ApH in the Thylakoids with a Spin Labeling Technique

Measurements of ApH in the Thylakoids with a Spin Labeling Technique 

The uptake of TA in response to the light-induced acidification of the thylakoid lumen reveals itself as the increase in the EPR signal from TA molecules in the bulk phase of the intrathylakoid volume (Fig. 5.12, upper curve). This process is inhibited in the presence of the ionophore, i.e., gramicidin D (Fig. 5.12, lower curve). 

The calibration curve (Fig. 5.13) shows that the broadening effect should manifest itself only if TA concentrations in the lumen, [TA]i , becomes higher than the threshold level [TA] = 2.5 mM, i.e., if [TA]i [TA]. Thus, for every fixed value of ApH there should exist a critical concentration of TA in the suspension, [TA]J, below which [TA]i [TA], and thus the 

The results of the measurements of the ApH (external pH 8.0) with the spin label TA under various conditions of chloroplast functioning are summarized in Table 5.1 [70, 82]. 

In the course of noncylic electron transport driven by simultaneously functioning PSl and PS2, the values of ApH in the state of photosynthetic control (without added ADP, state 4) obtained with a spin label technique, ApH 2.S-2.9, are in a good agreement with that determined with other pH-indicating amines [63-67, 83-86]. Similar values of ApH were also ob- 

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