Proton Strong Coupling in Heteronuclear Systems. Theoretical and Experimental Evaluation 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 |(ω_I ± πJ_IS) - ω_I′| ≤ 20πJ_II′, where I and I′ are protons and S the heterospin coupled only to I. When the above condition applies, coherences of type S_αI_β (α, β = x, y, z) undergo oscillatory transfer to S_αI′_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 t_m 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 J_II′/J_IS = 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 Δω_I.