What is described is a compound having the formula wherein R1 is branched alkyl of the structure (CH3(CH2)m)2CH—, wherein m is 2 or 3; R2 is a linear alkyl of 1, 2, 3, 4, 5, 6, 7, 8, or 9 carbons, a branched alkyl of 3, 4, 5, 6, 7, 8, or 9 carbons, or an alkenyl or alkynyl of 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11 carbons; R3 is —(CH2)p—, wherein p is 2, 3, 4, 5 or 6; R4 and R5 are the same or different, each a hydrogen, or a linear or branched alkyl of 1, 2, 3, 4, 5 or 6 carbons; L1 and L2 are the same or different, of the structure —(CH2)n—, wherein n is 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, or 18; L3 is a bond; X1 is —CO—O— whereby -L2-CO—O—R2 is formed; X2 is S or O; and X3 is —CO—O— whereby -L1-CO—O—R1 is formed; and wherein m+n+p is 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, or 27; or a pharmaceutically acceptable salt thereof.