Monosaccharide isomerism defines structural and spatial variations within carbohydrates characterized by identical molecular formulas ($C_nH_{2n}O_n$) but distinct atomic arrangements or configurations. The theory classifies these variants into two primary categories: structural isomers, which differ in functional group connectivity (e.g., aldose versus ketose), and stereoisomers, which possess the same bonding sequence but vary in spatial orientation around asymmetric chiral centers. In carbohydrate chemistry specifically, this classification governs the biological specificity of enzymes regarding D- vs L-enantiomers, optical rotation properties (dextrorotatory/levorotatory), and epimeric relationships defined by configuration changes at specific carbon atoms excluding the anomeric center.
Monosaccharide stereochemistry is organized by the D/L system, in which the configuration of the highest-numbered (penultimate) chiral center alone assigns a sugar to the D or L series; this is a pur…