Abstract: The subject invention pertains to methods of acetylation of the Fc region of immunoglobulins, particularly Lys248 of the heavy chain of human Immunoglobulin G (IgG) using a novel Fc-III derived peptide. The acetylation reaction with the phenolic ester with an azide or alkyne enables site-selective functionalization of Lys248 with a bioorthogonal reactive group for further derivatization. Further methods are provided to synthesize an antibody-lipid conjugate that allows for the construction of an immunoliposome that can target cells expressing oncogenes, including HER2+ cells, and a bispecific antibody complex (bsAbC) linking two distinct antibodies. The bsAbC can induce an effector-cell mediated cytotoxicity at nanomolar concentrations.