The present invention provides lipids that are advantageously used in lipid particles for the in vivo delivery of therapeutic agents to cells. In particular, the invention provides lipids having the following structure (I) wherein R 1 and R 2 are each independently for each occurrence optionally substituted C 10 -C 30 alkyl, optionally substituted C 10 -C 30 alkenyl, optionally substituted C 10 -C 30 alkynyl, optionally substituted C 10 -C 30 acyl, or -linker-ligand； R 3 is H, optionally substituted C 1 -C 10 alkyl, optionally substituted C 2 -C 10 alkenyl, optionally substituted C 2 -C 10 alkynyl, alkylhetrocycle, alkylphosphate, alkylphosphorothioate, alkylphosphorodithioate, alkylphosphonates, alkylamines, hydroxyalkyls, É-aminoalkyls, É-(substituted)aminoalkyls, É-phosphoalkyls, É-thiophosphoalkyls, optionally substituted polyethylene glycol (PEG, mw 100-40K), optionally substituted mPEG (mw 120-40K), heteroaryl, heterocycle, or linker-ligand； E is O, S, N(Q), C(O), N(Q)C(O), C(O)N(Q), (Q)N(CO)O, O(CO)N(Q), S(O), NS(O)2N(Q), S(O)2, N(Q)S(O)2, SS, O=N, aryl, heteroaryl, cyclic or heterocycle； and, Q is H, alkyl, É-aminoalkyl, É-(substituted)aminoalky, É-phosphoalkyl or É-thiophosphoalkyl.
