Improvement of HSQC and HMBC Experiments by J-modulation, t1-noise Reduction and Spectral Domain Selection. Application to the Study of Nosiheptide

Abstract

Modifications of the HSQC and HMBC pulse sequences and their application to the study of the antibiotic nosiheptide are reported. Reduction of t1-noise in the 1H–15N HSQC experiment is achieved by the combination of a BIRD-relaxation preparation period and introduction of a purging spin-locking pulse. This tends to weaken signals from protons bound to 14N nuclei. The insertion of a J-modulation period allows discrimination between cross peaks from NH and NH2 groups. Assignment problems caused by a low resolution in the 1H–13C HMBC spectra are solved by means of selective excitation of crowded spectral domains. In the absence of dedicated hardware, selective pulses were produced by DANTE sequences. Resonance assignment of nosiheptide matches previously reported data, established by means of isotopically enriched material. The unreported chemical shifts of the two primary amide protons were determined. The methods described will find valuable applications in problems of similar complexity for which isotopic enrichment is impossible.