In this Letter we explore the relationship between the radiative constant, k(r), and the lineshape of an emission spectrum for fluorescent organic molecules, in particular tryptophan, We make a number of judicious approximations to the expression given by Strickler and Berg in conjunction with the log-normal function for the lineshape. Our result is a closed form expression for k(r) in terms of the inverse maximum wavelength lambda(max)(-1) which compares well with recent experimental findings for tryptophan residues in proteins.