A method for determining protein binding specificity using a screen of a peptide library is provided. The method can be used to determine binding specificity for human NAD+-dependent deacetylase SIRT1, and to identify the most efficiently deacetylated peptide sequences. The method can be also used to screen a combinatorial H4 histone N-terminal tail peptide library to examine the binding preferences of a α-phos (S1) H4 antibody toward all known possible H4 histone modification states.