TRANSMEMBRANE HELIX STABILITY - THE EFFECT OF HELIX-HELIX INTERACTIONS STUDIED BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

We have measured, using infrared spectroscopy, the hydrogen/deuterium exchange rates of the amide protons in the photosynthetic antenna of R hodospirillum rubrum. These measurements were made not only on the int act protein in detergent solution but also on two dissociated forms (B 820 and B777). We have, on the basis of our knowledge of the structure of this protein, been able to assign the various groups of amide prot ons that exchange with different time constants to distinct regions of the protein. The most protected group of protons that we observe exch anging with time constants near 6000 min we assign to the transmembran e helices. The slow exchange rates measured for the amide protons of t he transmembrane helices of this protein in detergent solution may ind icate a destabilization of the helices in detergent solution compared with the membrane. This group of protons is progressively destabilized by stepwise dissociation of the antenna protein, and this destabiliza tion is greater than we can account for by increases in solvent access ibility. We suggest that the observed loss of amide proton protection in the transmembrane helices as they are dissociated might be due to a n increase in the helix flexibility and breathing motions as interacti ons between helices are reduced.