Spectral changes of the B800-850 antenna complex from Ectothiorhodospira sp. induced by detergent and salt treatment.

We have studied the effects of the detergent lauryl dimethylamine N-oxide and NaCl in the near infrared absorption spectra of the B800-850 antenna complex from Ectothiorhodospira sp. Strong spectral changes were induced on the BChl850 band by the lauryl dimethylamine N-oxide consisting of a blue shift, from 857 to 839-837 nm, and a hypochromism. No significant effects were detected on the BChl800 band in the same conditions. The changes were reversible after removing most of the detergent from the sample. Depending upon the detergent concentration in the solution, NaCl was also able to reverse the blueshift and increase the intensity of the 850 nm band close to the native values. Moreover, we have been able to separate both phenomena. Addition of 0.350 M NaCl after sample incubation with 0.15% (v/v) lauryl dimethylamine N-oxide for 30 min allowed a 9-10 nm redshift with no significant hyperchromism of the lowest energy band. We explained the overall effect of the detergent assuming that the lauryl dimethylamine N-oxide bound to the hydrophobic moiety of the complex and caused some protein conformational changes which affected the BChl850 domain without affecting that of the BChl800. The NaCl was able to circumvent these effects, most probably by acting directly on the BChl850 molecules or on the protein structure surrounding them.