The previously synthesized, terminally blocked heptapeptide Ac-Aib-ATANP-Aib-Aib-ATANP-Aib- Aib-OMe (1a), where ATANP is (S)-2-amino-3-[1- (1,4,7-triazacyclononane)]propanoic acid and Aib is ␣-aminoisobutyric acid, which is soluble in neutral water where it largely adopts a 3 10 -helical conformation, has been studied, as bimetallic complex [metal ions: Cu(II)
, Ni(II), Zn(II)], for the transphosphorylation catalysis of the RNA-model substrate 2-(hydroxypropyl)-p-nitrophenyl phosphate (HPNP). A detailed analysis was carried out with the Zn(II) dinuclear complex. Comparison with the mononuclear Zn(II) complex with 1,4,7-triazacyclononane (3) points to cooperativity between the two Zn(II) ions in the process catalyzed by 1a-2Zn(II). On the contrary, the dinuclear Zn(II) complex of dipeptide Ac-(ATANP) 2 -OMe (2), lacking any ordered conformation, is less active than 3-Zn(II).
The kinetic analysis suggests the following: (a) the peptide is conformationally very robust and does not loose activity up to 50°C; (b) the substrate binds to the peptide-Zn(II) complex, although not all modes of complexation allow us to take advantage of the cooperativity between the two metal centers. The maximum rate acceleration estimated at pH 7 for the fully bound substrate is ca. 200-fold compared with the uncatalyzed process.