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. 2026 Aug 6;11(32):47528–47535. doi: 10.1021/acsomega.6c01793

Structural Modification of Silk Fibroin via Acylation for Biobased Plastics

Momoka Kuwabara 1, Shota Akioka 1, Keiichi Noguchi 1, Yasumoto Nakazawa 1,*
PMCID: PMC13491925  PMID: 42626566

Abstract

Silk fibroin (SF) exhibits excellent mechanical properties closely associated with its crystalline structure. Silk resin is an important plastic material for environmental applications. In this study, we investigated how different carbon chains affect the crystalline structure and physical properties of bulk SF resins. SF was modified with five carboxylic anhydrides of varying chain lengths and steric hindrance and was fabricated into bulk resins via thermal compression molding. Nuclear magnetic resonance spectroscopy revealed that long or bulky carbon chains altered the secondary structure of SF. X-ray diffraction demonstrated the expansion of the spacing between β-sheets and changes in the stacked sheet structure depending on the carbon chain structure, as supported by differential scanning calorimetry through altered thermal stability. Mechanical testing showed differences in bending strength and modulus, with SF modified by a tert-butyl-containing acyl group maintaining high strength despite reduced crystallinity. Overall, carbon chain modification effectively tunes the crystallinity and macroscopic properties of bulk SF resins, enabling the development of biobased plastics from silk resources.

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Introduction

Petroleum-based plastics such as polyethylene terephthalate and polyethylene require nonrenewable fossil resources, and their manufacturing and disposal both damage the environment through greenhouse gas emission and plastic pollution, respectively. − Recently, biobased and biodegradable plastics have emerged as promising alternatives, − and many renewable materials such as poly­(lactic acid) and poly­(hydroxyalkanoate) have been investigated. However, these polymers are produced from edible crops including corn and sugar cane, raising concerns about competition with food production. , Moreover, bioplastics currently account for only ∼1% of the global plastic production (over 4.5 billion pounds annually). To scale up bioplastics production, diverse biomass resources should be used, especially nonedible resources and waste materials. −

Silk from domesticated silkworms (Bombyx mori) is an attractive biobased alternative to petroleum-based high-strength materials owing to its excellent mechanical properties and biodegradability. In addition, silk can be processed into various forms including fibers, films, sponges, gels, and bulk materials , for a wide range of applications. However, industrial silk processing generates a significant amount of waste such as damaged cocoons, silk reeling waste, and fiber scraps. , Utilizing waste silk in biobased plastics could simultaneously reduce silk waste and petroleum consumption, thus contributing to sustainable resource use.

Silk consists of a fibrous protein called silk fibroin (SF). , The structure of SF comprises highly ordered crystalline domains formed by repetitive GAGAGS sequences (G = Gly, A = Ala, and S = Ser), together with semicrystalline regions in which Ser residues are partially replaced by Tyr (=Y) or Val (=V). , These regions aggregate through hydrogen bonds within and between SF molecules, hydrophobic interactions, and van der Waals forces, forming a strong crystal structure based on a secondary structure composed of β-sheets. , This crystal structure directly affects the physical properties, thermal processability, and biodegradability of silk-based materials. ,−

We hypothesize that modifying SF with different carbon chains could tune the crystal structure and thereby physical properties of SF, broadening its application scope. Specifically, the acylation of SF introduces carbon chains onto the hydroxyl groups of Ser and Tyr side chains; these sites contribute to crystal structure formation. Many researchers have demonstrated that the acylation of natural polymers can impart dimensional stability, hydrophobicity, and thermoplasticity. , Iwata et al. have clarified that long acyl chains significantly influence the thermoplasticity, mechanical properties, and recyclability of cellulose into which long/short acyl chains (aromatic 3-pentadecylphenoxy acetyl and aliphatic stearoyl group/acetic group) were introduced. Acyl-chain-modified SF has also been partially investigated. Asakura et al. demonstrated that the modified acetic group improves the dimensional stability and water resistance of SF through the enhancement of its hydrophobic interaction using powder and fiber as the target material. However, these studies focus on functionalization rather than the structure-dependent effects of acyl chains on material modification. Moreover, the relationship between the acyl chain and SF crystal structures has not been systematically investigated.

