Mixed isomers of carboxytetramethylrhodamine free acid. Tetramethylrhodamine (TMR) is one of the fluorophores most often used for preparing peptide, protein, nucleotide and nucleic acid conjugates. TMR dyes have been widely used as acceptors for FAM fluorophores in a variety of FRET studies. The dye can be coupled to primary or secondary amines via standard peptide chemistry.