A lipid represented by the formula:

wherein each of R¹ and R² is an alkyl or alkenyl chain having between about 8 to about 24 carbon atoms; n=0-20; L is selected from the group consisting of (i) —X—(CO)—Y—, (ii) —X—(CO)—, and (iii) —X—CH₂—, wherein X and Y are independently selected from oxygen, NH, and a direct bond; and Z is a weakly basic moiety that has a pK of less than about 7.4 and greater than about 4.0 is described.