Two-Dimensional Nuclear Magnetic Resonance: Exploiting Spin Echoes To Maximize Information Content by Suppression of Diagonal Peaks in Homonuclear Experiments

Mechanistic investigation of enzymatic degradation of polyethylene terephthalate by nuclear magnetic resonance

The biocatalytic degradation of polyethylene terephthalate (PET) by thermophilic microbial enzymes has recently emerged as an option for a future eco-friendly recycling process for plastic waste, as it occurs under mild conditions and requires no harmful additives. In this chapter, we present a brief overview of solution and solid-state nuclear magnetic resonance (NMR) spectroscopic methods for the characterization of composition and chemical microstructure of PET and also associated chain dynamics over multiple time scales. Such detailed information provides an understanding of the enzymatic PET degradation mechanism by polyester hydrolases at the molecular level.

Spin-echo based diagonal peak suppression in solid-state MAS NMR homonuclear chemical shift correlation spectra

The feasibility of using the spin-echo based diagonal peak suppression method in solid-state MAS NMR homonuclear chemical shift correlation experiments is demonstrated. A complete phase cycling is designed in such a way that in the indirect dimension only the spin diffused signals are evolved, while all signals not involved in polarization transfer are refocused for cancellation. A data processing procedure is further introduced to reconstruct this acquired spectrum into a conventional two-dimensional homonuclear chemical shift correlation spectrum. A uniformly ¹³C, ¹⁵N labeled Fmoc-valine sample and the transmembrane domain of a human protein, LR11 (sorLA), in native Escherichia coli membranes have been used to illustrate the capability of the proposed method in comparison with standard ¹³C-¹³C chemical shift correlation experiments.

Analyses of Complex Mixtures by F1 Homo-Decoupled Diagonal Suppressed Total Correlation Spectroscopy

A diagonal suppressed F₁ decoupled total correlation spectroscopy(TOCSY) experiment is developed for analyses of complex mixtures. In 2D homonuclear correlation, assignment of the cross peaks is crucial for structure elucidation. However, when cross peaks are close to the diagonal peaks in overcrowded spectral regions, their assignment becomes tedious. In complex mixtures, the presence of multiple spectra along with broad and complex proton multiplets owing to homonuclear scalar couplings degrade the resolution to the extent that assignment of various cross peaks becomes tedious or impossible. Herein, a diagonal suppressed total correlation technique with F₁ decoupling is presented to improve the resolution of the cross peaks. The resolution of the cross peaks is improved by both diagonal suppression as well as the collapse of the multiplets to singlets. Application of the method to a few mixtures of organic compounds reveals better identification of the cross peaks relative to other TOCSY variants.

Diagonal free homonuclear correlation using heteronuclei at natural abundance

Homonuclear correlated spectroscopy such as COSY and TOCSY provides crucial structural information. In all homonuclear correlation, the most intense peaks are represented by the diagonal. As a result, the useful cross peaks close to the diagonal get obscured by the huge tails of diagonal peaks. Herein, we show that by editing the proton magnetization by a 13C nucleus in natural abundance, it is possible to eliminate the inphase coherence or untransferred magnetization that leads to the diagonal peak while retaining the antiphase coherence or transferred magnetization required for creation of cross peak. After the coherence transfer step, the untransferred magnetization directly attached to 13C evolves under one bond heteronuclear coupling while the transferred transverse magnetization directly attached to remote 12C does not. As a result, the untransferred magnetization directly attached to 13C can be converted to an unobservable heteronuclear multiple quantum coherence leading to a diagonal free correlated spectrum with a sensitivity penalty of two orders of magnitude but comparable to HSQC kind of experiments at natural abundance. The method demonstrated for COSY and TOCSY allows all proton-proton correlations to be observed except the geminal proton-proton correlations. Further, protons directly attached to heteronuclei other than 13C must be scalar coupled to protons directly attached to 13C to have a detectable cross peak.