Investigating Interaction Dynamics of an Enantioselective Peptide-Catalyzed Acylation Reaction.

Modern nuclear magnetic resonance (NMR) methods like carbon relaxation dispersion in the rotating frame (C-R) and proton chemical exchange saturation transfer (H-CEST) are key methods to investigate molecular recognition in biomacromolecules and to detect molecular motions on the μs to s timescale, revealing transient conformational states. Changes in kinetics can be linked to binding, folding, or catalytic events. Here, we investigated whether these methods allow detection of changes in the dynamics of a small, highly selective peptide catalyst during recognition of its enantiomeric substrates.

Evolution of a Combined UV/Vis and NMR Setup with Fixed Pathlengths for Mass-limited Samples.

The investigation of reaction mechanisms is a complex task that usually requires the use of several techniques. To obtain as much information as possible on the reaction and any intermediates - possibly invisible to one technique - the combination of techniques is a solution. In this work we present a new setup for combined UV/Vis and NMR spectroscopy and compare it to an established alternative.

Quantifying Intermolecular Interactions in Asymmetric Peptide Organocatalysis as a Key toward Understanding Selectivity.

The kinetic resolution of -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 free energies of interactions of the peptide with both enantiomers of -cyclohexane-1,2-diol using nuclear magnetic resonance (NMR) spectroscopy. For this, we use advanced methods such as transverse relaxation (), diffusion measurements, saturation transfer difference (STD), and chemical shift (δ) analysis of peptide-diol mixtures upon varying their composition (NMR titrations).

Pd-Catalyzed Asymmetric Allylic Alkylation of Cyclobutenes: From Double Inversion to Double Retention.

The Pd-catalyzed allylic alkylation of 3,4-disubstituted, racemic cyclobutene electrophiles exhibits a highly unusual stereoselectivity that allows for controlling diastereo- and enantioselectivity only by the choice of ligand and independent of the configuration of the substrate. In order to shed light on the origin of stereoinduction, we performed a systematic mechanistic investigation, including preparation of various putative Pd-allyl intermediates, H/P NMR reaction monitoring, H-labeling studies, ESI-HRMS and P NMR analysis of reaction mixtures, and DFT structural computations. The mechanism disclosed exhibits several steps with stereospecificities deviating from the commonly accepted "double inversion rule": oxidative addition was found to follow a stereoconvergent course, giving -configured η-Pd-cyclobutene species as detectable on-cycle intermediates irrespective of the configuration of starting material, while the subsequent nucleophilic attack features a stereodivergent behavior.

Detection and Verification of a Key Intermediate in an Enantioselective Peptide Catalyzed Acylation Reaction.

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.

Ni Complex Formation and Protonation States at the Active Site of a Nickel Superoxide Dismutase-Derived Metallopeptide: Implications for the Mechanism of Superoxide Degradation.

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.