BENT LEY GLASS 831 



For example, methylene blue in the triplet state oxidizes ferrous to 

 ferric, but leuco methylene blue in the ground state is itself oxidized 

 by ferric iron. Acitlity constants of such molecules as naphthols, 

 naphthylamines, and acridine may differ, in the first excited singlet 

 state, by as nmch as 6 pK units from those of the ground state; but 

 surprisingly the trij)let states are not very different from the ground 

 state in this respect. In general, the distinctive physical structure 

 and chemical properties of the triplet state make it essential to meas- 

 ure them directly instead of trying to make predictions from the 

 corresponding properties of the ground state. 



Excited States of Molecules of Biological Interest 



Gregorio Weber initiated the discussion of the excited states of 

 proteins, states that result from the absorption of radiation and are 

 potentially capable of emitting it. Both electronic and vibrational 

 excited states occur, but little is known of the latter. The former 

 include both singlet and metastable states, the former with lifetimes 

 of millimicroseconds, the latter with much longer lifetimes of milli- 

 seconds to seconds. The singlet states are best understood. Those 

 states arising from absorption of light of wavelength 200 ni/x or longer 

 occur in three chemical species: aromatic amino acids; certain groups 

 occurring within such molecules as heme, FMN, FAD, and caro- 

 tenoids, or bound irreversibly or reversibly to proteins, as in DPNH; 

 and certain groups artificially attached to proteins, as in various con- 

 jugates such as fluorescein conjugates. 



Aromatic amino acids have two absorption bands in the 200-300 m^i 

 region of the spectrum. The weaker band, at the longer wavelength, 

 appears to represent a multipolar type of transition; the stronger 

 band, at the shorter wavelength, a dipolar transition. When dis- 

 solved in water, each aromatic amino acid exhibits a single fluores- 

 cence emission band, at 282 rufi in the case of phenylalanine, at 303 

 m^ in the case of tyrosine, and at 348 m^i, in the case of tryptophan. 

 These correspond to the fluorescence bands of benzene, phenol, and 

 indole, respectively; and actual study has been mostly of the parent 

 compounds. In different solvents the fluorescence maximum of phenol 

 remains virtually unchanged; but that of indole shifts to 325 m^ in 

 hexane and to 340 ni/x in the alcohols. This shift is due to dipole 

 interaction between solvent and excited solute molecules, and the 

 extent of the shift depends on the dipole strength and on a quantity 



