Diastereomeric ratio determination by high sensitivity band-selective pure shift NMR spectroscopy

Abstract

An NMR method is reported that allows diastereomeric ratios to be determined even in crowded spectra or where chemical shift differences are small compared to multiplet widths. Band-selective pure shift NMR collapses multiplets to singlets, greatly improving spectral resolution while largely retaining, or even enhancing, signal-to-noise ratio.

Pure Shift NMR Spectroscopy

“…Several J-refocusing elements are available, based on BIRD, 4 frequency-and spatially-selective refocusing as originally developed by Zangger and Sterk (ZS), 5 BSD, 6,7 and, in the anti z-COSY experiment and time-reversal methods, low-flip-angle pulse pairs. 8,9 Each of these J-refocusing elements reverses the effects of J, which are thus suppressed explicitly at a given evolution time.…”

“…This can be of particular use in the study of proteins and peptides 6,29 where the variety of chemical groups is limited and the chemical shift ranges of different classes of proton are well-defined, and for mixtures of isomers, where quantification is often hampered by overlapping multiplets. 7 The limitation of band-selective pure shift sequences, that signals within the bandwidth of the refocusing pulse are not decoupled from one another, is balanced by the increase in sensitivity that results from collapsing multiplets into singlets. Where BSD is used with real-time acquisition the experiment sensitivity typically exceeds that of standard 1 H acquisition.…”

“…28 It is also very effective with both ZS and a variant of the ZS method in which no field gradient is applied during the selective 180 • pulse. 6,7 The distinction between active and passive spins is then made purely according to chemical shift: the active spins are those that are refocused by the selective 180 • pulse, so the resultant spectrum for these spins has the effects of all couplings to protons outside this frequency range suppressed. This BSD (also known as band selective homonuclear (BASH) 29 decoupling, and homodecoupled band selective (HOBS) 6 NMR) method is well-suited to systems in which a narrow range of chemical shifts is of interest (e.g., protein signals) or in which a particular set of signals is of interest.…”

J‐Edited Pure Shift NMR for the Facile Measurement of nJHH for Specific Protons

“…[2] Pure shift spectra, which provide information on chemical shifts, have wide applications, such as, unravelingt he overlapped peaks, thereby giving access to diastereomeric ratio determination, [3] measurement of heteronuclear couplings, [4] ands of orth. The biggest drawback of pure shifts pectra is the loss of information on homonuclear scalar couplings;i ti sw ell known that homonuclear scalar couplings are important for obtaining the relative configurations and conformations of small molecules, according to Karplus. [5]…”

Ultraclean pure shift NMR

“…8 The artefacts arise because for sensitivity reasons the pure shift FID is respectively acquired or constructed in chunks of a few tens of ms duration (1/sw 1 ) each, while the scalar couplings are only perfectly refocused at the midpoint of each chunk. [8][9][10][11][12] The small amount of J evolution during the rest of the chunk causes a weak modulation of the signal envelope with period 1/sw 1 (as do a variety of other perturbations, including instrumental imperfections). On Fourier transformation this modulation leads to each peak acquiring small periodic sidebands at intervals sw 1 .…”