A small, catalytically active metallopeptide (Nim SOD, m SOD=ACDLAC), which was derived from the nickel superoxide dismutase (NiSOD) active site was employed to study the mechanism of superoxide degradation, especially focusing on the protonation states of the Ni donor atoms, the proton source, and the role of the N-terminal proton(s). Therefore, the Ni -metallopeptide was studied at various pHs and temperatures using UV/Vis and NMR spectroscopy. These studies indicate a strong reduction of the pK of the Ni -ligating donor atoms, resulting in a fully deprotonated Ni active-site environment. Furthermore, no titratable proton could be observed within a pH ranging from 6.5 to 10.5. This rules out a recently discussed adiabatic proton tunneling-like hydrogen-atom transfer process for the metallopeptides, not found in the native enzyme. Furthermore, variable-temperature H NMR measurements uncovered an extended hydrogen-bond network within the Ni active site of the metallopeptide similar to the enzyme. With respect to the deprotonated Ni active site, the residual N-terminal proton, which is a prerequisite for catalytic activity, cannot act as proton source. Most likely, it stabilizes the Ni -coordinated substrate in an end-on fashion, thus allowing for an inner-sphere electron transfer. Lastly, and unlike the enzyme, the catalytic rate constant of superoxide degradation by the metallopeptides was determined to be strongly pH dependent, suggesting bulk water to be directly involved in proton donation, which in turn strongly suggests the N-terminal histidine to be the respective proton donor in the enzyme.

Until now, the intermediate responsible for the acyl transfer of a highly enantioselective tetrapeptide organocatalyst for the kinetic resolution of trans-cycloalkane-1,2-diols has never been directly observed. It was proposed computationally that a π-methylhistidine moiety is acylated as an intermediate step in the catalytic cycle. In this study we set out to investigate whether we can detect and characterize this key intermediate using NMR-spectroscopy and mass spectrometry. Different mass spectrometric experiments using a nano-ElectroSpray Ionization (ESI) source and tandem MS-techniques allowed the identification of tetrapeptide acylium ions using different acylation reagents. The complexes of trans-cyclohexane-1,2-diols with the tetrapeptide were also detected. Additionally, we were able to detect acylated tetrapeptides in solution using NMR-spectroscopy and monitor the acetylation reaction of a trans-cyclohexane-1,2-diol. These findings are important steps towards the understanding of this highly enantioselective organocatalyst.

The kinetic resolution of trans-cyclohexane-1,2-diol with a lipophilic oligopeptide catalyst shows extraordinary selectivities. To improve our understanding of the factors governing selectivity, we quantified the Gibbs energies of interactions of the peptide with both enantiomers of trans-cyclohexane-1,2-diol using nuclear magnetic resonance (NMR) spectroscopy. For this we use advanced methods such as transverse relaxation (R2), diffusion measurements, saturation transfer difference (STD), and chemical shift (δ) analysis of peptide-diol mixtures upon varying their composition (NMR titrations). The methods employed give comparable and consistent results: The molecular recognition by the catalyst is approx. 3 kJmol−1 in favor of the preferentially acetylated (R,R)-enantiomer in the temperature range studied. Interestingly, the difference of 3 kJ mol−1 is also confirmed by results from reaction monitoring of the acylation step under catalytic conditions, indicating that this finding is true regardless of whether the investigation is performed on the acetylated species or on the free catalyst. To arrive at these conclusions the self-association of both catalyst and substrate in toluene were found to play an important role and thus need to be taken into account in reaction screening.

For more than twenty years chemists use peptidic organocatalysts to transfer acyl groups onto alcohols. The goal was and still is the increase of selectivity towards the substrate of choice. One carefully designed example is a highly enantioselective tetrapeptide organocatalyst for the kinetic resolution of trans-cycloalkane-1,2-diols. Until now, the intermediate responsible for the acyl transfer of this tetrapeptide has never been directly observed. It was proposed computationally that a π-methylhistidine moiety is acylated as an intermediate step in the catalytic cycle. In this study we set out to investigate whether we can detect and characterize this key intermediate using NMR-spectroscopy and mass spectrometry. Different mass spectrometric experiments using a nano-ElectroSpray Ionization (ESI) source and tandem MS-techniques allowed the identification of tetrapeptide acylium ions using different acylation reagents. The complexes of trans-cyclohexane-1,2-diols with the tetrapeptide were also detected. Additionally, we were able to detect acylated tetrapeptides in solution using NMR-spectroscopy and monitor the acetylation reaction of a trans-cyclohexane-1,2-diol. These findings are important steps towards the understanding of this organocatalyst and its high enantioselectivity.