Fabrication of the resin form is important toward substituting silk for plastic materials in a wide range of fields. To this end, silk resin molding methods have recently been investigated, , such as hot-press molding used for conventional petroleum-based plastics; these methods have demonstrated promising productivity. However, the structure and properties of chemically modified SF resins have not been explored in literature.

In this study, SF reacted with five carboxylic anhydrides to selectively modify the amino acids involved in crystallization such as Ser and Tyr with different chain lengths and bulkiness. Here, five types of acylated SF were fabricated into bulk resin, and structural and physical property analyses revealed the effects of modifying different carbon chain structures.

Materials and Methods

Materials

Sodium carbonate (Na2CO3, guaranteed reagent), lithium bromide (LiBr), acetic anhydride, propionic anhydride, butyric anhydride, and N,N-dimethylformamide (DMF) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). Isobutyric anhydride and pivalic anhydride were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Acetone (guaranteed reagent) was purchased from Kanto Chemical Co., Inc. (Tokyo, Japan).

Preparation of SF Aqueous Solution

The aqueous SF solution was prepared as previously reported. Silk fibers from B. mori cocoons were immersed in 0.02 M Na2CO3 aqueous solution and stirred at 95 °C for 30 min. The remaining silk fibers were washed with ultrapure water several times to remove sericin, and pure SF fibers were obtained by drying at 37 °C for 24 h. The SF fibers were then dissolved in 9.0 M LiBr aqueous solution at a concentration of 10% (w/v) at 37 °C for 1 h. The solution was dialyzed against ultrapure water until LiBr was completely removed, and the insoluble residues were removed by centrifugation. The remaining SF aqueous solution had a concentration of 3.5%–5.0% (w/v).

Fabrication of Acylated SF Resins

Before preparing the SF resins, the SF aqueous solution was adjusted to 4% (w/v) concentration and freeze-dried in a silicone mold (21 mm × 21 mm × 13 mm) for 72 h to obtain sponges. These sponges were cut into cylindrical shapes with a diameter of 20 mm. The acylation reaction is illustrated in Figure . SF sponges (300 mg) were reacted with the desired anhydride (acetic, propionic, butyric, isobutyric, or pivalic anhydride; volume = 30 mL) and DMF (30 mL) as a solvent at 80 °C for 3 h. Acetic, propionic, and butyric anhydrides have different carbon chain lengths, whereas isobutyric and pivalic anhydrides have bulky substituents. The acylated sponges were washed sequentially with DMF, acetone, and ultrapure water to remove remaining reactants and dried to obtain the acylated sponges. Additionally, the amino groups of basic amino acids (7–8%) in SF may also react with carboxylic anhydrides. However, these residues are predominantly located in amorphous regions. Furthermore, carboxyl groups of acidic amino acids (∼25%) are present on SF but are unlikely to undergo the reaction.

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Acylation of serine and tyrosine residues of SF with five carboxylic anhydrides for resin modification.

Based on previous reports, , the sponges were loaded into a stainless-steel die with a diameter of 20 mm and processed in a hot press (H300–05, AS One, Japan) at 170 °C and 30 MPa. Thereafter, the assembly was pressurized to 10 MPa and cooled to 40 °C to obtain modified resins (denoted as ESF resins; Table ). The unmodified resin is denoted as SF resin.

1. Sample Codes and Carboxylic Anhydrides Used for SF Modification.

sample code carboxylic anhydride
ESF-A acetic anhydride
ESF-P propionic anhydride
ESF-B butyric anhydride
ESF-isoB isobutyric anhydride
ESF-Pi pivalic anhydride
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Structural Analysis

The chemical structure and secondary structure of the ESF and SF resins were analyzed using Fourier-transform infrared spectroscopy (FTIR) and 13C nuclear magnetic resonance (NMR). The FTIR spectra were collected using an FTIR-6600 instrument (JASCO, Tokyo, Japan) with a single-reflection ZnSe ATR attachment (ATR PRO670H–S, JASCO, Tokyo, Japan). A total of 64 scans were recorded with a resolution of 4 cm–1 and a scanning range of 500–4000 cm–1. The 13C cross-polarization/magic angle spinning (CP/MAS) NMR spectra were collected on a solid-state NMR spectrometer (ECX-400, JEOL, Tokyo, Japan) using the following pulse sequence parameters: pulse delay time, 5 s; spinning rate, 8.5 kHz; contact time, 2 ms; resonant frequency, 400 MHz; and accumulation time, 4096. The obtained free induction decay was Fourier-transformed to obtain the 13C NMR spectra, which were then analyzed via spectral deconvolution using Gaussian functions.

