Selection of reporter group

Intrinsic reporter groups are already present in an essential component of the reaction under study. They may be located within the protein, a prosthetic group or cofactor, such as flavins (3), NADH (2), pyridoxal-5 -phosphate (4). or a substrate or other ligand (5). 

The indole group of tryptophan (Trp) has a higher molar extinction coefficient than the phenolic and phenyl side chains of tyrosine (Tyr) and phenylalanine (Phe), respectively (see Table 1, Chapter 12). Thus although its quantum yield is similar to that of lyr, its fluorescence emission is much mote intense. Furthermore, its excitation spectrum overlaps the emission spectrum of Tyr and therefore fluorescence resonance energy transfer (FRET, see Chapters 2 and 3) from lyr to Trp occurs readily when both residues are in close proximity (i.e. located in the same protein molecule) and favourably orientated. The intrinsic fluorescence of a protein is therefore dominated by the contribution from Trp 

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