Bioinspired superoxide-dismutase mimics: The effects of functionalization with cationic polyarginine peptides

Continuing a bio-mimetic approach, we have prepared peptide conjugates of a superoxide dismutase (SOD) mimic [MnL](+) (where HL=N-(2-hydroxybenzyl)-N,N'-bis[2-(N-methylimidazolyl)methyl]ethane-1,2-diamine), namely [MnL'-Arg(n-1)](n+) (where n=2, 4, 7 and 10) and [MnL'-Gly1](+). [MnL'-Arg(n-1)](n+) contained cationic residue(s) that emulate the electrostatic channel of the enzyme. Physicochemical methods showed that functionalization at the secondary amine of HL did not impair coordination to Mn(II) with association constants (Kassoc) between 1.6 and 3.3×10(6)M(-1). The Mn(III)/Mn(II) redox potential of the conjugates was between 0.27 and 0.30V vs SCE, slightly higher than [MnL](+) under the same conditions, but remain at a value that facilitates O2(-) dismutation. The catalytic rate constant (kcat) of the dismutation for the series was studied using a direct stopped-flow method, which showed that for compounds with the same overall charge, the alkylation of the secondary amine of [MnL](+) (kcat=5.0±0.1×10(6)M(-1)s(-1)) led to a lower value (i.e. for [MnL'Gly](+), kcat=4.2±0.1×10(6)M(-1)s(-1)). However, under the same conditions, kcat values between 5.0±0.4×10(6)M(-1)s(-1) and 6.6±0.1×10(6)M(-1)s(-1) were determined for [MnL'-Arg(n-1)](n+) conjugates, indicating that the cationic residue(s) compensated for the loss in activity. Analysis of the effect of ionic strength on the kcat strongly suggested that not all the charges were involved, but only the closest ones electrostatically influenced the SOD active metal centre.