Formation of Nanocomplexes between Carboxymethyl Inulin and Bovine Serum Albumin via pH-Induced Electrostatic Interaction

As a functional polysaccharide, inulin was carboxymethylated and it formed nanocomplexes with bovine serum albumin (BSA). The success of obtaining carboxymethyl inulin (CMI) was confirmed by a combination of Fourier transform Infrared (FT-IR), Raman spectroscopy, gel permeation chromatography (GPC), and titration. The effects of pH and ionic strength on the formation of CMI/BSA nanocomplexes were investigated. Our results showed that the formation of complex coacervate (pH(φ1)) and dissolution of CMI/BSA insoluble complexes (pH(φ2)) appeared in pH near 4.85 and 2.00 respectively. FT-IR and Raman data confirmed the existence of electrostatic interaction and hydrogen bonding between CMI and BSA. The isothermal titration calorimetry (ITC) results suggested that the process of complex formation was spontaneous and exothermic. The complexation was dominated by enthalpy changes (∆Η < 0, ∆S < 0) at pH 4.00, while it was contributed by enthalpic and entropic changes (∆Η < 0, ∆S > 0) at pH 2.60. Irregularly shaped insoluble complexes and globular soluble nanocomplexes (about 150 nm) were observed in CMI/BSA complexes at pH 4.00 and 2.60 while using optical microscopy and atomic force microscopy, respectively. The sodium chloride suppression effect on CMI/BSA complexes was confirmed by the decrease of incipient pH for soluble complex formation (or pH(c)) and pH(φ1) under different sodium chloride concentrations. This research presents a new functional system with the potential for delivering bioactive food ingredients.