Napin-enriched fraction
The α-helix content of 32.1% ± 0.4% (Figure 6) determined for camelina napin was similar to that reported for B. napus napin (Pantoja-Uceda, 2003; Wanasundara, 2011). The spectral peaks resembling β-sheets (1627-1638 cm-1) and β-turns (1674-1684 cm-1) most likely originated from bond vibrations responsible for short, extended chains attached to helical motifs. These peaks resembled those observed for cruciferin (Figure 6); however, they do not originate from true β-structures in napin. This phenomenon is commonly observed in other helical proteins, such as hemoglobin and cytochrome C (Byler and Susi, 1986). Camelina napin showed far-UV CD spectra typical of a helical protein similar to described by Greenfield (2006) with -θ peaks at ~222 nm and ~208 nm and a + θ peak at ~195 nm (Figure 7B). The secondary structural features of napin did not show differences in magnitude with changes in pH (Figure 7B and Table 4). Napin from B. napus is also structurally stable in response to changes in pH (Perera et al., 2016). The increase in S0 at pH 3 and the decrease at pH 10 (Table 4), although much less than that observed with cruciferin, suggested some degree of structural change for napin. Use of intrinsic fluorescence to probe napin tertiary structure details during pH changes was not possible due to the very low number of Trp and Tyr residues. No thermal transition peak above or below 100°C was observed for camelina napin dry powder or slurries at pH 3, 7 and 10. However, Boyle et al. (2018) reported that temperatures of ~104-107°C with lower enthalpy values (ΔH = ~0.2-3.0 J g-1) are required for denaturation of camelina napin. The high thermal-stability nature of crucifer napin at a temperature of >100°C (ΔH = ~9-14 J g-1) is reported elsewhere for B. napus (Krzyzaniak et al., 1998). The lower sensitivity of napin structure to changes in pH and its high stability in response to heat is a distinct contrast to the properties of cruciferin found in the SSPs of Brassicas, including camelina.
Cruciferin and napin, and to a lesser extent vicilins and OBPs, comprise the bulk of the camelina meal proteins. Camelina protein is rich in S-containing amino acids, contains proteins that are soluble at acidic pH, and is structurally stable at low pH and high temperatures, making camelina seed protein similar to that from other economically-important crucifers. Protein products rich in cruciferin and/or napin could be obtained from camelina using scalable processes described forBrassica oilseeds to benefit camelina value chain development. Unique properties of individual proteins (cruciferin and napin) could position camelina alongside commercial ventures being developed for canola/rapeseed protein (https://www.meritfoods.com/canola.protein/; https://napiferyn.com/; https://www.dsm.com/corporate/solutions/nutrition-health/canolapro-plant-protein.html). The ability to change the camelina seed protein profile and consequently influence the quality attributes of SSP fractions (Lyzenga et al, 2019) may cater to specific nutritional or functional applications. Seed coat mucilage is also a valuable hydrocolloid (Soukoulis et al., 2018); however, removal or reduction of camelina seed coat mucilage is critical for high protein recoveries. Therefore, effective technologies for recovering seed coat mucilage are necessary to bring this valuable co-product into an integrated valorization scheme. OBs that are stabilized by OBPs may also have economic value in food, feed and personal care applications (https://www/botaneco.com/food-and-feed).