The present invention provides a compound comprising the structure shown in Formula I for use in delivering a therapeutic nucleic acid, one or more drug agents, or a biologically active agent to a cell, €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ R 3 -(C=O)-Xaa-Z-R 4 €ƒ€ƒ€ƒ€ƒ€ƒFormula I wherein Xaa is histidine, 1-methylhistidine, pyridylalanine, lysine, arginine, homoarginine, nor-norarginine, ornithine, homolysine, asparagine, N ethylasparagine, glutamine, 4-aminophenylalanine, and nor-arginine, or an N-methylated version thereof； R 3 is independently a lipophilic tail derived from a naturally-occurring or synthetic lipid, phospholipid, glycolipid, triacylglycerol, glycerophospholipid, sphingolipid, ceramide, sphingomyelin, cerebroside, or ganglioside, wherein the tail may contain a steroid； or a substituted or unsubstituted C(3-22)alkyl, C(6-12)cycloalkyl, C(6-12)cycloalkyl-C(3-22)alkyl, C(3-22)alkenyl, C(3-22)alkynyl, C(3-22)alkoxy, or C(6-12)alkoxy-C(3-22)alkyl； R 4 is independently a lipophilic tail derived from a naturally-occurring or synthetic lipid, phospholipid, glycolipid, triacylglycerol, glycerophospholipid, sphingolipid, ceramide, sphingomyelin, cerebroside, or ganglioside, wherein the tail may contain a steroid； or a substituted or unsubstituted C(3-22)alkyl, C(6-12)cycloalkyl, C(6-12)cycloalkyl-C(3-22)alkyl, C(3-22)alkenyl, C(3-22)alkynyl, C(3-22)alkoxy, or C(6-12)alkoxy-C(3-22)alkyl； wherein both R 3 and R 4 are lipophilic tails； and Z is NH, O, S, -CH 2 S-, -CH 2 S(O)-, or an organic linker consisting of 1-40 atoms selected from hydrogen, carbon, oxygen, nitrogen, and sulfur atoms； and salts thereof.
