An Assessment of Phase Transitions in Soybean Membranes

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Abstract
Phase transitions were measured in vesicles of phospholipids, alone and in various combinations, and in pelleted mitochondrial membranes, using thermal (DSC [differential scanning calorimetry]) and optical methods to consider their possible involvement in chilling injury of soybeans (G. max [L.] Merr. cv. Wayne 1977). Saturated phospholipids showed clear transitions in the temperature range of 50.degree. C to near 0.degree. C. When mixtures of 2 phospholipids were examined, there was a marked lowering and broadening of the transition peaks and a shift in the transition temperatures to intermediate temperatures. The unsaturated phospholipids that occur naturally in soybeans showed no detectable phase transitions in this temperature range, alone or in combinations. Examination of the polar lipids from soybean asolectin revealed no transitions in the biological temperature range: the additions of cations such as Ca2+ and La3+ did not evoke a detectable phase transition in them. Mitochondrial membrane pellets showed no transitions. The application of these 2 direct methods of examination of membrane components without the addition of foreign agents did not support the suggested occurrence of a bulk phase transition which could be related to chilling injury in soybeans.