Trimethylamine N-oxide

Trimethylamine N-oxide (TMAO or TMANO) is a naturally occurring osmolyte that occurs in saltwater fish, sharks and rays, molluscs, and crustaceans. They use this osmolyte, along with free amino acids, to reduce the 3% saltiness of seawater to the desirable 1% of dissolved solids inside cells. TMAO decomposes to trimethylamine which is the main odor molecule giving rise to the characteristic fishy smell.

TMAO is used in protein folding experiments to counteract the unfolding effects of urea.