AbstractTwo Tyr residues are supposed to play a crucial role in the regulation of protein tyrosine kinases of the Src family. Autophosphorylation of Src Tyr416 correlates with enzyme activation, while phosphorylation of C‐terminal Tyr527 by Csk gives rise to inactive forms of Src kinases.It has previously been demonstrated that the Src‐like tyrosine kinase expressed by the oncogenelyndisplays a particularly high affinity (Km20 μm) toward the dimeric linear and cyclic derivatives of the heptapeptide H‐Glu‐Asp‐Asn‐Glu‐Tyr‐Thr‐Ala‐OH which reproduces the main autophosphorylation site of most of the Src enzymes. Under the experimental conditions used only one Tyr residue of the dimeric sequence can be phosphorylated [P. Ruzza, A. Calderan, B.Filippi, B. Biondi, A. Donella Deana, L. Cesaro, L. A. Pinna&G. Borin (1995)Int. J. Peptide Protein Res. 45, 529–539].The present study addresses the problem of the efficiency displayed by Lyn towards the two Tyr residues located at positions 5 and 12 of the dimeric peptide. To this purpose, two tetradecapeptides were synthesized by the classical solution method, each containing one of the two Tyr residues alternatively replaced by Phe, and the corresponding univocal cyclic form. A possible correlation between the different structural properties induced by the modifications of the native sequence and the ability of the peptides to act as Lyn substrates was noted. The kinetic data obtained indicate that Lyn phosphorylates the residues located at different positions in the two linear analogues differently. In particular, while the Tyr5, Phe12 derivative presents aKmvalue similar to those obtained for the dimeric linear and cyclic unmodified analogues, theKmvalue of the Phe5, Tyr12 derivative is two‐fold higher than those found for the above‐mentioned peptides. Moreover, as previously reported for the linear and cyclic dimeric forms of the native sequence, in the mono‐tyrosine containing series of dimers the still conformationally flexible cyclic derivative shows a phosphorylation efficiency two‐fold higher than those found for the linear derivatives. © 1998 European Peptide