Determination of the number of detergent molecules associated with the reaction center protein isolated from the photosynthetic bacterium Rhodopseudomonas viridis

Abstract

Detergent‐free reaction center (RC) proteins from the photosynthetic bacterium Rhodopseudomonas viridis were obtained using Bio‐Beads SM‐2. With these RCs, the amount of detergent molecules associated with the protein was measured by determining the detergent concentration at which re‐solubilization occurred as a function of the RC concentration. For N,N dimethyl dodecylamine‐N‐oxide (LDAO), Triton X‐100 and β‐octylglucoside 260 ± 30,105 ± 10 and 360 ± 100 detergent molecules were necessary to dissolve the protein, respectively. With this technique we have studied the effect of the amphiphilic molecule 1,2,3‐heptanetriol, which is essential in the crystallization process of these RCs. Addition of 5% 1,2,3‐heptanetriol reduces the value for LDAO to 120 ± 20 LDAO/RC, supporting the notion that crystallization of the RCs is promoted by increasing the number of protein‐protein contacts.