2-Nonyl-3-hydroxy-4-quinolone-d 4 (C9-PQS-d 4 ) is deuterium labeled 2-Nonyl-3-hydroxy-4-quinolone. 2-nonyl-3-hydroxy-4-Quinolone (C9-PQS) is a quinolone compound produced by P. aeruginosa and other related bacterias. 2-nonyl-3-hydroxy-4-Quinolone is a quorum sensing (QS) signal molecule that controls the expression of many virulence genes as a function of cell population density.