SIDNEY SHI1'RL\ AND NATHAN O. KAPLAN 



147 



0.9 



A S-DPN 

 B S-DPNH 



300 



340 



380 



420 



WAVELENGTH (m/x) 



Fig. 2. Comparison of the absorplion spectra ot oxidized and reduced tliio- 

 nicotinamide Dl'N. Curve A. absorption spectrum of 9.3 X •*'"' '^^ thionicotina- 

 mide-DPX) (S-DPX) in 0.1 M pfiosphate buffer j)H 9.2. Curve B, absorption spec- 

 trum of 9.3 X If*"" ^^^ thionicotinamide DI'NH (S-DFNH) obtained by enzymatic 

 reduction of the oxidized analogue, using 1 X •0"'' ^^^ •i"''*" 'i^'-'' alcohol dehy- 

 drogenase and 2 X 1^'"° '^^ ethanol in O.I M phosphate buflcr, /;H 9.2. 



Although the thionicotinamide analogue of DPNH is very weakly 

 fluorescent and thereby does not permit studies of intramolecular 

 energy transfer, spectrophotometric analysis has allowed a determina- 

 tion of the interacting groups within the molecule. A comparison 

 of the absorption spectrum of the thionicotinamide analogue of 

 DPNH in water and propylene glycol is shown in Fig. 5. It can 

 readily be observed that the inflection at 295 ni/x in water has been 

 shifted to .HOO mfx and is more jjrominent in propylene glycol. 



From a study of the jjosition of the absorption maximum of 

 thionicotinamide as a function of the dielectric constant of the 

 solvent, it appears that the 300 ni/x band results from an n -> 77* 

 transition, as suggested by Kasha (]?>). A schematic diagram of the 



