Question #45005

A non-reducing trisaccharide $\mathbf{X}$ ($C_{18}H_{32}O_{16}$) is subjected to the following enzymatic and chemical analyses at 298 K and pH 7.4:

1. Complete exhaustive methylation of $\mathbf{X}$ followed by acid hydrolysis yields equimolar amounts of:

   • 2,3,4,6-tetra-$O$-methyl-D-galactose
   • 2,3,4,6-tetra-$O$-methyl-D-glucose
   • 2,3,6-tri-$O$-methyl-D-glucose

2. Hydrolysis of $\mathbf{X}$ with lactase ($\beta$-galactosidase) yields D-galactose and a disaccharide $\mathbf{Y}$. $\mathbf{Y}$ does not reduce Tollens' reagent and is completely resistant to maltase ($\alpha$-glucosidase).

3. Hydrolysis of $\mathbf{X}$ with emulsin ($\beta$-glucosidase) yields D-glucose and a disaccharide $\mathbf{Z}$. $\mathbf{Z}$ reduces Tollens' reagent and undergoes mutarotation.

Based on these observations, identify the correct IUPAC structure of the trisaccharide $\mathbf{X}$.