n. WEBER S?> 



Aromatic Amino Acids 

 riic al)s()ii)ti()n specLiuni ol each ot the aromatic amino acids 

 presents in the region ol 200-3()() ny tAvo absoriHion bands, a weak 

 one at a h)nger wavelength and a stronger one at a shorter wavelength. 

 These appear to correspond respectively to the two types of Piatt's 

 transitions: L (nuiltipolar, weak) and B (dipolar, strong) . The fluores- 

 cence of each compound consists in each case (23) ot a single inire- 

 solved synnnetric band which in water solution has a maximum at 282 

 m/x in phenylalanine, 303 m^^ in tyrosine, and 348 ni/x in tryptophan. 

 Entirely similar bands are observed in the parent compounds benzene, 

 phenol, and indole. The variations of the fluorescence spectrum 

 with solvent composition are best studied with the parent compounds 

 because of the insolubility of the dipolar molecules in non-polar 

 media. In water, none of the spectra show any sign of vibrational 

 structure; and this is true also of phenol and indole even when in 

 hexane solution, in ^vhich the vibrational structure of the indole 

 absorption spectrum is clearly discernible. While phenol shows vir- 

 tually no change in the position of the fluorescence maximimi in the 

 different solvents, the fluorescence maximum of indole undergoes 

 marked changes, appearing at 325 m^ in hexane and 340 mjx in 

 methanol and other alcohols. These variations are due to dipole 

 interaction between the solvent molecules and the excited molecule, 

 and can be used with the help of a theory of Lippert (11) to deter- 

 mine the increase in dipole moment in the excited state over that 

 of the groimd state. The spectral shift is found to be dependent upon 

 the dipole strength and upon the quantity 



^ 2i) + 1 2«2 + 1 



where D is the dielectric constant and ti the refractive index of the 

 medium, a quantity that determines the interaction energy be- 

 tween the excited molecular dipole and the induced or permanent 

 dipoles of the solvent molecules, according to the theory of Onsager 

 (14) . Conversely, if the dipole strength in the excited state is known, 

 by the determination of the fluorescence spectrum in a series of sol- 

 vents the A/ value of an unknown mediimi can be determined. Thus 

 in a protein the polarizability jiroperties of the region surrounding the 

 residues may be estimated from the fluorescent spectrum, and the same 

 considerations apply to groups either chemically attached or physically 

 adsorbed (e.g., Weber, 27) . The quantum yield of the aromatic 

 amino acids in water solution is 0.04 for phenylalanine, 0.21 for 



