Graft copolymers (SPI-g-NH-PAMPS) were prepared by a graft reaction between the free carboxylic acid groups of soy protein isolate (SPI) and the amino groups of poly(2-acrylanmido-2-methyl propane sulfonic acid) (H2N-PAMPS) using 1-(3-(dimethylamino)propyl)-3-ethyl-carbodiimide hydrochloride/N-hydroxysuccinimide as the condensing agents in a buffer solution.H2N-PAMPS and SPI-g-NH PAMPS were fully characterized by 1H-NMR

FTIR and 13C-NMR spectroscopies.The aqueous solution properties of SPI and SPI-g-NH-PAMPS were studied by zeta potentiometer

UV-Visible spectroscopy

fluorescence spectrometer

dynamic laser light scattering (DLS).The charge of SPI-g-NHPAMPS measured by zeta potentiometer was negative within the whole pH range studied

which was due to the ionization of sulfonic groups of grafted chains.UV-Visible spectroscopy measurements demonstrated that the solubility of SPI was improved after the graft reaction

especially around the isoelectric point of SPI where it was increased by about 40%

because of the grafted hydrophilic H2N-PAMPS chains.Fluorescence probing studies with 1-anilinonaphthalene-8-sulfonic acid (ANS) in aqueous solution revealed that the surface hydrophobicity of SPI-g-NH-PAMPS was much weaker than that of SPI.The average hydrodynamic radius (h) of SPI aggregates measured by dynamic laser light scattering (DLS) was increased from about 140 nm to about 200 nm.TEM studies indicated that SPI-g-NH-PAMPS aggregates presented a unique core-shell structure because hydrophilic PAMPS long chains surrounded on the surface of SPI globulins

and formed more hydrophilic shell

and the average size of SPI-g-NH-PAMPS aggregates was also bigger than that of SPI aggregates.The graft reaction using EDC provides a new method to modify SPI and expands the application fields of SPI.