Fluorescence investigation of a specific structure formed by aggregation of transglycosylated stevias: solubilizing effect of poorly water-soluble drugs.

Spray-dried particles (SDPs) with indomethacin (IND) and alpha-glycosyl transferase-treated stevia (Stevia-G) indicated extremely high dissolution rates and apparent solubility compared to particles of a ground mixture and a physical mixture of IND/Stevia-G. The apparent solubility of IND from SDPs was 5-fold higher than that of untreated IND. We investigated the solubility-enhancing effect of Stevia-G towards hydrophobic materials by using fluorescence spectroscopy. 1-Naphthoic acid (1-NPA) and pyrene were used as hydrophobic fluorescence probes. Although both the ground mixture and SDPs of 1-NPA/Stevia-G exhibited halo patterns in powder X-ray diffraction, the fluorescence spectra of 1-NPA solely in SDPs indicated a monomolecularly dispersed state of 1-NPA. The plot of the pyrene I(1)/I(3) ratio versus the Stevia-G concentration showed a sigmoidal curve as a function of the Stevia-G concentration, suggesting the existence of a hydrophobic environment around pyrene molecules under high Stevia-G concentrations. The critical micelle concentration calculated from the pyrene I(1)/I(3) plot was about 16mg/mL. Based on results from the static quenching plots, the micellar aggregation number of Stevia-G was estimated as ca.15. Therefore, the hydrophobic steviol-skeleton of Stevia-G made a hydrophobic core around a hydrophobic molecule. This specific structure formed by Stevia-G molecules led to an enhancement of the apparent solubility of poorly water-soluble drugs.