Fluorescence of Ce 3+ aqua ion is shown to be much more intense than that of other rare earth ions

and the fluorescence intensity is in good proportional to its concentration which may be changed in rather wide range The properties of Ce 3+ aqua ion may be employed to determine its concentration As tRNA strongly quench fluorescence of Ce 3+

fluorescence titration was applied to Ce 3+ tRNA systems

from which the binding number and association constants of the system were deduced with Scatchard plot It was found that there are two classes of Ce 3+ binding site in tRNA with the association constant of 10 8 mol/L and 10 6 mol/L

respectively

and the binding number and association constants were slightly reduced by addition of spermine.