Wheat-Based Glues in Conservation and Cultural Heritage: (Dis)solving the Proteome of Flour and Starch Pastes and Their Adhering Properties

Abstract

Plant-based adhesives, such as those made from wheat, have been prominently used for books and paper-based objects and are also used as conservation adhesives. Starch paste originates from starch granules, whereas flour paste encompasses the entire wheat endosperm proteome, offering strong adhesive properties due to gluten proteins. From a conservation perspective, understanding the precise nature of the adhesive is vital as the longevity, resilience, and reaction to environmental changes can differ substantially between starch- and flour-based pastes. We devised a proteomics method to discern the protein content of these pastes. Protocols involved extracting soluble proteins using 0.5 M NaCl and 30 mM Tris-HCl solutions and then targeting insoluble proteins, such as gliadins and glutenins, with a buffer containing 7 M urea, 2 M thiourea, 4% CHAPS, 40 mM Tris, and 75 mM DTT. Flour paste's proteome is diverse (1942 proteins across 759 groups), contrasting with starch paste's predominant starch-associated protein makeup (218 proteins in 58 groups). Transformation into pastes reduces proteomes' complexity. Testing on historical bookbindings confirmed the use of flour-based glue, which is rich in gluten and serpins. High levels of deamidation were detected, particularly for glutamine residues, which can impact the solubility and stability of the glue over time. The mass spectrometry proteomics data have been deposited to the ProteomeXchange, Consortium (http://proteomecentral.proteomexchange.org) via the MassIVE partner repository with the data set identifier MSV000093372 (ftp://MSV000093372@massive.ucsd.edu).

Keywords: book conservation; gliadin; gluten; leather; starch; wheat.

Figure 1

(A) Total number of protein groups and protein matches in fractions 1 and 2 of raw starch and starch glue. (B) Number of protein matches in fraction 1, fraction 2, and in both (common) in raw starch and starch glue. (C) Main protein groups identified in fractions 1 and 2 of raw starch and starch glue. (D) Number of protein matches in raw starch and starch glue and in both (common) from fraction 1 and fraction 2. Fraction 1 was performed using 0.5 M NaCl 30 mM Tris-HCl pH 8 buffer at room temperature for 2 h. Fraction 2 was performed using 7 M urea, 2 M thiourea, 4% CHAPS, 40 mM Tris, 75 mM DTT buffer overnight at 4 °C. Parameters are 1% FDR, protein score >50, minimum two peptides.

Figure 2

(A) Total number of protein groups and protein matches identified in fractions 1 and 2 of raw flour and flour glue. (B) Number of protein matches in fraction 1, fraction 2, and in both (common) in raw flour and flour glue. (C) Main protein groups identified in fractions 1 and 2 of raw flour and flour glue. (D) Number of protein matches in raw flour and flour glue, and in both (common) from fractions 1 and fraction 2. Fraction 1 was performed using 0.5 M NaCl 30 mM Tris-HCl pH 8 buffer at room temperature for 2 h. Fraction 2 was performed using 7 M urea, 2 M thiourea, 4% CHAPS, 40 mM Tris, and 75 mM DTT buffer, overnight at 4 °C. Parameters are 1% FDR, protein score >50, minimum two peptides.

Figure 3

(A) The bar graph shows the abundance of the total number of protein groups (black) and total proteins (gray) when fractions 1 and 2 are combined across four samples: raw starch, starch glue, raw flour, and flour glue. Notably, the raw flour combined sample exhibits the highest number of total proteins. In contrast, starch glue combined samples show the lowest counts in both metrics, proteins, and protein groups. (B) Profiling of specific protein groups across the combined samples. The stacked bar graph shows the number of protein groups in the same combined samples.

Figure 4

(A) Categories of proteins identified in the leather samples V0, 46A, 49A, and 55A. Among the samples, 49A and 55A show a high wheat protein concentration. (B) Main wheat proteins identified in the leather samples compared to the starch and flour pastes.

Figure 5

Number of protein matches identified that are shared with starch or flour pastes in (A) binder 49A and (B) binder 55A.

Figure 6

Percentage of deamidation calculated for Asn N and Gln Q in (A) leather and egg proteins and (B) wheat proteins. The total number of N (asparagine) and Q (glutamine) residues on which the calculation was based is indicated above each bar. Milk peptides are not included due to the low number of peptides containing N and Q for calculation.

Figure 7

Asparagine deamidation in wheat samples. Percentage of deamidation calculated for Asn N in key protein families. The total number of N (asparagine) residues on which the calculation was based is indicated above each bar.

Figure 8

Asparagine and glutamine deamidation of wheat proteins in the bookbinding samples. The percentage of deamidation calculated for Asn N and Gln Q is shown for gluten and nongluten proteins. The total number of N and Q residues on which the calculation was based is indicated above each bar.