The crystal structures of the ESF and SF resins were evaluated by X-ray diffraction (XRD) using an automated multipurpose X-ray diffractometer (Rigaku, Tokyo, Japan) in the transmission mode with Cu Kα radiation (λ = 1.54184 Å). Irradiation was conducted at 45 kV and 200 mA, and the scanning rate was 4° min–1. The crystal face spacing (d) was calculated using Bragg’s law.

Thermal Analysis

Thermal analysis was performed on the ESF and SF resins by differential scanning calorimetry (DSC) using a DSC60plus instrument (SHIMADZU, Kyoto, Japan) under a dry nitrogen gas flow of 50 mL min–1. Each measurement was conducted using 4.5–5.0 mg of sample, which was sealed in a 40 μL aluminum pan. The samples were heated from 25 to 350 °C at a heating rate of 10 °C min–1 (n = 5).

Mechanical Property Tests

The mechanical properties of the ESF and SF resins were analyzed using the three-point bending test on an AUTOGRAPH AGS-X instrument (SHIMADZU, Kyoto, Japan) with a center distance of 16 mm and testing speed of 1 mm min–1. The resin was cut into small plate-shaped test pieces (0.7–0.8 mm in width, 16–20 mm in length, and 0.2 mm in thickness). The breaking stress, elongation, and Young’s modulus were calculated using the obtained stress–strain curves (n = 5). The Young’s modulus was defined as the slope of the stress–strain curve within the stress interval of 10–20 MPa.

Statistical Analysis

All data were presented as the mean ± SD and analyzed by one-way analysis of variance (one-way ANOVA), followed by Tukey’s multiple comparisons test. Statistical significance was considered at p < 0.05 vs. SF, as indicated by “*”.

Results and Discussion

Structural Analysis

To confirm carbon chain modification via the acylation reaction, the FTIR spectra of SF and ESF resins were compared (Figure (A)). ESF exhibited changes in all characteristic absorption bands associated with the acylation reaction, such as the CO stretching vibration of esters at 1760–1720 cm–1 and the C–O stretching vibration of esters at 1200–1100 cm–1 (Figure (B)). Moreover, the absorption bands of hydroxyl groups at 3650–3200 cm–1 decreased in intensity upon acylation (Figure (A)), implying fewer hydroxyl groups in the Ser and Tyr residues of ESF resins because these are acylation reaction sites. These results indicate that carbon chains were successfully added onto these hydroxyl groups.

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FTIR spectra of ESF and SF resins: (A) original and (B) expanded spectra in the range of 1800–1100 cm–1.

The 13C CP/MAS NMR spectra are shown in Figure . All the spectra exhibited several peaks that were assigned based on the chemical shifts of different carbon nuclei in Ala, Gly, Ser, and Tyr residues. Additionally, peaks of the modified carbon chains (−CH3, −CH2–, and −CH ) were observed at 5–40 ppm for each ESF resin. These peaks directly confirm the successful modification of SF with carbon chains. After acylation, the Ser Cα peak shifted to a higher field by 1.7–2.7 ppm and appeared as a shoulder-type peak. This was due to the increased electron density at Cα caused by the polarization of the Cα–Cβ bond. Such a result is consistent with a previous report on the preparation of acylated SF powder. In contrast, the chemical shift and line shapes of the Ser Cβ peak did not change significantly between SF and ESF. In the CP/MAS NMR method, components with high molecular mobility show weak or no CP signals. Hence, the lack of change in the chemical shift of Ser Cβ from SF to ESF was likely due to increased molecular mobility. Furthermore, the Tyr Cζ peak partly shifted to a higher field by 5.3–6.2 ppm, and the Tyr Cε peak partly shifted to a lower field by 6.0–6.8 ppm upon acylation (Figure S1). These changes were due to the polarization of the Cζ and Cε bonds within the aromatic ring of Tyr, indicating that carbon chains were introduced at the Ser and Tyr side chains of ESF.

