Poster Presentation 23rd International Society of Magnetic Resonance Conference 2023

Two-dimensional nuclear magnetic resonance homonuclear correlation spectroscopy (2D NMR COSY) provides insights into structural information about molecules by tracing J coupling correlation. Its application has been extended by implementing pure shift technique^[1], which enhances spectral resolution.^[2] However, existing doubly pure shift 2D COSY experiments still encounter challenges. Low signal intensity may limit the accuracy of experimental results, and complex data processing is required for analysis. We propose an NMR protocol, CTPSRJ-COSY (Constant-Time Pure Shift with Retrieved J Coupling COSY), combining constant-time and J-resolved evolution (Figure 1). With routine pulse elements for 2D pure shift extraction, the CTPSRJ-COSY enables the recording of doubly pure shift COSY spectra and maintains a high decoupling signal intensity analogous to conventional NMR signals without additional data processing to reconstruct extra pure shift dimension. Moreover, it facilitates the retrieval of J couplings, enabling multiplet structure analysis.

Strychnine (Figure 2a) dissolved in chloroform-d is carried out to illustrate the feasibility of the CTPSRJ-COSY. For strychnine, some coupled proton pairs with slight chemical shift differences are hardly identified in the conventional 2D COSY spectrum (Figure 2b) because their diagonal and cross peaks are close to each other and even overlapped together caused by J coupling splitting, seeing observed peaks in the spectral region between 4.00 and 4.35 ppm. Fortunately, the CTPSRJ-COSY experiment is applied to record the pure shift-based 2D COSY spectrum, and the CS processing^[3] is adopted for 3D acquisition acceleration. In the doubly pure shift COSY spectrum (Figure 2c), all peaks are observed as singlets in a high spectral resolution manner. We can readily determine coupling correlation information of coupled proton pairs for molecular structure analysis on strychnine, even for coupled proton pairs with slight chemical shift differences such as 23α and 23β. Moreover, the CTPSRJ-COSY provides an additional 2D J-resolved spectrum for multiplet structure analysis (Figure 4d). This demonstration indicates that the CTPSRJ-COSY method effectively elucidates molecular structures and multiplet structure analysis for samples containing complicated J coupling systems and crowded 1D NMR resonances.