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 wherein: R1 and R2 are each independently for each occurrence optionally substituted C10-C30 alkyl, optionally substituted C10-C30 alkenyl, optionally substituted C10-C30 alkynyl, optionally substituted C10-C30 acyl, or -linker-ligand; R3 is H, optionally substituted C1-C10 alkyl, optionally substituted C2-C10 alkenyl, optionally substituted C2-C10 alkynyl, alkylhetrocycle, alkylphosphate, alkylphosphorothioate, alkylphosphorodithioate, alkylphosphonates, alkylamines, hydroxyalkyls, ω-aminoalkyls, ω-(substituted)aminoalkyls, ω-phosphoalkyls, ω-thiophosphoalkyls, optionally substituted polyethylene glycol (PEG, mw 100-40 K), optionally substituted mPEG (mw 120-40 K), heteroaryl, heterocycle, or linker-ligand; and E is C(O)O or OC(O).