Lipids and compositions for the delivery of therapeutics

A lipid of formula I

• or a salt or isomer thereof, wherein
• R1 and R2 are each independently for each occurrence optionally substituted C10-C20 alkyl, optionally substituted C10-C20 alkenyl, or optionally substituted C10-C20 alkynyl;
• R3 is independently for each occurrence H, optionally substituted C1-C10 alkyl, optionally substituted alkylheterocycle, optionally substituted heterocyclealkyl, optionally substituted amino, optionally substituted alkylamino, optionally substituted di(alkyl)amino, optionally substituted aminoalkyl, optionally substituted alkylaminoalkyl, or optionally substituted di(alkyl)aminoalkyl;
• X and Y are each independently —O— or alkylene;
• A1 and A2 are each independently —O— or —CH2—;
• Z is N, or C(R3);
• and
• m and n are each independently 0 to 5, where m and n taken together result in a 5, 6, or 7 member ring.