| Abstract
| - We have measured the temperature dependence of the FT-IR spectra of bacteriorhodopsin(bR) as a function of the pH and of the divalent cation regeneration with Ca2+ and Mg2+. It has beenfound that although the irreversible melting transition shows a strong dependence on the pH of the nativebR, the premelting reversible transition at 78−80 °C shows very little variation over the pH range studied.It is further shown that the acid blue bR shows a red-shifted amide I spectrum at physiological temperature,which shows a more typical α-helical frequency component at 1652 cm-1 and could be the reason for theobserved reduction of its melting temperature and lack of an observed premelting transition. Furthermore,the thermal transitions for Ca2+- and Mg2+-regenerated bR (Ca-bR and Mg-bR, respectively) each showa premelting transition at the same 78−80 °C temperature as the native purple membrane, but the irreversiblemelting transition has a slight dependence on the cation identity. The pH dependence of the Ca2+-regeneratedbR is studied, and neither transition varies over the pH range studied. These results are discussed in termsof the cation contribution to the secondary structural stability in bR.
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