Disclosed are lipids that are advantageously used in lipid particles for the in vivo delivery of therapeutic agents to cells. In particular, is provided lipids having the following structure XXXIII 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, alky lhetro cycle, alkylphosphate, alkylphosphorothioate, alkylphosphorodithioate, alkylphosphonates, alkylamines, hydroxyalkyls, &omega-aminoalkyls, &omega-(substituted)aminoalkyls, &omega -phosphoalkyls, &omega -thiophosphoalkyls, optionally substituted polyethylene glycol (PEG, mw 100-40K), optionally substituted mPEG (mw 120-40K), heteroaryl, heterocycle, or linker-ligand and E is C(O)O or OC(O).
