Controlling the Chiral Inversion Reaction of the Metallopeptide Ni-Asparagine-Cysteine-Cysteine with Dioxygen

Abstract

Synthetically generated metallopeptides have the potential to serve a variety of roles in biotechnology applications, but the use of such systems is often hampered by the inability to control secondary reactions. We have previously reported that the NiII complex of the tripeptide LLL-asparagine-cysteine-cysteine, LLL-NiII-NCC, undergoes metal-facilitated chiral inversion to DLD-NiII-NCC, which increases the observed superoxide scavenging activity. However, the mechanism for this process remained unexplored. Electronic absorption and circular dichroism studies of the chiral inversion reaction of NiII-NCC reveal a unique dependence on dioxygen. Specifically, in the absence of dioxygen, the chiral inversion is not observed, even at elevated pH, whereas the addition of O2 initiates this reactivity and concomitantly generates superoxide. Scavenging experiments using acetaldehyde are indicative of the formation of carbanion intermediates, demonstrating that inversion takes place by deprotonation of the alpha carbons of Asn1 and Cys3. Together, these data are consistent with the chiral inversion being dependent on the formation of a NiIII-NCC intermediate from NiII-NCC and O2. The data further suggest that the anionic thiolate and amide ligands in NiII-NCC inhibit Cα–H deprotonation for the NiII oxidation state, leading to a stable complex in the absence of O2. Together, these results offer insights into the factors controlling reactivity in synthetic metallopeptides.

Embedding the Ni-SOD Mimetic Ni-NCC within a Polypeptide Sequence Alters the Specificity of the Reaction Pathway

“…Absorbance and CD spectra of Ni-GGNCC were deconvoluted using a minimum number of Gaussian bands, in order to provide a quantitative comparison to the amine/amide Ni-NCC species (Figure 2 and Table 1). 3 MS data reveal the Ni complex of each pentapeptide is at the expected mass (e.g. Ni-GGNCC, m/z = 506.22, calculated = 506.02), and a nickel titration supports the 1:1 metal to peptide ratio (Figure S1).…”

“…The CD spectral features differ from those of the all-L tripeptide, which can be observed only in the absence of oxygen. 3 The transitions in the pentapeptide complex are shifted to higher energy because the amine nitrogen ligand from the N-terminus in the tripeptide is replaced by an amide in the pentapeptides. The extinction coefficients observed for the electronic transitions of Ni II -GGNCC are also reduced relative to those of Ni II -NCC, which is most likely also reflective of the structural differences between these complexes.…”

“…As described in reference 3, Ni II -NCC reacts rapidly with O 2 to initiate chiral inversion. Thus, the original electronic absorption and CD data reported for Ni II -NCC are not reflective of authentic Ni II -NCC.…”

“…This inversion results in a complex with activity comparable to other peptide mimics. 1-3 Here we compare the differences in reactivity of the Ni-NCC tripeptide alone to Ni-NCC embedded within longer sequences, to examine the importance of chiral inversion to superoxide scavengiving activity.…”

Metal complexes of amino acids and peptides

“…295 3.3 Metal complexes of sulphur containing peptides Thioether, thiolate and disulfide moieties are the most frequent functionalities for the occurrence of sulphur atoms in amino acids and peptides. 304 The formation of metal-sulfur bonded complexes was also suggested in the reaction of Au(I) and Au(III) with angiotensin peptide on the basis of mass spectrometric measurements. platinum metals).…”