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Expanded 13C CP/MAS NMR spectra of the ESF and SF resins in the range of 0–80 ppm. Full view of the spectra is shown in Figure S2. In the spectra of ESF, “*” indicates the modified carbon chain.

In the Ala Cβ region, peaks at ∼16.7 ppm were assigned to the random coil and β-turn, and those at ∼19.6 and ∼22.2 ppm to the β-sheet structures, according to previous reports. The ratio of (random coil + β-turn):β-sheet in ESF and SF was determined by deconvolution of the Ala Cβ peaks (Figure ). Compared with SF, ESF-A and ESF-P with shorter introduced carbon chains show little change in the ratio of secondary structures. ESF-Pi shows a larger proportion of the peak at 16.7 ppm, which is assigned to random coil + β-turn, indicating a decrease in crystallinity. This result suggests that introducing bulky carbon chains, such as the tert-butyl-containing acyl group in the case of ESF-Pi, inhibits crystal structure formation. In contrast, ESF-B and ESF-isoB show a higher proportion of peaks assigned to β-sheets, indicating that these modified carbon chains, with the specific length and hindrance, effectively strengthen local hydrophobic interactions and promote the formation of β-sheet structures. Moreover, the components attributed to the β-sheet structure in the Ala Cβ peak correspond to the methyl groups of alanine located between the β-sheets. This indicates that β-sheet stacking changes with the proportion of β-sheet structure components calculated from the 13C CP/MAS NMR spectra. Accordingly, the introduced acyl groups enhance sheet stacking in ESF-B and ESF-isoB, whereas stacking was inhibited in ESF-Pi.

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Ratios of random coil + β-turn and β-sheet in ESF and SF resins, as determined by the deconvolution of the Ala Cβ peaks.

The XRD patterns are shown in Figure . Previous XRD analyses of SF resins reported that the β-sheet structure is characterized by peaks at 2θ = 9.6°, 20.8°, and 24.9° (d = 9.2 Å, 4.3 Å, and 3.6 Å, respectively). , Referring to previous XRD results for the crystal structure of SF, , the peak at ∼9° is considered to be a single (010) reflection corresponding to the intersheet distance of ∼9.5 Å. Moreover, the peak at ∼20° is a combination of (020) and (210) reflections corresponding to the ∼4.7 Å and ∼4.4 Å inter-sheet and β-sheet distances, respectively. All the ESF resins except ESF-Pi exhibited strong peaks at ∼9° (010) and ∼18.9° (020) corresponding to the inter-sheet rather than SF resin, and there were differences between peak positions at 2θ ≈ 9° (010) among the resins. Hence, the modification of carbon chains by acylation and their length or steric hindrance influences the formation of crystal structure, especially β-sheet stacking. The SF peak at ∼20.8° (210) is attributed to the β-sheet formed by hydrogen-bonded SF molecular chains, specifically from the spacing of 4.7 Å between β-strands in the β-sheet. , All resins exhibited the peak around the same angles, suggesting the retention of the β-sheet. In contrast, ESF-A, ESF-P, ESF-B, and ESF-isoB showed remarkable diffraction peaks at 17°, 24°, and 26°, indicating structural change due to the introduction of carbon chains. These peaks were attributed to overlapping contributions from the inter-sheet, disordered sheet, and amorphous contents. A more detailed analysis is necessary to discuss their structural characterization.

