Thio‐Bromo “Click” Reaction Derived Polymer–Peptide Conjugates for Their Self‐Assembled Fibrillar Nanostructures

Abstract

Back Cover: Thio‐bromo based “click” chemistry is highlighted for the synthesis of polymer‐peptide conjugates of PEG‐based thiolated polymers (PEG‐SH) and an acyl‐brominated Aβ17‐20 peptide (Br‐LVFF), in‐turn selforganizing into fibrillar structured materials. This is reported by Wolfgang H. Binder and co‐workers in article number 2000048.

Bifunctional Peptide–Polymer Conjugate-Based Fibers via a One-Pot Tandem Disulfide Reduction Coupled to a Thio-Bromo “Click” Reaction

“…After confirming the complete disulfide reduction, to the same reaction the brominated peptide 5 was added under inert conditions while changing the final reaction solvent to an aqueous binary mixture H 2 O/THF with a ratio 2:1 (v/v), additionally adding triethylamine (Et 3 N) base to enable catalysis of the thio-bromo click reaction. Under these conditions, the reaction completed within 2 h and the final product, the bifunctional peptidic conjugate 6 , was obtained and subsequently purified further in a yield of 62% via preparative GPC (Figure A), a method recently exploited by our group. , Note that in the view of synthetic hurdles due to purification struggles associated with the standard purification methods, which has vexed scientists for decades, the herein exemplified preparative GPC-based method is found to be trivial for complex peptide–polymer conjugates (such as here due to the high susceptibility of the amyloidogenic peptide Aβ 17–20 to supramolecular aggregation in solution), in turn increasing the yield of the conjugate highly effective in terms of production time and cost. Figure C for the analytical GPC chromatograms evidenced the appearance of a monomodal RI trace for the peptide conjugate 6 with R v ∼ 7.65 mL ( M n,GPC = 3800 g mol –1 , Đ = 1.10), which displayed a higher molecular weight compared to that of peptide 5 ( R v ∼ 8.94 mL, M n,GPC = 750 g mol –1 , Đ = 1.06) and the in situ generated free thiolated PEG polymer 7 ( R v ∼ 8.02 mL, M n,GPC = 2400 g mol –1 , Đ = 1.07) but lower compared to that of the intermediate polymer 4 ( R v ∼ 6.86 mL, M n,GPC = 9350 g mol –1 , Đ = 1.62).…”

“…The successive formation of the desired conjugate 6 was further confirmed by HPLC studies (Figure S13, Supporting Information) and via structural analysis by 1 H NMR spectroscopy (Figure C), which exhibited the characteristic major proton signals for the peptidic constituent FFFF (at δ 7.29–7.05 ppm for four phenyl ring protons and at δ 3.16–2.60 ppm of four benzyl group protons, of tetraphenylalanine) and for the PEGylated polymer segment (at δ 3.54 ppm for their repeating unit −OCH 2 CH 2 −) (see also Figure S14, Supporting Information, for the 13 C NMR spectrum). Moreover, ESI-TOF MS investigations were undertaken to fully prove the chemical composition of conjugate 6 (Figure ), where the observed major isotopic peak maximum of 1 + charged series, as a typical example at m / z = 2819.1678 g mol –1 , corresponds to [M + Na + CH 3 OH] + with a degree of polymerization ( n = 29) successfully simulated to match the desired ion …”

“…For the preparation of peptide–polymer conjugate, we started from the readily prepared 2-bromo-acyl-derivative of tetraphenylalanine peptide, Br-FFFF-OMe ( 5 ), documented in our recent article for conjugation to yield the PEGylated conjugate MeO-FFFF-PEG-FFFF-OMe ( 6 ). The intermediate peptidic derivatives (Figures S1–S6, Supporting Information) and the final bromo-peptide 5 (Figures S7 and S8, Supporting Information) were synthesized via conventional solution-phase peptide synthesis (Scheme S1, Supporting Information) .…”

“…Important thereto is the proper choice of a common solvent for both, the peptide and the polymer, as well as mild reaction conditions, in line with the subsequent potential for biological application. We have previously described , a synthetic methodology to prepare monofunctional polymer–peptide conjugates, taking advantage of poly­(ethylene glycol) (PEG) , and the phenylalanine-based hydrophobic tetrapeptide FFFF, inspired by the amyloid-β­(Aβ)­protein core sequence FF (Aβ 19–20 ) explored by Gazit and co-workers. − Our focus in this work is on the synthetic methodology development and the preparation of α- and ω-chain-ends-modified PEGylated peptidic conjugates FFFF-PEG-FFFF (with C-terminal methyl ester units) that are able to self assemble into β-sheet-enriched fibrous structures. − The synthesis of such β-sheet fibril-based bifunctional conjugates and peptide-PEG-peptides has been previously reported by Hamley et al. for the dipeptide YY based conjugate YY-PEG-YY (with N -terminal NH 2 or Fmoc units) and by Besenius et al for the pentapeptide FHFHF based conjugate FHFHF-PEG-FHFHF (with acetylated N -termini), employing conventional amide coupling chemistry .…”

“…for the dipeptide YY based conjugate YY-PEG-YY (with N -terminal NH 2 or Fmoc units) and by Besenius et al for the pentapeptide FHFHF based conjugate FHFHF-PEG-FHFHF (with acetylated N -termini), employing conventional amide coupling chemistry . We here exploit the less explored metal-free thio-bromo based “click” chemistry approach ,− (compared to various other types of thiol-based click reactions, such as the thiol-ene/yne/maleimide/-conjugation − ) for the synthesis of peptidic polymer conjugates, which plausibly is more promising for bioapplications than the utilization of, e.g., the copper­(I) metal-catalyzed azide-alkyne cycloaddition (CuAAC). , The methodology developed here can lead to the widening of the applicability of such unexplored click approach because of its following significant advantages: (1) It represents the first example of the implementation of thio-bromo click reaction directly coupled to a one-pot tandem disulfide reduction of disulfide-bridged PEG polymers (thus improving the ease of implication of such polymers by unveiling this critical issue of disulfide formation). (2) The illustrated coupling reaction commenced in aqueous solution for the synthesis of conjugates (therefore, the strategy can be conveniently adopted for the polymer conjugation with various other bioimolecules such as proteins, mRNAs, etc.…”