VISCOSITY AND REACTION RATES IN BACTERIA 365 



In Fig. 6 are light- induced difference spectra for K, rubrum suspen- 

 sions in the presence and absence of 67 per cent glycerol. It is seen 

 that there is a definite shift in the peak from 428 to 422 m/i. This is 

 attributed to the observed decrease of the rate of oxidation of cyto- 

 chrome b (or o-rhp) (13) measured at 428 ran and to the increase in 

 the rateof oxidation of cytochrome of type c in the presence of glycerol. 

 The shift of the band position observed in R. rubnim makes it very 

 unlikely that the effects of glycerol upon the reaction kinetics are an 

 optical artifact. 



Effects of glycerol upon absorption spectra. 



Suspension of R. rubnim in 67 per cent glycerol causes a 9 per 

 cent decrease of absorbancy in the prominent 549 m/i carotenoid band 

 but no shifts of the peaks. In the region 750-900 nxn there is a decrease 

 in absorbancy in Chromatiurn and R. rubrum of 34 per cent at 800 m)Lt 

 and 35 per cent at 890 m/i , respectively. This decrease in absorbancy 

 would be expected to decrease the rates of the light-induced reactions. 

 The data of Fig. 3 may underestimate the increase of rate that actually 

 occurs. 



DISCUSSION 



Mechanism of the electron transfer. 



The lack of inhibition by increased viscosity of the light-induced 

 oxidation of cytochrome of type c in Chromatiurn confirms the lack of 

 temperature dependence of this reaction (5) and supports the idea that 

 electron transfer between cytochrome of type c and bacterial chloro- 

 phyll is a collision-independent event. In R.rubrum simildiV responses 

 to glycerol indicate a similar reaction between cytochrome and bac- 

 terial chlorophyll. 



In confirmation of the temperature dependence of the light-off re- 

 action (5), the light-off reaction is highly sensitive to effects of 

 glycerol. We believe that glycerol increases the viscosity at the site 

 of action of the enzymes transporting reducing equivalents to cyto- 

 chromes of type c and interferes with collision mechanisms involved 

 in electron transport. 



Relation to the quantum requirement for cytochrome oxidation. 



The apparent increase in the velocity of the light-induced oxidation 

 of cytochrome of type c in Chromatiurn in the presence of glycerol 

 suggests that an even higher quantum efficiency of the reaction might 

 be obtainable under these circumstances and represents a factor that 

 is worthy of consideration not only in purple bacteria (4,14,15) but also 

 in green cells. If we accept, without further consideration of possible 

 experimental artifacts, the twofold increase of initial velocity illus- 



