84 



One of the parameters of excitation which may have a major 

 biological importance is lifetime. The lifetime of rhodamin is 

 short, of the order of 10"--10~" seconds. A greater number of sub- 

 stances have been tested for their ability to prolong this lifetime, 

 with negative results. Only two substances were found to be ca- 

 pable of extending it at a low concentration and extending it 

 strongly into the dimension of seconds: these are glutathione and 

 pyridoxine. Solutions of rhodamin containing 0.01 AI glutathione 

 showed a strong red phosphorescence in the slow moving phos- 

 phoroscope and showed an afterglow if illumination was suddenly 

 disconnected. In 10 ^ M concentration the effect was but somewhat 

 weaker. Oo had only a moderate quenching effect so that it can be 

 expected not to interfere with this long life excitation at the low 

 Oo concentrations of tissues. 



That the lengthening of lifetime was actually due to the SH 

 group could be shown by the fact that glutathione lost activity on 

 oxidation of this group and that the same effect was produced by 

 cysteine but not by cystine. There was only one difference between 

 the action of cysteine and glutathione: while the former acted in 

 acid solution only (dissolved as cystine HCl) and was inactive 

 at neutral reaction, glutathione show^ed the opposite pH de- 

 pendence, being active at neutral but not at acid reaction. Cysteine 

 ethyl ester was found to be inactive indicating that the action of 

 the SH group, somehow, depends on the structure of the whole 

 molecule involving the COOH group and its dissociation. Pos- 

 sibly nature has built the cysteine into a tripeptide and produced 

 glutathione to make the substance more soluble and to enable the 

 SH to do its job at the physiological neutral reaction. 



The lifetime of triplet excitations varies within wide limits. 

 Rhodamin has a short one, quinidine and acridine orange a long 

 one. The longer the lifetime originally, the easier it is prolonged 

 into the 0.1 or 1 second dimension. Accordingly it was found that 

 while this effect could be achieved in rhodamin with three sub- 

 stances only, a greater number of kno\\n or unknown cell con- 



