The effect of the detergent cetyltrimethylammoniumchloride on the phase behaviour of dipalmitoylphosphatidylcholine: A high precision calorimetric study.

The influence of the linear, positively charged detergent cetyltrimethylammoniumchloride (CTAC1) on the well known phase behaviour of fully hydrated dipalmitoylphosphatidylcholine (DPPC) has been studied along fourteen isopleths of the phase diagram using high sensitivity scanning microcalorimetry. Particular interest was focused towards the effects of very small amounts of detergent down to the region of 1 CTAC1 per 10⁴ DPPC-molecules.Even starting with such low concentrations distinct changes of the thermograms are detectable by high sensitivity DSC. Essentially, the phase diagram is cut into four domains between the two limiting cases of pure phospholipid multilamellar dispersion and isotropic solution of detergent micelles, respectively. At the present stage these four regions in the phase diagram are identified as follows : 1. 0–1 Mol % CTAC1: a modified lamellar phase 2. 1–50 Mol % CTAC1: a two-phase region with this modified lamellar phase coexisting with a newly formed detergent-rich phase. 3. 50–90 Mol % CTAC1: large mixed micelles. 4. > 90 Mol % CTAC1: an isotropic solution of small mixed phospholipid/detergent micelles.