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the tertiary structure of a protein is structure that comes from interactions between the unique side chains (R groups) of different amino acids within a single protein chain. These interactions can be from one side chain to another sidechain or from one sidechain to the backbone of another, but you have to have at least 1 side chain involved. For single-chain proteins (monomeric proteins), this is the highest level of structure, but proteins with multiple chains (multimeric or oligomeric proteins) also have quaternary structure which comes from interactions between amino acids on the different chains.