Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Enzymatic activity of purified NS2B(H)-NS3pro was assayed against commercially available synthetic peptide substrates containing either three non-prime side residues, Boc-GRR-amc and Boc-LRR-amc; four residues, Ac-nKRR-amc, Ac-nKRR-amc; and five residues, Pyr-RTKR-amc, and a previously described internally quenched DEN NS3 substrate, Abz-(R)4SAGnY-amide [22] (all purchased from Peptides International, KY, USA).
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