Lipids and compositions thereof

A lipid represented by Formula I:

• or a pharmaceutically acceptable salt thereof, wherein:
• R′ is absent, hydrogen, or C1-C6 alkyl;
• provided that when R′ is hydrogen or C1-C6 alkyl, the nitrogen atom to which R′, R1, and R2 are all attached is positively charged;
• R1 and R2 are each independently hydrogen, C1-C6 alkyl, or C2-C6 alkenyl;
• R3 is selected from the group consisting of C12 alkylene, C11 alkylene, C10 alkylene, C9 alkylene, C8 alkylene, C6 alkylene, C5 alkylene, C4 alkylene, C3 alkylene, C2 alkylene, C1 alkylene, C12 alkenylene, C11 alkenylene, C10 alkenylene, C9 alkenylene, C8 alkenylene, C7 alkenylene, C6 alkenylene, C8 alkenylene, C4 alkenylene, C3 alkenylene, and C2 alkenylene;
• R4 is C1-C16 unbranched alkyl, C2-C16 unbranched alkenyl, or
• wherein:
• R4a and R4b are each independently C1-C16 unbranched alkyl or C2-C16 unbranched alkenyl;
• R5 is absent, C1-C8alkylene, or C2-C8 alkenylene;
• R6a and R6b are each independently C7-C16 alkyl or C7-C16 alkenyl;
• provided that the total number of carbon atoms in R6a and R6b as combined is greater than 15;
• X1 and X2 are each independently-OC(═O)—, —SC(═O)—, —OC(═S)—, —C(═O)O—, —C(═O)S—, —S—S—, —C(Ra)=N—, —N═C(Ra)—, —C(Ra)=NO—, —O—N═C(Ra)—, —C(═O)NRa—, —NRaC(═O)—, —NRaC(═O)NRa—, —OC(═O)O—, —OSi(Ra)2O—, —C(═O)(CRa2) C(═O)O—, or OC(═O)(CRa2)C(═O)—;
• wherein:
• Ra, for each occurrence, is independently hydrogen or C1-C6 alkyl;
• and
• n is an integer selected from 1, 2, 3, 4, 5, and 6.