoc- Trp(Boc)-2-chlortrityl resin, Fmoc-Phe-2-chlortrityl resin; Fmoc-Val-2-chlorotrityl resin, Fmoc-Pro-2-chlorotrityl resin, Fmoc-Arg(Pbf)-2-chlorotrityl resin and Fmoc-Glu(iBu)-2- chlorotrityl resin. Synthesis of the fully protected peptide fragment The synthesis was carried out on a Syro-peptide synthesizer (MultiSynTech GmbH) using 24 to 96 reaction vessels. In each vessel 0.04 mMol of the above resin were placed and the resin was swelled in CH2CI2 and DMF for 15 min, respectively. The following reaction cycles were programmed and carried out: Step Reagent Time 1 CH2CI2, wash and swell (manual) 1 x 3 min 2 DMF, wash and swell 2 x 30 min 3 20percent piperidine/DMF 1 x 5 min and 1 x 15 min 4 DMF, wash 5 x 1 min 5 3.5 eq Fmoc amino acid/3.5 eq HOAt in DMF + 3.5 eq PyBOP/7 eq DIPEA or 3.5 eq DIC 1 x 40 min 6 3.5 eq Fmoc amino acid/DMF + 3.5 eq HATU or PyBOP or HCTU + 7 eq DIPEA 1 x 40 min 7 DMF, wash 5 x 1 min 8 20percent piperidine/DMF 1 x 5 min and 1 x 15 min 9 DMF, wash 5 x 1 min 10 CH2CI2, wash (at the end of the synthesis) 3 x 1 min Steps 5 to 9 are repeated to add each amino-acid residue. After the termination of the synthesis of the fully protected peptide fragment, one of the procedures A - E, as described herein below, was adopted subsequently, depending on which kind of interstrand linkages, as described herein below, were to be formed. Finally, the peptides were purified by preparative reverse phase LC-MS, as described herein below. Procedure A: Cyclization and work up of a backbone cyclized peptide having no interstrand linkage Cleavage, backbone cyclization and deprotection After assembly of the linear peptide, the resin was suspended in 1 mL of 1percent TFA in CH2CI2 (v/v; 0.14 mMol) for 3 minutes. After filtration the filtrate was neutralized with 1 mL of 20percent DI PEA in CH2CI2 (v/v; 1.15 mMol). This procedure was repeated four times to ensure completion of the cleavage. An alternative cleavage method comprises suspension of the resin in lmL of 20percent HFIP in CH2CI2 (v/v; 1.9 mMol) for 30 minutes, filtration and repetition of the procedure. The resin was washed three times with 1 mL of CH2CI2. The CH2CI2 layers containing product were evaporated to dryness. The fully protected linear peptide was solubilised in 8 mL of dry DM F. Then 2 eq of HATU and 2 eq of HOAt in dry DM F (1-2 mL) and 4 eq of DIPEA in dry DM F (1-2 mL) were added to the peptide, followed by stirring for ca. 16 h. The volatiles were removed by evaporation. The crude cyclic peptide was dissolved in 7 mL of CH2CI2 and washed three times with 4.5 mL 10percent acetonitrile in water (v/v). The CH2CI2 layer was then evaporated to dryness. To fully deprotect the peptide, 7 mL of cleavage cocktail TFA/DODT/thioanisol/H20 (87.5 :2.5:5:5) or TFA/TIS/H20 (95:2.5 :2.5) were added, and the mixture was kept for 2.5-4 h at room temperature until the reaction was completed. The reaction mixture was evaporated close to dryness, the peptide precipitated with 7 mL of cold Et20/pentane and finally washed 3 times with 4 mL of cold Et20/pentane. Procedures Bl and B2: Cyclization and work up of a backbone cyclized peptide having a disulfide interstrand linkage Bl: Formation of a disulfide interstrand linkage using DMSO After cleavage, backbone cyclization and deprotection of the linear peptide, as described in the corresponding section of procedure A, th... |
Yield | Reaction Conditions | Operation in experiment |
---|---|---|
