New strategy for RDC‐assisted diastereotopic proton assignment using a combination of J‐scaled BIRD HSQC and J‐scaled BIRD HMQC/HSQC

Abstract

A new strategy to assign diastereotopic protons was developed on the basis of residual dipolar couplings (RDCs) collected in compressed poly(methyl methacrylate) (PMMA) gels. A combination of 2D J-scaled BIRD HSQC and J-scaled BIRD HMQC/HSQC NMR experiments was used to collect the RDC data. In the proposed strategy, the first experiment is used to measure (1)D(CH) for methine groups, the sum of (1)D(CHa) + (1)D(CHb) for methylene groups and the average (1)D(CH3) value for methyl groups. In turn, the small molecule alignment tensor is calculated using these D values without the a priori assignment of CH(2) diastereotopic protons. The D values of each individual CH bond (CHa and CHb) of each methylene group in the molecule are then predicted using the calculated alignment tensor and these values compared with the results from the HMQC/HSQC experiment, leading to their unambiguous assignment. This strategy is demonstrated with the alkaloid strychnine that contains five methylene groups with diastereotopic protons, and our results fully agree with the previously reported assignment using combinations of permutated assignments.

Suppression of phase and amplitude J(HH) modulations in HSQC experiments

“…Although organic compounds under isotropic conditions present a narrower range of J values, these intervals are commonly found when measuring J splittings in anisotropic conditions. For instance, it has been reported that experimental 2 T(HH) ( 2 J(HH) + 2 D(HH)) coupling values up to 30–40 Hz in magnitude and 1 T(CH) ( 1 J(CH) + 1 D(CH)) in the range of 100–180 Hz are measured in the case of strychnine dissolved in a CDCl 3 /poly‐methylmethacrylate gel …”

Computer‐Assisted 3D Structure Elucidation (CASE‐3D): The Structural Value of 2JCH in Addition to 3JCH Coupling Constants

“…A total of 32 2 J CH and 47 3 J CH unsigned coupling constants were extracted for a sample of strychnine in CDCl 3 from a single IPAP‐PIP‐HSQMBC experiment processed to yield α and β subspectra . The assignment was taken from previously published work . Most coupling constants could be determined with high precision compared to the accuracy of the DFT calculations.…”

Isotropic/Anisotropic NMR Editing by Resolution‐Enhanced NMR Spectroscopy

“…Besides, the determination of the tiny 1 D (CH) of +0.3 Hz in H‐22 due to the very similar 1 J (C22‐H22) and 1 T (C22‐H22) values is only feasible due to its observable Δδ( 13 C) of +6.7 Hz. As recently reported, 1 J CH and 1 T CH measurements could also be performed along the indirect F1 dimension of complementary J CH ‐resolved HSQC ( J ‐HSQC) spectra . Figure A and 5B compare the SA‐PIP‐HSQC and J ‐HSQC cross‐peaks for the H13 and H15b protons, respectively, of 1 .…”