Tryptophan Phosphorescence in Proteins

A large number of proteins have now been reported to phosphoresce at room temperature. A recent survey of 40 proteins revealed that about 75 % of them showed lifetimes longer than 1 ms.(ro) 

The phosphorescence lifetimes of various proteins at room temperature are given in Table 3.1. Some variability in the lifetimes reported from lab to lab is evident, possibly due to different enzyme preparation, removal of oxygen (see below), or other conditions. Nevertheless, when measured under the same conditions, it is apparent that the tryptophan lifetimes vary dramatically from protein to protein. Alkaline phosphatase exhibits the longest lifetime from a protein in solution with a lifetime of 1.5—1.7 s at 22°C, approaching the lifetime of 5.5 s at 77 K. The lifetime of free indole in solution is 15—30 /is at 22°C.(38 39) Therefore, in the absence of other quenching mechanisms, the lower limit for the phosphorescence lifetime of a fully exposed tryptophan moiety in a protein should be about 20 /is. 

The range of six orders of magnitude for lifetimes of tryptophan phosphorescence in proteins at room temperature is larger than for fluorescence. The lower limit for fluorescence lifetime is about 0.5 ns, while the upper limit is 8 ns.(21 Typical values range from 3 to 5 ns. 

From an experimental viewpoint, the wide range of phosphorescence lifetimes is advantageous for the study of proteins. It means that it should be 

---

Sdwnerte, I. A.. Sled. D. G., and Gafhi, A., 1997, Time resolved room tenycaatuie tryptophan phosphorescence in proteins. Medh oifrfiiOmoL27t 49-71. 

---