Reconstruction of NMR Relaxation Rates from Coarse-Grained Polymer Simulations

NMR relaxometry is a powerful and well established experimental approach to characterize dynamic processes in soft matter systems. All-atom (AA) resolved simulations are typically employed to gain further microscopic insights while reproducing the relaxation rates $R_1$. However, such approaches are limited in time and length-scales that hinder modeling of systems like long polymer chains or hydrogels. Coarse-graining (CG) can overcome this barrier at the cost of loosing atomistic details that impede the calculation of NMR relaxation rates. Here, we address this issue by systematic characterization of $R_1$ while performing systematic measurements on a PEG-H$_2$O mixture at two different levels of details: AA and CG. Remarkably, we show that NMR relaxation rates $R_1$ obtained at the CG level obey the same trends when compared to AA calculations, but with a systematic offset. We find this offset to be due to, on the one hand, the lack of an intra-monomer component and, on the other hand, the inexact positioning of the spin carriers. We show that the offset can be corrected for quantitatively by reconstructing a posteriori the atomistic details from the CG trajectories.