Science, Tech, Math › Science Mercapto Group Definition Share Flipboard Email Print The left-hand side of this chemical group (cysteine) is a mercapto group. Todd Helmenstine Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated March 24, 2019 Definition: A mercapto group is a fuctional group containing a sulfur atom bonded to a hydrogen atom. General formula: -SH Also Known As: thiol group, sulfanyl group Examples: The amino acid cysteine contains a mercapto group.