Is Gum Arabic a Good Emulsifier Due to CH•••π Interactions? How Urea Effectively Destabilizes the Hydrophobic CH•••π Interactions in the Proteins of Gum Arabic than Amides and GuHCl?

The photophysical studies of gum arabic (GA) in the presence of urea, 1,3-dimethylurea (DMU), tetramethylurea (TMU), guanidine hydrochloride (GuHCl), formamide (FA), acetamide (AA), and dimethyl formamide (DMF) were carried out by monitoring the emission, three-dimensional emission contour, and time-correlated fluorescence lifetime techniques. On addition of only 1 X 10(-3) M urea, 75.0% of the fluorescence of GA is quenched, while the same occurs in GuHCl at 3.0 M. FA quenched 50% of the fluorescence of GA at 5.0 M. However, DMU, TMU, AA, and DMF resulted in a fluorescence enhancement. The unusual fluorescence trends reveal the existence of CH center dot center dot center dot pi interactions in the proteins of GA. The experimental results and the structural aspects of proteins in GA led us to propose that the aggregation of polyproline helices in GA, through several CH center dot center dot center dot pi interactions, would have a major role to play in the emulsification mechanism of GA.