What is described is a compound of formula III or IV wherein R1 is a branched alkyl with 12 to 20 carbons; R2 is a linear alkyl with 5 to 10 carbons or a branched alkyl with 12 to 20 carbons; L1 and L2 are each a bond or a linear alkylene having 1 to 3 carbon atoms, when L3 is a bond; L1 and L2 are each a linear alkylene or alkenylene 5 to 18 carbon atoms, when L3 is an alkylene of 5 to 18 carbons; X is S or O; R3 is of a lower alkyl; and R4 and R5 are the same or different, each a lower alkyl; or a pharmaceutically acceptable salt thereof.