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XRD patterns of ESF and SF resins. Dashed vertical lines indicate positions of the β-sheet structure, as previously reported. ,

The peak of SF at a low angle (∼9°) is attributed to the inter-sheet, specifically to the spacing between the β-sheets (d = 9.5 Å). , In all the ESF resins, this peak shifted to a lower angle. The introduced carbon chains increased the spacing between β-sheets (Table ). Compared with SF, this spacing was 2.7–2.8 Å wider in ESF-A and ESF-P and 4.2–4.4 Å wider in ESF-B and ESF-isoB. Moreover, there is no significant peak in ESF-Pi (although the exact value was difficult to determine). Among the five carbon chains in this study, the tert-butyl-containing acyl group introduced by pivalic acid inhibited the formation of a regular crystal structure, leading to structural disorder and the retention of the amorphous. In contrast, the spectra of ESF-B and ESF-isoB exhibit a slightly sharper peak around the same position, suggesting strengthened hydrophobic interactions between SF molecules and more ordered β-sheet packing. We hypothesize that introducing longer and bulkier carbon chains that have higher hydrophobicity, such as ESF-B and ESF-isoB, strengthens local hydrophobic interactions and forms highly regular crystal structures while expanding the spacing between the β-sheets.

2. Spacing between the β-Sheets of ESF and SF Resins Calculated from Bragg’s Law.

sample code spacing between the β-sheets (Å)
SF 9.56
ESF-A 12.35
ESF-P 12.21
ESF-B 13.70
ESF-isoB 13.96
ESF-Pi impossible to calculate the peak top
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Thermal Stability

Thermal analysis was conducted to measure the thermal behavior of the samples. From the DSC thermograms of SF and ESF resins (Figure S3), all samples showed a broad endothermic dehydration peak between 50 and 100 °C. Additional endothermic peaks observed within 280–305 °C were assigned to the mixed component of crystal relaxation and thermal decomposition of each resin.

Figure shows the temperature (T m+d) values identified from the top of the endothermic peak observed at 280–305 °C in the DSC thermograms. T m+d was employed in this study as an indirect indicator of the structural stability of the SF because both the thermal decomposition and crystal relaxation influence the structural stability of polymers. ESF-A, ESF-P, and ESF-Pi showed much lower T m+d than SF, especially with a reduction of ∼6 °C in ESF-Pi. In contrast, ESF-B and ESF-isoB showed higher T m+d than SF. Thus, as shown by the XRD results (Figure and Table ), the introduction of acyl groups in ESF-A and ESF-P changed the crystalline structure, which is consistent with the reduced thermal stability observed in the DSC results. For ESF-B and ESF-isoB, hydrophobic interactions may have outweighed the inhibitory effects induced by steric hindrance on crystal structure formation. For all the resins except ESF-Pi, T m+d increased in the order ESF-A < ESF-P (<SF) < ESF-B < ESF-isoB, suggesting its association with hydrophobicity because longer and bulkier carbon chains offer local areas with stronger hydrophobic interactions. In contrast, ESF-Pi with both higher hydrophobicity and steric hindrance showed the lowest T m+d value, suggesting that the bulky tert-butyl group strongly inhibits crystal structure formation.

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Temperature corresponding the top of the endothermic peaks between 280–305 °C (T m+d) in the DSC thermograms of ESF and SF resins. *: p < 0.05, †: p < 0.1.

The thermal processing of SF is difficult because thermal decomposition occurs well before melting. A significant decrease in crystal relaxation temperature should impart thermoplasticity to SF, expanding its application scope as a plastic material. Therefore, it is important to test whether introducing carbon chains with large steric hindrance (such as the tert-butyl-containing acyl group and bulkier group than that) improves the thermal properties and thermal processability of silk-based plastics.

Structural analysis via 13C CP/MAS NMR and XRD have demonstrated that carbon chain modification alters the crystal structure. These results imply that carbon chains with stronger steric hindrance than hydrophobic interactions tend to inhibit crystal structure formation. In contrast, carbon chains with stronger hydrophobic interactions than steric hindrance could have the opposite effect. The changes in T m+d identified from DSC results show the same trend as structural analyses, possibly indicating crystal structure stability. To accurately evaluate the endothermic peaks assigned to the crystal relaxation temperature of SF, DSC measurements must be conducted, ideally under ultrafast heating conditions.

