Nuclear Singlet Relaxation by Chemical Exchange

Abstract

The population imbalance between nuclear singlet states and triplet states of strongly coupled spin-1/2 pairs, also known as nuclear singlet order, is well protected against several common relaxation mechanisms. We study the nuclear singlet relaxation of 13C pairs in aqueous solutions of 1,2-13C2 squarate, over a range of pH values. The 13C singlet order is accessed by introducing 18O nuclei in order to break the chemical equivalence. The squarate dianion is in chemical equilibrium with hydrogen-squarate (SqH−) and squaric acid (SqH2) characterised by the dissociation constants pKa1 = 1:5 and pKa2 = 3:4. Surprisingly, we observe a striking increase in the singlet decay time constants TS when the pH of the solution exceeds ~ 10, which is far above the acid-base equilibrium points. We derive general rate expressions for chemical-exchange-induced nuclear singlet relaxation and provide a qualitative explanation of the TS behaviour of the squarate dianion. We identify a kinetic contribution to the singlet relaxation rate constant which depends explicitly on kinetic rate constants. Qualitative agreement is achieved between the theory and the experimental data. This study shows that infrequent chemical events may have a strong effect on the relaxation of nuclear singlet order.

31P nuclear spin singlet lifetimes in a system with switchable magnetic inequivalence: experiment and simulation

“…It has recently been shown that chemical exchange phenomena, such as reversible protonation, can induce singlet relaxation through modulation of isotropic chemical shifts 39 . We do not anticipate a significant contribution from this mechanism in the current study, since the solvent is aprotic.…”

Long-lived states of methylene protons in achiral molecules

“…However, long-lived states (LLSs) have lifetimes T LLS that can be substantial longer than T 1 (1)(2)(3). The discovery of LLSs and the invention of methods to excite them have opened new perspectives, in particular for revealing interactions between potential drugs and target proteins (4)(5)(6), for the observation of hyperpolarized metabolites (7)(8)(9)(10), for probing slow chemical exchange (11), for determining rates of slow diffusion (12,13), for storing nuclear hyperpolarization (14)(15)(16)(17), for selecting signals of interest in peptides and proteins (18,19), and for detecting signals of metabolites in magnetic resonance imaging (MRI) (19)(20)(21)(22).…”

Limits of the quantum cognition hypothesis: 31P singlet order lifetimes of pyrophosphate from experiment and simulation

“…Isotopic incorporation was confirmed by mass spectrometry to be 96% 18 O4, 96% 18 O3. For NMR experiments, the tetrabasic sodium uPPi was formulated as a 30 mM solution in deuterium oxide plus 10 equivalents of potassium hydroxide, in order to minimize proton exchange, which can accelerate singlet relaxation (11), and minimize interactions with sodium ions in solution, which our results seem to indicate also accelerates relaxation of both longitudinal and spin order (Fig. S1, Supplementary Materials).…”

“…We note, however, that many of the experimental conditions used for our relaxation measurements are different from those that would be encountered in a biological system. First, our experiments were performed at a relatively high pD, to limit deuteron exchange, whereas faster exchange at physiological pD values would be expected to reduce the TS and possibly T1 relaxation times (11). In addition, the nature of the counterion played a role in the relaxation measurements, and the longest T1 times were observed with an excess of K + ions relative to the Na + ions from the synthesized uPPi tetrasodium salt.…”