Figure 5.12 Phosphoserine.

According to the diff 1-2 rule, we should have expected to see the pKa 4.13 (carboxylate) and 8.75 (amine), but the liposome titration shows something else. Instead, we have an ''anomalous'' additional shift of +1.37 for the carboxylic group and a +2.75 for the amine group. These extra shifts are due to the negatively charged surface of the liposomes! We can estimate, using Eq. (5.5), that when the carboxylic group is titrated in the PS liposome, the surface charge is — 81 mV (pH 5.5), and when the amine group is titrated, the surface charge drops to —163 mV (pH 11.5). Conversely, if we had a way of estimating surface charge,

TABLE 5.3 Critically Selected Experimental Liposome-Water Partition Coefficients


ta (°C)



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