MOLECULAR-BASIS OF FILM FORMATION FROM A SOYBEAN PROTEIN - COMPARISONBETWEEN THE CONFORMATION OF GLYCININ IN AQUEOUS-SOLUTION AND IN FILMS

Fourier transform infrared spectroscopy has been used to investigate t he conformational changes of glycinin, a major storage protein of soyb ean seeds, upon film-forming. The results show that the secondary stru cture of glycinin is mainly composed of a beta-sheet (48%) and unorder ed (49%) structures. The amide I band of glycinin in film-forming cond itions, i.e. in alkaline media and in the presence of plasticizing age nt, reveals the conversion of 18% of the secondary structure of the pr otein from the beta-sheet (6%) and random coil (12%) to the alpha-heli cal conformation due to the helicogenic effect of the ethylene glycol used as the plasticizing agent. Conformational changes also occur upon the him-forming process leading to the formation of intermolecular hy drogen-bonded beta-sheet structures. Results obtained from other plant families indicate that, whatever the origin and conformation of prote in, formation of films leads to the appearance of intermolecular hydro gen-bonded beta-sheet structures, suggesting that this type of structu re might be essential for the network formation in films. Thus, it is hypothesized that, in the him state, intermolecular hydrogen bonding b etween segments of beta-sheet may act as junction zones in the film ne twork. This study reveals for the first time that there is a close rel ationship between the conformation of proteins and the mechanical prop erties of films. (C) 1998 Elsevier Science B.V. All rights reserved.