Final answer:
D-glucose and D-fructose differ most in three-dimensional structure due to one being an aldose and the other a ketose, which affects the placement of the carbonyl group and overall molecular shape.
Step-by-step explanation:
The stereochemistry of monosaccharides greatly influences their three-dimensional structure. When comparing D-glucose, D-galactose, and D-fructose, all of which have the formula C6H12O6, their structures differ due to different arrangements of atoms and stereoisomers known as anomers.
These differences are particularly pronounced between D-glucose and D-fructose because fructose is a ketose sugar with the carbonyl group (C=O) on the second carbon (ketone group), while glucose is an aldose sugar with the carbonyl group on the first carbon (aldehyde group).
D-galactose and D-glucose share a very similar structure except for the arrangement at the fourth carbon atom; galactose is a C4-epimer of glucose. This difference, however, is not as drastic in the three-dimensional shape as the difference between glucose and fructose.
Overall, when comparing glucose, galactose, and fructose, it is the structural difference between aldose and ketose forms that result in the most significant change in three-dimensional shape, making D-fructose and D-glucose the two monosaccharides that differ most in three-dimensional structure.