Previously we demonstrated for several examples that peptides having a general internal sequence R(N)-Yaa-Ser/Thr-Xaa-His-Zaa-R(C) (Yaa = Glu or Ala, Xaa = Ala or His, Zaa = Lys, R(N) and R(C) = any N- and C-terminal amino acid sequence) were hydrolyzed specifically at the Yaa-Ser/Thr peptide bond in the presence of Ni(II) ions at alkaline pH (Krezel, A., Mylonas, M., Kopera, E. and Bal, E. Acta Biochim. Polon. 2006, 53, 721-727 and references therein). Hereby we report the synthesis of a combinatorial library of CH(3)CO-Gly-Ala-(Ser/Thr)-Xaa-His-Zaa-Lys-Phe-Leu-NH(2) peptides, where Xaa residues included 17 common alpha-amino acids (except Asp, Glu, and Cys) and Zaa residues included 19 common alpha-amino acids (except Cys). The Ni(II)-dependent hydrolysis at 37 and 45 degrees C of batches of combinatorial peptide mixtures randomized at Zaa was monitored by MALDI-TOF mass spectrometry. The correctness of library-based predictions was confirmed by accurate measurements of hydrolysis rates of seven selected peptides using HPLC. The hydrolysis was strictly limited to the Ala-Ser/Thr bond in all library and individual peptide experiments. The effects of individual residues on hydrolysis rates were quantified and correlated with physical properties of their side chains according to a model of independent contributions of Xaa and Zaa residues. The principal component analysis calculations demonstrated partial molar side chain volume and the free energy of amino acid vaporization for both Xaa and Zaa residues and the amine pK(a) for Zaa residues to be the most significant empirical parameters influencing the hydrolysis rate. Therefore, efficient hydrolysis required bulky and hydrophobic residues at both variable positions Xaa and Zaa, which contributed independently to the hydrolysis rate. This relationship between the peptide sequence and the hydrolysis rate provides a basis for further research, aimed at the elucidation of the reaction mechanism and biotechnological applications of Ni(II)-dependent peptide bond hydrolysis.