The phosphorescence properties of tryptophan (Trp), tyrosine (Tyr), pheny-lalanine (Phe), proline (Pro), and histidine (His) were investigated. The phosphorescence of Trp is the strongest (high quantum yield). Tyr is the next (about 1/10 of Trp) and Phe, Pro, His are weak (about 1/100 of Trp). The spectra of Tyr and Phe are short (λex 284 and 276, λem 390 and 386 nm respectively), but Trp, Pro, His are long (λex 290, 308 and 320 nm, respectively). The phosphorescence lifetime of Phe, Trp, are the longest (about 7 s), Tyr, Pro and His are short (2.84,1.31 and 0.49 s respectively). The phosphorescence spectra of amino acids in different alcohol (methanol, ethanol, n-propanol, n-butanol) are changed little, but the lifetime becomes shorter as the polarity of alcohol decreasing. The effect of pH on the spectra of amino acids were studied. The stokes energy loss and the excited pK*a were also measured.
