A bioinspired and degradable riboflavin-containing polypeptide as a sustainable material for energy storage

Abstract

Inspired by Nature, we present a polypeptide-based organic redox-active material constructed from renewable feedstocks, L-glutamic acid (an amino acid) and riboflavin (vitamin B₂), to address challenges with start-to-end-of-life management in energy storage systems (ESSs). The amino acid was utilized to establish a degradable polymer backbone, along which many copies of riboflavin were incorporated to serve as the redox-active pendant groups that enabled energy storage. The overall synthesis involved the ring-opening polymerization (ROP) of an l-glutamic acid-derived N-carboxyanhydride (NCA) monomer, followed by side chain activation with azides and, finally, click coupling to achieve installation of alkyne-functionalized riboflavin moieties. The steric bulkiness and rich chemical functionality of riboflavin resulted in synthetic complexities that required reaction optimization to achieve the desired polymer structure. Electrochemical characterization of the resultant riboflavin polypeptide, in organic electrolyte, showed quasireversible redox activity with a half-wave potential (E_1/2) of ca. -1.10 V vs. ferrocene/ferrocenium (Fc/Fc⁺). Cell viability assays revealed biocompatibility, as indicated by negligible cytotoxicity for fibroblast cells. The polypeptide design, consisting of labile amide backbone linkages and side-chain ester functionalities that tethered the riboflavin units to the backbone, enabled hydrolytic degradation to recover building blocks for future upcycling or recycling. This bioinspired strategy advances the development of degradable redox-active polymers and promotes sustainable materials design for circular energy storage technologies.

Keywords: bioderived redox-active material; controlled ring-opening polymerization; degradable polymers; sustainable energy storage.

Fig. 1.

(A) Structural design of polypeptides carrying redox-active pendant groups. (B) Riboflavin was selected as a potential electroactive alternative to viologen to enhance the sustainability and biocompatibility of the resulting redox-active polypeptide.

Fig. 2.

Synthetic schemes for the stagewise and overall route for the production of a riboflavin-functionalized redox-active polypeptide as an electroactive material.

Fig. 3.

SEC traces of the resulting riboflavin-functionalized polypeptides, 7 from different CuAAC conditions listed in Table 1.

Fig. 4.

(A) CuAAC of 6 and 2 conducted at room temperature under N₂ atmosphere using 5 mol% CuI and 10 mol% THPTA, and images of the resulting product under visible and UV light. (B) ¹H NMR spectra (500 MHz, DMSO-d₆) of PANLG₅₀ (6, black trace), 3-propargyl RBTA (2, red trace), and riboflavin polypeptide (7, blue trace). (C) ATR-FTIR spectra of PANLG₅₀, 3-propargyl RBTA, and riboflavin polypeptide. The highlighted region showed the disappearance of the azide peak from 6 and the terminal alkyne peak from 2 after CuAAC.

Fig. 5.

(A) Cyclic voltammograms of riboflavin polypeptide (1 mM in 0.5 M TBAPF₆/PC) collected at different scan rates (2×, with the second of each scan rate cycle being shown), after a 10 mv/s (3×) preconditioning. (B) Peak current vs. square-root of scan rate from the cyclic voltammograms. (C) Redox state of riboflavin (neutral form) and flavosemiquinone (radical anion). (D) Cottrell plots from chronoamperometry of riboflavin polypeptide. The Inset shows the potential steps and the current responses.

Fig. 6.

(A) A schematic diagram of the acidic degradation study of riboflavin polypeptide (7). Figures were made at BioRender.com. (B) Degradation species detected by LC–MS.

Fig. 7.

(A) Fluorescence confocal microscopy images of Live/Dead test for the degradation products at 100 µg/mL. Images show live cells (green, stained with calcein AM) and dead cells [red, stained with ethidium homodimer-1 (EthD-1)] of fibroblast cells after incubation with degradation products of riboflavin polypeptide, 7, riboflavin analog, 10, and control (unexposed). (Scale bar, 200 µm.) (B) Quantitative cell viability assessment by MTS assay for the degradation products at 100 µg/mL. Viability percentages were normalized to the control group, calculated from different confocal microscopy areas with error bars showing SD (n = 3).