The disclosure features novel lipids and compositions involving the same. Lipid nanoparticles (e.g., empty LNPs or loaded LNPs) include a novel lipid as well as additional lipids such as phospholipids, structural lipids, and PEG lipids. Lipid nanoparticles (e.g., empty LNPs or loaded LNPs) further including therapeutic and/or prophylactics such as RNA are useful in the delivery of therapeutic and/or prophylactics to mammalian cells or organs to, for example, regulate polypeptide, protein, or gene expression.