ABSTRACT
As described in detail in Chapter 2, only three amino acids are uorescent in the range where conventional uorescence instrumentation can be utilized: phenylalanine, tyrosine, and Trp (Table 7.1). Trp is the least abundant of all uorescent amino acids: the occurrence frequency of phenylalanine, tyrosine, and Trp is 3.6:3:1, respectively (Brooks et al. 2002). Yet, Trp is the most useful for imaging because of its attractive spectral features-with excitation and emission maxima at 280 and 350 nm, respectively, it has relatively high molar absorptivity (5500 M−1 cm−1) and modest quantum yield (Φ) of 0.13 in the 300-400 nm range (Chen 1967). Excited-state uorescence of Trp decays biexponentially with an average lifetime of 3.03 ns (Figure 7.1). e uorescence of phenylalanine over the same spectral range suffers from lower Φ(0.024) and molar absorptivity (150 M−1 cm−1) (Chen 1967).
