title: Impact of freeze-drying on ionization of sulfonephthalein probe molecules in trehalose-citrate systems.
abstract: "pH memory," i.e., correlation between pH of solution before freeze-drying and chemical reactivity in the freeze-dried state, has been reported in many systems. In this study, the "pH memory" is explored by comparing the extent of protonation of sulfonephthalein probe molecules, bromophenol blue, bromocresol green, and chlorophenol red, in aqueous solution in the pH range of 3.4-6.0 and in the resulting freeze-dried amorphous matrix (lyophile) containing trehalose and sodium citrate buffer. The protonation of the probe molecules was measured in the lyophiles by diffuse reflectance visible spectroscopy, and compared with that in the solution before drying. The protonation of the indicators in the amorphous matrix correlated with solution pH, that is, an increase in solution pH resulted in a progressive decrease in the indicator protonation in the corresponding lyophile. However, the protonation was consistently higher in the lyophile than in the corresponding solution. The Hammett acidity function of lyophiles was calculated based on the extent of protonation of the probe molecules. Protonation of the probe molecules and the Hammett acidity function depended not only on prelyophilization solution pH, but also on the residual water content and the presence of amorphous sugar in the lyophile.
authors.affiliation: Department of Pharmaceutics, University of Minnesota, 308 Harvard St. SE, Minneapolis, 55455, USA.
authors.forename: Ramprakash
authors.initials: R
authors.lastname: Govindarajan
authors.forename: Koustuv
authors.initials: K
authors.lastname: Chatterjee
authors.forename: Larry
authors.initials: L
authors.lastname: Gatlin
authors.forename: Raj
authors.initials: R
authors.lastname: Suryanarayanan
authors.forename: Evgenyi Y
authors.initials: EY
authors.lastname: Shalaev
mesh_terms: D001961:Bromcresol Green / Q000737:chemistry*; D001978:Bromphenol Blue / Q000737:chemistry*; D002021:Buffers; D002951:Citrates / Q000737:chemistry*; D005612:Freeze Drying; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D010637:Phenolsulfonphthalein / Q000031:analogs & derivatives* / Q000737:chemistry; D000077559:Sodium Citrate; D014199:Trehalose / Q000737:chemistry*
chemical_list: D002021:Buffers; D002951:Citrates; D007202:Indicators and Reagents; D001978:Bromphenol Blue; D000077559:Sodium Citrate; C007016:chlorophenol red; D001961:Bromcresol Green; D014199:Trehalose; D010637:Phenolsulfonphthalein