US Patent 6974589 Neutral-cationic lipid for nucleic acid and drug delivery

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—(C═O)—Y—, (ii) —X—(C═O)—, 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.