Tryptophan

Faurobert E, Otto-Bruc A, Chardin P, Chabre M - "Tryptophan W207 in transducin T alpha is the fluorescence sensor of the G protein activation switch and is involved in the effector binding" EMBO J 12(11):4191-8 (1993)
http://www.ncbi.nlm.nih.gov/htbin-post/Entrez/query?uid=8223434&form=6&db=m&Dopt=r