Partition of amphiphilic molecules into phospholipid vesicles and human erythrocyte ghosts: measurements by ultraviolet difference spectroscopy

Abstract

Molar partition coefficients for chlorpromazine and methochlorpromazine between phospholipid vesicles or human erythrocyte ghosts and buffer were determined by UV difference spectroscopy. The partition coefficients between small unilamellar egg phosphatidylcholine vesicles and buffer at pH 7.4 were 4.4 .times. 105 for chlorpromazine and 0.8 .times. 105 for methochlorpromazine, determined with 10 .mu.M amphiphile. An increase in the partition of chlorpromazine into vesicles is seen as the pH is increased to the pKa of chlorpromazine at 9.2. Chlorpromazine also partitions preferentially into fluid-phase phospholipid compared to solid-phase phospholipid. Molar partition coefficients between unsealed human erythrocyte ghosts and buffer at pH 8.0 with 10 .mu.M amphiphile were determined to be 6.5 .times. 105 for chlorpromazine and 2.5 .times. 105 for methochlorpromazine. Difference spectroscopy is an equilibrium technique that does not require separation of bound from free amphiphile, as do many other methods of determining membrane-buffer partition coefficients. This method is useful for any amphiphile that has an appreciable absorbance below its critical micelle concentration and whose absorbance is sensitive to environment.