Note Evidence of two-step deprotonation of D -mannitol in aqueous solution Ernestas Gaidamauskas, a, * Eugenijus Norkus, b J urat _ e Vaic ˇi  unien _ e, b Debbie C. Crans, c Tapani Vuorinen, d Jan _ e Jac ˇiauskien _ e b and Gintaras Baltr unas a a Vilnius University, Faculty of Chemistry, Naugarduko 24, LT-03225 Vilnius, Lithuania b Institute of Chemistry, Laboratory of Catalysis, A. Gos ˇtauto 9, LT-01108 Vilnius, Lithuania c Colorado State University, Department of Chemistry, Fort Collins, CO 80523-1872, USA d Helsinki University of Technology, Department of Forest Products Technology, Vuorimiehentie 1A, FIN-02150 Espoo, Finland Received 22 July 2004; received in revised form 24 March 2005; accepted 24 March 2005 Abstract—Deprotonation of D-mannitol was studied in aqueous basic solutions by means of potentiometry and 13 C NMR spectro- scopy. Two-step dissociation in the pH range from 12 to 13.8 was shown, and successive dissociation constants K a1 and K a2 were determined. In a solution with ionic strength I = 1.0 M (NaOH + NaNO 3 )pK a1 = 13.1 ± 0.1 and pK a2 = 13.8 ± 0.2. With increasing ionic strength from 0.75 to 3.0 M, both pK a1 and pK a2 values decrease. Deprotonation-induced chemical shifts in pH-variable 13 C NMR spectra show that the OH-groups next to internal carbon atoms C-3 and C-4 dissociate to a greater extent compared to OH- groups next to external carbon atoms C-1 and C-6. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: pK a ; 13 C NMR; Potentiometry; Metal ion chelation D-Mannitol (1) is a widespread hexitol found in a vari- ety of plants, algae and fungi. It is endogenous in humans 1–3 and is a widely accepted food additive. 4 Although its biological role is not fully understood, evi- dence suggests that D-mannitol is an important interme- diate in the physiology of plants, 5,6 animals 7,8 and humans. 9–12 In aqueous solution D-mannitol adopts a flat zigzag conformation, 13 and sequesters metal ions forming surprisingly stable chelates. 14 D-Mannitol com- plexes with oxometallates are stable in a wide pH range, 14 however, complexes with simple non-oxo metal ions are rather weak in acidic and neutral media 15 owing to the weak acidic D-mannitol properties. 16,17 Stable non-oxo metal complexes with D-mannitol exist only in strongly basic solutions where hexitol deprotonation takes place. 14,18,19 Acidic properties of D-mannitol were shown, 20–25 and one-proton per D-mannitol molecule deprotonation is widely accepted. 22–25 Reported pK a1 values 20,22–25 range from 13.1 24 to 13.7. 25 Two-step deprotonation was also suggested, 21 however, no pK a2 value was reported. Series of studies have shown that D-mannitol chelates metal ions in bidentante, tetraden- tate and even hexadentate fashion, 26–30 suggesting that deprotonation of multiple D-mannitol OH-groups is possible in strongly basic solution. Since no step-wise deprotonation constants for D-mannitol were known, D-mannitol deprotonation in strongly basic solutions by means of potentiometry and 13 C NMR spectroscopy was investigated in the present study. OH HO OH OH OH OH 1 0008-6215/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carres.2005.03.006 * Corresponding author. Tel.: +370 5 213 1572; fax: +370 5 233 0987; e-mail: ernestas.gaidamauskas@chf.vu.lt Carbohydrate RESEARCH Carbohydrate Research 340 (2005) 1553–1556