Unveiling an NMR-Invisible Fraction of Polymers in Solution by Saturation Transfer Difference

The observation of signals in solution NMR requires nuclei with sufficiently large transverse relaxation times (T₂). Otherwise, broad signals embedded in the baseline afford an invisible fraction of nuclei (IF). Based on the STD (saturation transfer difference) sequence, IF-STD is presented as a quick tool to unveil IF in the ¹H NMR spectra of polymers. The saturation of a polymer in a region of the NMR spectrum with IF (very short ¹H T₂) results in an efficient propagation of the magnetization by spin diffusion through the network of protons to a visible-invisible interphase with larger ¹H T₂ (STD_on). Subtracting this spectrum from one recorded without saturation (STD_off) produces a difference spectrum (STD_off-on), with the nuclei at the visible-invisible interphase, that confirms the presence of an IF. Analysis of a wide collection of polymers by IF-STD reveals IF more common than previously thought, with relevant IF figures when STD > 0.4% at 750 MHz. A fundamental property of the IF-STD experiment is that the signal is generated within a single state comprising polymer domains with different dynamics, as opposed to several states in exchange with different degrees of aggregation. Contrary to a reductionist visible-invisible dichotomy, our results confirm a continuous distribution of nuclei with diverse dynamics. Since nuclei observed (edited) by IF-STD at the visible-invisible interphase are in close spatial proximity to the IF (tunable with the saturation time), they emerge as a privileged platform from which gaining an insight into the IF itself.