International Journal of Biological Macromolecules 51 (2012) 640–646 Contents lists available at SciVerse ScienceDirect International Journal of Biological Macromolecules jo u rn al hom epa ge: www.elsevier.com/locate/ijbiomac Dynamic rheological properties of native and cross-linked gliadin proteins Rosane M.D. Soares a,∗ , Maria I. Lionzo a , Nadya P. Da Silveira a , Patricia Rayas-Duarte b , Valdir Soldi c a Institute of Chemistry, Universidade Federal do Rio Grande do Sul, 91501-970, Porto Alegre, Brazil b Food and Agricultural Products Research Center, Oklahoma State University (OSU), Stillwater, OK 74078-3035, United States c Department of Chemistry, Universidade Federal de Santa Catarina, 88040-900, Florianópolis, Brazil a r t i c l e i n f o Article history: Received 14 March 2012 Received in revised form 4 June 2012 Accepted 27 June 2012 Available online 4 July 2012 Keywords: Gliadin Oscillatory rheology Globular protein a b s t r a c t A comparison of cross-linked and native gliadin suspensions, with respect to the state of protein globular structure was carried out using small-angle X-ray scattering (SAXS), dynamic light scattering (DLS) and rheological analysis. Gliadin suspensions were also analyzed in the presence and absence of glycerol. DLS analysis showed that R h increased only with gliadin/EDC/NHS suspensions. However, Kratky plots revealed that gliadin and gliadin/l-cysteine maintained their globular shape even in absence or presence of glycerol. Rheological experiments revealed that gliadin and gliadin/l-cysteine suspension exhibited a similar profile with three main domains, and a sol–gel transition. Gliadin/EDC/NHS did not present any sol–gel transition, and this fact corroborates with DLS results and the hypothesis of lower protein–protein interaction, which are in agreement with G  > G  . © 2012 Elsevier B.V. All rights reserved. 1. Introduction Gliadins are important storage proteins in wheat grain, which provide viscous character and strain hardening to wheat gluten. These proteins are formed by a nonrepetitive domain rich in - helix structure and by a heterogeneous repetitive domain rich in -reverse turns. They have excellent film forming character via solution casting using water and/or ethanol as co-solvent. In this sense, the unique physical and biochemical properties of gliadin make it suitable for numerous non-food applications such as biodegradable plastics, membranes and also as a material for biomedical applications [1–8]. There have been many investigations of the role of gluten, gliadin and glutenin interactions, although only some recent inves- tigations relate the behavior of gliadin with polyanions, membranes and polymers [9–11]. As a consequence, many other factors as solu- tion preparation, solvent, concentration, mixing procedure, cross linker addition, and pH of solution have large effects on measured rheological properties and have a lack of studies. All factors play key roles in the formation of a cohesive and con- tinuous matrix in the materials to provide excellent mechanical properties and variations in any of these parameters can result in systems with varying rheological properties. Generally, these stud- ies are focused on dough functional parameters and the ability of dough to leaven [12,13] while others have investigated interac- tions between polymer and protein blends [14]. Nevertheless, the ∗ Corresponding author. Tel.: +55 51 3308 6291; fax: +55 51 3308 7304. E-mail address: soaresr@iq.ufrgs.br (R.M.D. Soares). effect of protein modifications after cross linking reaction has been rarely investigated. Dynamic rheological parameters have helped to elucidate the nature of gliadin, its conformation and molecular interactions. Sun et al. [5] have examined the rheological behav- ior of gliadin solutions with different concentrations of aqueous propanol. The authors observed that gliadin solutions behaved as Newtonian fluid and are not spherical shaped, with a molecular size that increases with temperature, due to improved solvation. Studies with gliadin gels prepared from propanol/water sol- vents at pH 9.3 have shown a network composing of cross-linked strands (Song et al., 2009) [6]. The interaction between gliadin and polysaccharide has shown that this protein facilitates the hydrophobic association of methylcellulose molecules upon heat- ing. However, the interaction between gliadin and glutenin, two different proteins, has shown that for both protein fractions, linear and non-linear rheological properties were strongly dependent on concentration, and as a consequence, gliadin is an important factor to adjust gluten’s viscoelastic properties [12]. In a previous work [15] we have shown the effect of different cross linkers on steady shear properties of modified gliadin suspen- sions. Suspensions of gliadin modified by EDC have shown slightly higher values of viscosity when compared to unmodified proteins. This behavior was mostly associated to a singular agglomeration, effect of EDC in the modified gliadins. Also, the EDC is a water sol- uble condensing reagent with large monovalent ions, such as Cl − . Due to their diffused surface charges causing weak electric fields, these ions were unable to polarize water molecules beyond the first layer of hydration and consequently, the viscosity of this system was lower. Another interesting way to investigate the global struc- ture of proteins is the information about the secondary and tertiary 0141-8130/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijbiomac.2012.06.041