Photon Factory Activity Report 2014 #32 (2015) B

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10010-1

BL-10C/2014G12710-2

I

Nanostructure of hydrated wheat protein gliadin as revealed by SAXS
10-310-4

2Nobuhiro Sato1,*, Aoi Matsumiya2, Yuki Higashino2, Satoshi Funaki2, Yuki Kitao, Yojiro Oba1,
1
12 10
1010
Rintaro Inoue , Masaaki Sugiyama , and Reiko Uradeq / nm
1
Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan
2
Graduate School of Agriculture, Kyoto University, Uji, Kyoto 611-0011, Japan
10-5-6-1

0-12 Experiment
Dough was prepared by mixing and kneading wheatflour (Super KingTM, Nisshin Flour Milling, Inc.) with 0.5
M NaClaq. It was repeatedly kneaded in distilled waterand the third and sixth washing solutions were collected
and centrifuged.
Then, NaCl was added to thesupernatant to precipitate gliadins.
The SAXS measurements were carried out on the BL10C beam line of the Photon Factory. Solutions were put
in a 1-mm thick aluminum cell with 20-m thick quartz
windows, while hydrated solids were put in a 1-mm thick
PTFE sandwich cell with windows of 7.5-m thickKapton® films (TORAY-DuPont). The X-ray wavelength
was 0.1488 nm, the sample-to-detector distance wasaround 2 m, and the observed q-range was 0.06–2.5 nm.
The scattered beam was detected with R-AXIS VII(RIGAKU) or PILATUS 2M (DECTRIS). The typical Xray exposure time was 300 s.
3 Results and Discussion
Gliadin is soluble in water below 10 wt%. Figure 1 left
shows SAXS profiles of gliadin aqueous solutions. At avery dilute concentration (0.1 wt%), the scattering curve
can be analyzed by three-component Guinier relationship.
The lowest Rg value obtained by the Guinier analysis isfound to be 5.68 nm, which corresponds to a monomeric

101

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10 wt%

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10-1 1 wt%

100

10-2

10-1

15 wt%

40 wt%

II

1 Introduction
The physical properties of wheat dough are muchaffected by those of its component, gluten. Gluten ismainly composed of two proteins: glutenin and gliadin.
The former is responsible for elasticity; the latter is forviscosity. Glutenin forms high-molecular-weight network
structure connected with intermolecular disulphide bonds.
In contrast, gliadin is a monomeric protein withintramolecular disulphide bonds. Gliadin had been
known to be insoluble in water and had usually been
extracted from dough with aqueous ethanol or dilute acids.
However, Ukai et al. found that gliadin can be extracted
from NaCl-containing dough with distilled water [1]. It is
expected that the behavior of gliadin extracted in water
resembles more closely that of gliadin in real dough.
Although gliadin is soluble in water at low concentrations,
it is no longer soluble and forms hydrated solids at highconcentrations. Nanostructure of such condensed matterscan effectively be disclosed by small-angle X-ray
scattering (SAXS). In this study, we investigated thenanostructure of gliadin assembly in distilled water over awide concentration range by SAXS.

10-3

10-2

0.1 wt%

10-4

10-3

10-5

10-4

10-6

10-1

100q /nm-1

10-5

70 wt%
10-1

100q / nm-1

Fig. 1 SAXS profiles of gliadin aqueous solutions (left)
and gliadin hydrated solids (right). In the right figure,profiles are vertically shifted for clarity.
gliadin molecule. It is also suggested that the other larger
components correspond to dimers and oligomers. A small
shoulder appears at q = 0.21 nm-1 at 1 wt%, and shifts to0.13 nm-1 at 10 wt%. This shoulder is indicative ofinterparticle interference between aggregated gliadin
domains. The domain separation distance calculated with
the Bragg’s equation is found to be 30 and 48 nm for 1
and 10 wt%, respectively.
Figure 1 right displays SAXS profiles of hydrated
solids. The SAXS profile for 15 wt% solids is quite
similar to that of the 10 wt% solution. In spite ofdifferent physical states, the SAXS profiles are similar toeach other, indicating that the nanostructure of these two
samples are quite alike. At 40 wt%, the shoulder at 0.13
nm-1 diminishes and new broad shoulder appears at 0.45
nm-1 (= 14 nm in real space). This shoulder indicatesdensity fluctuation inside the aggregated gliadin domains.
Finally, this shoulder diminishes at 70 wt%, and theupturn at the low-q region becomes prominent due to theformation of large aggregates. The poor water content
leads to poor free space inside the domains and reduces
the density fluctuation
As described above, it is demonstrated that withincreasing concentrations SAXS profiles change
continuously, but the assembly states are largely different
according to the interparticle interference and density
fluctuation.
References[1] T. Ukai, Y. Matsumura, R. Urade, J. Agric. FoodChem., 56, 1122 (2008).
* sato-n@rri.kyoto-u.ac.jp