Mechanical Properties

Figure shows the bending stress–strain curves of the SF and ESF resins. All bending stress–strain curves of the SF and ESF resins are shown in Figure S4. The shape of the curve changed after acylation. Figure shows the physical property parameters calculated from the bending stress–strain curves, where (A), (B), and (C) represent the maximum stress, breaking elongation, and Young’s modulus, respectively. Compared with that of SF, the Young’s modulus decreased after acylation, implying that all the ESF resins became more flexible. The introduction of carbon chains may have weakened intermolecular forces between the β-sheets of SF, as suggested by the spacing changes shown in Table , thereby potentially enhancing the mobility of its molecular chain segments. ESF-A and ESF-P with short chain lengths exhibited significantly lower maximum stress and breaking elongation than other resins. The inter-sheet spacing between the sheets in ESF-A and ESF-P is smaller than that of ESF-B and ESF-isoB (Table ), indicating ESF-A and ESF-P have a denser stacking the retention of the amorphous than ESF-B and ESF-isoB, which is expected to exhibit higher resin strength; however, the opposite trend was observed. Furthermore, as the chain length increases, local hydrophobic interactions theoretically become stronger, which could contribute to the higher strength of ESF-B and ESF-isoB. These results imply the importance of balancing the mobility of molecular chain segments and local hydrophobic interactions around introduced acyl groups. However, these two groups, separated by carbon chain length (ESF-A and ESF-P/ESF-B and ESF-isoB), differ in the proportion of β-sheet structures (Figure ) and thermal stability (Figure ). Therefore, the decrease in the bending maximum stress and elongation at break cannot be definitively related to local hydrophobic interactions and the mobility of molecular chain segments. Thus, further discussion is needed based on hierarchical structural analysis. Moreover, ESF-Pi showed a different trend from the structural analysis prediction: its mechanical strength was maintained despite inhibited crystal structure formation, suggesting that ESF-Pi had a small crystal domain size. SF is known to achieve higher strength and toughness when the β-crystals are confined to a few nanometers in size. In the case of ESP-Pi, the superior physical properties are likely due to the significant steric hindrance of the tert-butyl-containing carbon chain, which potentially reduces the crystal size. This possibility should be further verified by structural analysis at the nanometer scale using small-angle X-ray scattering measurements. The results for ESF-Pi imply that modification with bulky carbon chains can inhibit crystal structure formation without compromising the mechanical strength.

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Representative bending stress–strain curves of ESF and SF resins.

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Mechanical property parameters calculated from the bending stress–strain curve. (A) Maximum stress, (B) breaking elongation, and (C) Young’s modulus.

Conclusion

Five types of acylated SF resins with varying crystal structures were prepared by introducing carbon chains with different lengths and steric hindrance. 13C CP/MAS NMR analysis revealed that long or bulky carbon chains induced significant changes in the secondary structure. Especially, carbon chains with a much higher steric hindrance inhibited the formation of crystal structures, resulting in the retention of the amorphous state. Moreover, XRD results showed alterations in the crystal structure, especially the expansion of the spacing between β-sheets after acylation and changes in β-sheet stacking. DSC data also supported these structural changes. Three-point bending tests revealed the differences in the physical properties of the resins associated with the changes in their crystal structure. Remarkably, the tert-butyl-containing pivaloyl group with large steric hindrance maintained mechanical strength of resin despite inhibiting crystal structure formation, whereas butyryl- and isobutyryl groups with moderate chain length and number of methyl groups promoted crystallization, thereby relatively maintained mechanical strength.

These analyses confirmed the successful alteration of the SF crystal structure through carbon chain modification, demonstrating the potential for controlling the crystal structure and physical properties of SF resins. This study promotes the broad-scale application of silk in the development of bio-based plastics.

Supplementary Material

ao6c01793_si_001.pdf (692.2KB, pdf)

Acknowledgments

This work was partially supported by the Bio-oriented Technology Research Advancement Institution (BRAIN), Project for Open Innovation Research and Commercialization Promotion (JPJ011937); JST, the Program on Open Innovation Platform for Industry-Academia Co-Creation (JPMJPF2104); and JSPS KAKENHI (24K03277, 23K13573).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c01793.

  • Detailed experimental procedures and additional characterization data (PDF)

†.

M.K. and S.A. contributed equally to this work. Y.N. conceived and designed the overall study and acquired the funding. M.K. and S.A. carried out the material synthesis and characterization. K.N. conducted solid-state NMR and X-ray diffraction (XRD) measurements. All authors contributed to data analysis and manuscript preparation. All authors have approved the final version of the manuscript.

The authors declare no competing financial interest.

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