Functionalization of single-walled carbon nanotubes (SWCNTs) is-necessitated in a number of conditions such as drug delivery, and here amino acid functionalization of SWCNTs is conducted within the framework of density functional theory. Functionalization efficiencies of Gly are largely determined by dopants; as a combined effect of atomic radius, electronic Configuration, and distortion to SWCNTs. Different functionalization sites in Gly have divergent interaction strengths with M/SWCNTS that decline as O-b > N > O-a, and this trend seems almost independent of the identity of metallic dopants. B/SWCNT behaves distinctly and prefers to the N site. Dopants affect principally interaction strengths, while: amino adds regulate significantly both functionalization configurations and interaction energies. Then focus is given to stabilization of zwitterionic amino adds due to enhanced interactions with the widely used zwitterionic drugs. All metallic dopants render zwitterionic Gly to be the most stable, and side chains in amino acids rather than dopants in M/SWCNTs cause more pronounced effects to zwitterionic stabilizations. Charge transfers between amino acids and M/SWCNTs are closely associated with zwitterionic stabilization effects, and different charge transfer mechanisms between M/SWCNTs and metal ions are interpreted. Thus, this work provides a comprehensive understanding of amino acid functionalization of M/SWCNTs.