A 4-nitrophenyl α-glycoside of isomaltose (Glcα1-6Glc), an α1-6-linked glucose disaccharide. Used as a chromogenic substrate for α-glucosidases and isomaltases with α1-6 linkage selectivity (e.g., sucrase-isomaltase, oligo-1,6-glucosidase, and other debranching α-glucosidases); enzymatic cleavage of the aglycone-proximal α-glucosidic bond releases 4-nitrophenol. 4-Nitrophenyl (pNP) glycosides are chromogenic enzyme substrates: cleavage of the glycosidic bond by the target enzyme releases free 4-nitrophenol, which is yellow under basic conditions (absorbance maximum ~405 nm), enabling straightforward spectrophotometric assays of enzyme activity in solution and in high-throughput screening formats.