Proton strong coupling in heteronuclear systems. Theoretical and experimental evaluation of the effects in quantitative analysis of SQC-NOESY spectra of biopolymers

Hydrogen-detected NOE-relayed heteronuclear correlation via single-quantum coherence spectra (SQC-NOESY) may be affected by second-order anomalies when |(ωI ± πJIS) − ωI′| ≤ 20πJII′, where I and I′ are protons and S the heterospin coupled only to I. When the above condition applies, coherences of type SαIβ (α, β = x, y, z) undergo oscillatory transfer to SαI′β coherences without the need for any pulse perturbation. Thus the INEPT transfers, as well as the t1 precession step of the SQC-NOESY scheme, will no longer be effective in sorting out only antiphase or transverse coherences of the proton spin directly coupled to the heteronucleus S. In practice the process leads to measurable amplitude contributions to both auto- and cross-peak volumes, despite the fact that the effects developed during the INEPT steps are often negligible and the theoretical net transfer expected from the t1 evolution is null. Since during tm and t2 (provided heteronuclear decoupling is applied) no effect is expected from the direct-heteronuclear-coupling operator, any strong-coupling contribution arising in these conditions can be computed using the specific parameters of the system under investigation. Thus auto- and cross-peak volumes can be corrected before internuclear distances are evaluated, In natural-abundance or slightly enriched 1H-13C biopolymer systems, assuming JII′/JIS = 0.06 and t1 = 10-20 ms, a correction amounting to 0-7% of the auto- and cross-peak volume sum should be applied to the connectivities of the strongly coupled pair, depending on ΔωI.