1. Peptide synthesis 1.1 General synthetic procedures A general method for the synthesis of the peptidomimetics of the present invention is exemplified in the following. This is to demonstrate the principal concept and does not limit or restrict the present invention in any way. A person skilled in the art is easily able to modify these procedures, especially, but not limited to, choosing a different starting position within the ring system, to still achieve the preparation of the claimed cyclic peptidomimetic compounds of the present invention. Coupling of the first protected amino acid residue to the resin . In a dried flask, 2-chlorotritylchloride resin (polystyrene, 1% crosslinked; loading: 1.4 mMol/g) was swollen in dry CH2CI2 for 30 min (7 mL CH2CI2 per g resin). A solution of 0.8 eq of the Fmoc-protected amino acid and 6 eq of DIPEA in dry CH2CI2/DMF (4/1) (10 mL per g resin) was added. After shaking for 2-4 h at rt the resin was filtered off and washed successively with CH2CI2, DMF, CH2CI2, DMF and CH2CI2. Then a solution of dry CH2CI2/MeOH/DIPEA (17:2:1) was added (10 mL per g resin). After shaking for 3 x 30 min the resin was filtered off in a pre-weighed sinter funnel and washed successively with CH2CI2, DMF, CH2CI2, MeOH, CH2CI2, MeOH, CH2CI2 (2x) and Et20 (2x). The resin was dried under high vacuum overnight. The final mass of resin was calculated before the qualitative control. Loading was typically 0.6 - 0.7 mMol/g. The following preloaded resins were prepared: Fmoc-Dab(Boc)-2-chlorotrityl resin, Fmoc-DDab(Boc)-2-chlorotrityl resin, Fmoc-Lys(Boc)-2-chlorotrityl resin, Fmoc- Trp(Boc)-2-chlortrityl resin, Fmoc-Phe-2-chlortrityl resin; Fmoc-Val-2-chlorotrityl resin, Fmoc-Pro-2-chlorotrityl resin, Fmoc-Arg(Pbf)-2-chlorotrityl resin and Fmoc-Glu(iBu)-2- chlorotrityl resin. Synthesis of the fully protected peptide fragment The synthesis was carried out on a Syro-peptide synthesizer (MultiSynTech GmbH) using 24 to 96 reaction vessels. In each vessel 0.04 mMol of the above resin were placed and the resin was swelled in CH2CI2 and DMF for 15 min, respectively. The following reaction cycles were programmed and carried out: Step Reagent Time 1 CH2CI2, wash and swell (manual) 1 x 3 min 2 DMF, wash and swell 2 x 30 min 3 20% piperidine/DMF 1 x 5 min and 1 x 15 min 4 DMF, wash 5 x 1 min 5 3.5 eq Fmoc amino acid/3.5 eq HOAt in DMF + 3.5 eq PyBOP/7 eq DIPEA or 3.5 eq DIC 1 x 40 min 6 3.5 eq Fmoc amino acid/DMF + 3.5 eq HATU or PyBOP or HCTU + 7 eq DIPEA 1 x 40 min 7 DMF, wash 5 x 1 min 8 20% piperidine/DMF 1 x 5 min and 1 x 15 min 9 DMF, wash 5 x 1 min 10 CH2CI2, wash (at the end of the synthesis) 3 x 1 min Steps 5 to 9 are repeated to add each amino-acid residue. After the termination of the synthesis of the fully protected peptide fragment, one of the procedures A - E, as described herein below, was adopted subsequently, depending on which kind of interstrand linkages, as described herein below, were to be formed. Finally, the peptides were purified by preparative reverse phase LC-MS, as described herein below. Procedure A: Cyclization and work up of a backbone cyclized peptide having no interstrand linkage Cleavage, backbone cyclization and deprotection After assembly of the linear peptide, the resin was suspended in 1 mL of 1% TFA in CH2CI2 (v/v; 0.14 mMol) for 3 minutes. After filtration the filtrate was neutralized with 1 mL of 20% DI PEA in CH2CI2 (v/v; 1.15 mMol). This procedure was repeated four times to ensure completion of the cleavage. An alternative cleavage method comprises suspension of the resin in lmL of 20% HFIP in CH2CI2 (v/v; 1.9 mMol) for 30 minutes, filtration and repetition of the procedure. The resin was washed three times with 1 mL of CH2CI2. The CH2CI2 layers containing product were evaporated to dryness. The fully protected linear peptide was solubilised in 8 mL of dry DM F. Then 2 eq of HATU and 2 eq of HOAt in dry DM F (1-2 mL) and 4 eq of DIPEA in dry DM F (1-2 mL) were added to the