THERMAL-STABILITY OF LIPID-DEPLETED PURPLE MEMBRANES AT NEUTRAL AND LOW PH VALUES

Differential scanning calorimetry was used to compare the thermal beha vior of native and delipidated purple membrane fragments at pH values corresponding to purple, blue and acid-purple forms. At neutral pH, de lipidation results in a 2.5- to 3-times increase in the cooperativity of the denaturational transition, accompanied by a minor increase in i ts temperature. At pH values below 5 the delipidated membranes exhibit considerably higher thermal stability than the native membranes. The reversible predenaturational transition observed in the native state i s not detectable upon delipidation. There is no strict correlation bet ween color changes upon acidification and deionization of either nativ e or delipidated purple membranes and their thermal stability.