NMR profiling of biomolecules at natural abundance using 2D 1H–15N and 1H–13C multiplicity-separated (MS) HSQC spectra

摘要

2D NMR ¹H–X (X = ¹⁵N or ¹³C) HSQC spectra contain cross-peaks for all XHn moieties. Multiplicity-edited ¹H–¹³C HSQC pulse sequences generate opposite signs between peaks of CH2 and CH/CH3 at a cost of lower signal-to-noise due to the ¹³C T2 relaxation during an additional 1/¹JCH period. Such CHn-editing experiments are useful in assignment of chemical shifts and have been successfully applied to small molecules and small proteins (e.g. ubiquitin) dissolved in deuterated solvents where, generally, peak overlap is minimal. By contrast, for larger biomolecules, peak overlap in 2D HSQC spectra is unavoidable and peaks with opposite phases cancel each other out in the edited spectra. However, there is an increasing need for using NMR to profile biomolecules at natural abundance dissolved in water (e.g., protein therapeutics) where NMR experiments beyond 2D are impractical. Therefore, the existing 2D multiplicity-edited HSQC methods must be improved to acquire data on nuclei other than ¹³C (i.e. ¹⁵N), to resolve more peaks, to reduce T2 losses and to accommodate water suppression approaches. To meet these needs, a multiplicity-separated ¹H–X HSQC (MS-HSQC) experiment was developed and tested on 500 and 700 MHz NMR spectrometers equipped with room temperature probes using RNase A (14 kDa) and retroviral capsid (26 kDa) proteins dissolved in 95% H2O/5% D2O. In this pulse sequence, the 1/¹JXH editing- period is incorporated into the semi-constant time (semi-CT) X resonance chemical shift evolution period, which increases sensitivity, and importantly, the sum and the difference of the interleaved ¹JXH-active and the ¹JXH-inactive HSQC experiments yield two separate spectra for XH2 and XH/XH3. Furthermore we demonstrate improved water suppression using triple xyz-gradients instead of the more widely used z-gradient only water-suppression approach.