supplementary materials

Stevia rebaudiana Bertoni (Asteraceae), first identified in 1905 by the botanist Bertoni, is a perennial bush indigenous to the mountainous regions of Paraguay and Brazil, and is well known to produce a mixture of high-potency sweet compounds that have been in the market since the 1970's as low-calorie sweeteners (Bertoni, 1905; Kinghorn, 2002). Stevioside is one of these low-calorie sweeteners. The native cultures of Paraguay and Brazil have safely used stevia for sweetening for centuries. The Joint FAO/WHO Expert Committee on Food Additives has established a monograph for Steviol Glycosides (JECFA, 2010). The United States Food and Drug Administration granted regulatory acceptance of Rebaudioside A (FDA, 2008) and mixtures of steviol glycosides (FDA, 2010) followed by the European Union approved steviol glycosides for marketing (OJ, 2011). The world demand for Stevia leaves is expected to exceed 6–8 million metric tonnes in the next 10 years. Stevioside and Rebaudioside A, also knownas Rebiana, are the major glycoside sweeteners from S. rebaudiana which have been commercialized for human consumption throughout the world (Prakash et al. 2008; Wölwer-Rieck, 2012).

Stevioside has several different polymorph forms (Prakash & Upreti, 2011). These polymorph forms include methanol or ethanol solvate, hydrate and amorphous. These different polymorphic forms would affect the physical properties like melting point, solubility and density. The powder X-ray patterns of these forms have been documented; however, no single-crystal results have been reported so far. Here we report the structure of stevioside methanol solvate.

The asymmetric unit contains one stevioside and four methanol molecules (Fig. 1). Stevioside has an aglycone portion and a sugar portion. The aglycone is a tetracyclic diterpene in which the four fused ring system consists of three six membered rings and one five membered ring. Rings A, B and C form chair conformations with the puckering parameter (Cremer & Pople, 1975) Q = 0.550 (2)Å, θ = 175.6 (2)° and φ= 255 (3)°; Q = 0.5753 (17)Å, θ = 166.72 (17)° and φ= 191.3 (8)°; Q = 0.6355 (17)Å, θ = 24.09 (16)° and φ= 281.3 (4)°, respectively. The five-membered ring D forms an envelope conformation with the puckering parameter Q = 0.4791 (19)Å, φ= 24.6 (2)°. These ring systems form the hydrophobic part of the molecule. The three sugars are all β-D glucopyranoses (E, F, G) and they all form chair conformations with the puckering parameters Q = 0.5660 (17)Å, θ = 9.94 (16)°, φ = 4.3 (10)°, Q = 0.6026 (17)Å, θ = 3.01 (16)°, φ= 72 (3)° and Q = 0.5861 (19) Å, θ =1.56 (18)°, φ= 358 (7)°, respectively. One sugar is attached to the carboxylic acid functional group at C4 while the other two sugars are linked as 2ρightarrow1 and attached to C13. The sugar attached to the carboxylic group at C4 has been shown to be linked on an axial position instead of an equatorial position.

Stevioside and four methanol molecules were stablized by an extensive hydrogen bonding network and a weak O4—H4O···O15 intramolecular interaction (Table 1). These hydrogen bonds and weak interaction further link two adjacent asymmetric units to form a hydrophilic layer. The aglycon part of the stevioside forms a hydrophobic nucleus. This bilayer system acts like an oil in water system (Fig. 2). This is believed to increase the solubility of stevioside.