peptide, followed by stirring for ca. 16 h. The volatiles were removed by evaporation. The crude cyclic peptide was dissolved in 7 mL of CH2CI2 and washed three times with 4.5 mL 10% acetonitrile in water (v/v). The CH2CI2 layer was then evaporated to dryness. To fully deprotect the peptide, 7 mL of cleavage cocktail TFA/DODT/thioanisol/H20 (87.5 :2.5:5:5) or TFA/TIS/H20 (95:2.5 :2.5) were added, and the mixture was kept for 2.5-4 h at room temperature until the reaction was completed. The reaction mixture was evaporated close to dryness, the peptide precipitated with 7 mL of cold Et20/pentane and finally washed 3 times with 4 mL of cold Et20/pentane. Procedures Bl and B2: Cyclization and work up of a backbone cyclized peptide having a disulfide interstrand linkage Bl: Formation of a disulfide interstrand linkage using DMSO After cleavage, backbone cyclization and deprotection of the linear peptide, as described in the corresponding section of procedure A, the deprotected cyclic peptide was treated w... |
Yield | Reaction Conditions | Operation in experiment |
---|---|---|
1. Peptide synthesis 1.1 General synthetic procedures A general method for the synthesis of the peptidomimetics of the present invention is exemplified in the following. This is to demonstrate the principal concept and does not limit or restrict the present invention in any way. A person skilled in the art is easily able to modify these procedures, especially, but not limited to, choosing a different starting position within the ring system, to still achieve the preparation of the claimed cyclic peptidomimetic compounds of the present invention. Coupling of the first protected amino acid residue to the resin . In a dried flask, 2-chlorotritylchloride resin (polystyrene, 1percent crosslinked; loading: 1.4 mMol/g) was swollen in dry CH2CI2 for 30 min (7 mL CH2CI2 per g resin). A solution of 0.8 eq of the Fmoc-protected amino acid and 6 eq of DIPEA in dry CH2CI2/DMF (4/1) (10 mL per g resin) was added. After shaking for 2-4 h at rt the resin was filtered off and washed successively with CH2CI2, DMF, CH2CI2, DMF and CH2CI2. Then a solution of dry CH2CI2/MeOH/DIPEA (17:2:1) was added (10 mL per g resin). After shaking for 3 x 30 min the resin was filtered off in a pre-weighed sinter funnel and washed successively with CH2CI2, DMF, CH2CI2, MeOH, CH2CI2, MeOH, CH2CI2 (2x) and Et20 (2x). The resin was dried under high vacuum overnight. The final mass of resin was calculated before the qualitative control. Loading was typically 0.6 - 0.7 mMol/g. The following preloaded resins were prepared: Fmoc-Dab(Boc)-2-chlorotrityl resin, Fmoc-DDab(Boc)-2-chlorotrityl resin, Fmoc-Lys(Boc)-2-chlorotrityl resin, Fmoc- Trp(Boc)-2-chlortrityl resin, Fmoc-Phe-2-chlortrityl resin; Fmoc-Val-2-chlorotrityl resin, Fmoc-Pro-2-chlorotrityl resin, Fmoc-Arg(Pbf)-2-chlorotrityl resin and Fmoc-Glu(iBu)-2- chlorotrityl resin. Synthesis of the fully protected peptide fragment The synthesis was carried out on a Syro-peptide synthesizer (MultiSynTech GmbH) using 24 to 96 reaction vessels. In each vessel 0.04 mMol of the above resin were placed and the resin was swelled in CH2CI2 and DMF for 15 min, respectively. The following reaction cycles were programmed and carried out: Step Reagent Time 1 CH2CI2, wash and swell (manual) 1 x 3 min 2 DMF, wash and swell 2 x 30 min 3 20percent piperidine/DMF 1 x 5 min and 1 x 15 min 4 DMF, wash 5 x 1 min 5 3.5 eq Fmoc amino acid/3.5 eq HOAt in DMF + 3.5 eq PyBOP/7 eq DIPEA or 3.5 eq DIC 1 x 40 min 6 3.5 eq Fmoc amino acid/DMF + 3.5 eq HATU or PyBOP or HCTU + 7 eq DIPEA 1 x 40 min 7 DMF, wash 5 x 1 min 8 20percent piperidine/DMF 1 x 5 min and 1 x 15 min 9 DMF, wash 5 x 1 min 10 CH2CI2, wash (at