Ultrafast <i>T</i><sub>1</sub>–<i>T</i><sub>1ρ</sub> NMR for Correlating Different Motional\nRegimes of Molecules

Nuclear magnetic\nresonance (NMR) relaxation times provide detailed\ninformation about molecular motions and local chemical environments.\nLongitudinal <i>T</i><sub>1</sub> relaxation time is most\noften sensitive to relatively fast, nano- to picosecond ranges of\nmolecular motion. Rotating frame <i>T</i><sub>1ρ</sub> relaxation time reflects a much slower, micro- to millisecond range\nof motion, and the motional regime can be tuned by changing spin-lock\nfield strength. Conventional methods for measuring <i>T</i><sub>1</sub> and <i>T</i><sub>1ρ</sub> relaxation\ntimes are time-consuming, since experiments must be repeated many\ntimes with incremented magnetization recovery or decay delay. In this\nwork, we introduce two novel and efficient NMR methods to correlate\nthe <i>T</i><sub>1</sub> and <i>T</i><sub>1ρ</sub> relaxation times. The first method, IR-SPICY, combines the conventional <i>T</i><sub>1</sub> inversion recovery (IR) with the single-scan <i>T</i><sub>1ρ</sub> detection-based spin-lock cycle (SPICY).\nThe second method, ultrafast (UF) IR-SPICY, allows measurement of\nwhole two-dimensional <i>T</i><sub>1</sub>–<i>T</i><sub>1ρ</sub> correlation data in a single scan,\nin a couple of seconds, based on spatial encoding of the <i>T</i><sub>1</sub> dimension. We demonstrate the performance of the methods\nby studying relaxation of water in porous silica and hydrogel samples,\nlatter acting as a model of the articular cartilage extracellular\nmatrix. The methods allow correlating different molecular motional\nregimes, potentially providing unprecedented information about various\nchemical and biochemical systems, such as structures and fluid dynamics\nin porous materials, macromolecular changes in tissues, and protein\ndynamics. One to three orders of magnitude shortened experiment time\nenable the studies of changing or degrading samples. Furthermore,\nthe single-scan approach may significantly facilitate the use of modern\nnuclear-spin hyperpolarization techniques to enhance the sensitivity\nof <i>T</i><sub>1</sub>–<i>T</i><sub>1ρ</sub> measurements by several orders of magnitude.