the end of the synthesis) 3 x 1 min Steps 5 to 9 are repeated to add each amino-acid residue. After the termination of the synthesis of the fully protected peptide fragment, one of the procedures A - E, as described herein below, was adopted subsequently, depending on which kind of interstrand linkages, as described herein below, were to be formed. Finally, the peptides were purified by preparative reverse phase LC-MS, as described herein below. Procedure A: Cyclization and work up of a backbone cyclized peptide having no interstrand linkage Cleavage, backbone cyclization and deprotection After assembly of the linear peptide, the resin was suspended in 1 mL of 1percent TFA in CH2CI2 (v/v; 0.14 mMol) for 3 minutes. After filtration the filtrate was neutralized with 1 mL of 20percent DI PEA in CH2CI2 (v/v; 1.15 mMol). This procedure was repeated four times to ensure completion of the cleavage. An alternative cleavage method comprises suspension of the resin in lmL of 20percent HFIP in CH2CI2 (v/v; 1.9 mMol) for 30 minutes, filtration and repetition of the procedure. The resin was washed three times with 1 mL of CH2CI2. The CH2CI2 layers containing product were evaporated to dryness. The fully protected linear peptide was solubilised in 8 mL of dry DM F. Then 2 eq of HATU and 2 eq of HOAt in dry DM F (1-2 mL) and 4 eq of DIPEA in dry DM F (1-2 mL) were added to the peptide, followed by stirring for ca. 16 h. The volatiles were removed by evaporation. The crude cyclic peptide was dissolved in 7 mL of CH2CI2 and washed three times with 4.5 mL 10percent acetonitrile in water (v/v). The CH2CI2 layer was then evaporated to dryness. To fully deprotect the peptide, 7 mL of cleavage cocktail TFA/DODT/thioanisol/H20 (87.5 :2.5:5:5) or TFA/TIS/H20 (95:2.5 :2.5) were added, and the mixture was kept for 2.5-4 h at room temperature until the reaction was completed. The reaction mixture was evaporated close to dryness, the peptide precipitated with 7 mL of cold Et20/pentane and finally washed 3 times with 4 mL of cold Et20/pentane. Procedures Bl and B2: Cyclization and work up of a backbone cyclized peptide having a disulfide interstrand linkage Bl: Formation of a disulfide interstrand linkage using DMSO After cleavage, backbone cyclization and deprotection of the linear peptide, as described in the corresponding section of procedure A, th... |
Yield | Reaction Conditions | Operation in experiment |
---|---|---|
1. Peptide synthesis 1.1 General synthetic procedures A general method for the synthesis of the peptidomimetics of the present invention is exemplified in the following. This is to demonstrate the principal concept and does not limit or restrict the present invention in any way. A person skilled in the art is easily able to modify these procedures, especially, but not limited to, choosing a different starting position within the ring system, to still achieve the preparation of the claimed cyclic peptidomimetic compounds of the present invention. Coupling of the first protected amino acid residue to the resin . In a dried flask, 2-chlorotritylchloride resin (polystyrene, 1percent crosslinked; loading: 1.4 mMol/g) was swollen in dry CH2CI2 for 30 min (7 mL CH2CI2 per g resin). A solution of 0.8 eq of the Fmoc-protected amino acid and 6 eq of DIPEA in dry CH2CI2/DMF (4/1) (10 mL per g resin) was added. After shaking for 2-4 h at rt the resin was filtered off and washed successively with CH2CI2, DMF, CH2CI2, DMF and CH2CI2. Then a solution of dry CH2CI2/MeOH/DIPEA (17:2:1) was added (10 mL per g resin). After shaking for 3 x 30 min the resin was filtered off in a pre-weighed sinter funnel and washed successively with CH2CI2, DMF, CH2CI2, MeOH, CH2CI2, MeOH, CH2CI2 (2x) and Et20 (2x). The resin was dried under high vacuum overnight. The final mass of resin was calculated before the qualitative control. Loading was typically 0.6 - 0.7 mMol/g. The following preloaded resins were prepared: Fmoc-Dab(Boc)-2-chlorotrityl resin, Fmoc-DDab(Boc)-2-chlorotrityl resin, Fmoc-Lys(Boc)-2-chlorotrityl resin, Fmoc- Trp(Boc)-2-chlortrityl resin, Fmoc-Phe-2-chlortrityl resin; Fmoc-Val-2-chlorotrityl resin, Fmoc-Pro-2-chlorotrityl resin, Fmoc-Arg(Pbf)-2-chlorotrityl resin and Fmoc-Glu(iBu)-2- chlorotrityl resin. Synthesis of the fully protected peptide fragment The synthesis was carried out on a Syro-peptide synthesizer (MultiSynTech GmbH) using 24 to 96 reaction vessels. In each vessel 0.04 mMol of the above resin were placed and the resin was swelled in CH2CI2 and DMF for 15 min, respectively. The following reaction cycles were programmed and carried out: Step Reagent Time 1 CH2CI2, wash and swell (manual) 1 x 3 min 2 DMF, wash and swell 2 x 30 min 3 20percent piperidine/DMF 1 x 5 min and 1 x 15 min 4 DMF, wash 5 x 1 min 5 3.5 eq Fmoc amino acid/3.5 eq HOAt in DMF + 3.5 eq PyBOP/7 eq DIPEA or 3.5 eq DIC 1 x 40 min 6 3.5 eq Fmoc amino acid/DMF + 3.5 eq HATU or PyBOP or HCTU + 7 eq DIPEA 1 x 40 min 7 DMF, wash 5 x 1 min 8 20percent piperidine/DMF 1 x 5 min and 1 x 15 min 9 DMF, wash 5 x 1 min 10 CH2CI2, wash (at the end of the synthesis) 3 x 1 min Steps 5 to 9 are repeated to add each amino-acid residue. After the termination of the synthesis of the fully protected peptide fragment, one of the procedures A - E, as described herein below, was adopted subsequently, depending on which kind of interstrand linkages, as described herein below, were to be formed. Finally, the peptides were purified by preparative reverse phase LC-MS, as described herein below. Procedure A: Cyclization and work up of a backbone cyclized peptide having no interstrand linkage Cleavage, backbone cyclization and deprotection After assembly of the linear peptide, the resin was suspended in 1 mL of 1percent TFA in CH2CI2 (v/v; 0.14 mMol) for 3 minutes. After filtration the filtrate was neutralized with 1 mL of 20percent DI PEA in CH2CI2 (v/v; 1.15 mMol). This procedure was repeated four times to ensure completion of the cleavage. An alternative cleavage method comprises suspension of the resin in lmL of 20percent HFIP in CH2CI2 (v/v; 1.9 mMol) for 30 minutes, filtration and repetition of the procedure. The resin was washed three times with 1 mL of CH2CI2. The CH2CI2 layers containing product were evaporated to dryness. The fully protected linear peptide was solubilised in 8 mL of dry DM F. Then 2 eq of HATU and 2 eq of HOAt in dry DM F (1-2 mL) and 4 eq of DIPEA in dry DM F (1-2 mL) were added to the peptide, followed by stirring for ca. 16 h. The volatiles were removed by evaporation. The crude cyclic peptide was dissolved in 7 mL of CH2CI2 and washed three times with 4.5 mL 10percent acetonitrile in water (v/v). The CH2CI2 layer was then evaporated to dryness. To fully deprotect the peptide, 7 mL of cleavage cocktail TFA/DODT/thioanisol/H20 (87.5 :2.5:5:5) or TFA/TIS/H20 (95:2.5 :2.5) were added, and the mixture was kept for 2.5-4 h at room temperature until the reaction was completed. The reaction mixture was evaporated close to dryness, the peptide precipitated with 7 mL of cold Et20/pentane and finally washed 3 times with 4 mL of cold Et20/pentane. Procedures Bl and B2: Cyclization and work up of a backbone cyclized peptide having a disulfide interstrand linkage Bl: Formation of a disulfide interstrand linkage using DMSO After cleavage, backbone cyclization and deprotection of the linear peptide, as described in the corresponding section of procedure A, th... |