Eriksson Shannon L, Lindale Jacob R, Li Xiaoqing, Warren Warren S
Department of Chemistry, Duke University, Durham, NC 27708, USA.
School of Medicine, Duke University, Durham, NC 27708, USA.
Sci Adv. 2022 Mar 18;8(11):eabl3708. doi: 10.1126/sciadv.abl3708. Epub 2022 Mar 16.
Signal amplification by reversible exchange (SABRE) creates "hyperpolarization" (large spin magnetization) using a transition metal catalyst and parahydrogen, addressing the sensitivity limitations of magnetic resonance. SABRE and its heteronuclear variant X-SABRE are simple, fast, and general, but to date have not produced polarization levels as large as more established methods. We show here that the commonly used theoretical framework for these applications, which focuses on avoided crossings (also called level anticrossings or LACs), steer current SABRE and X-SABRE experiments away from optimal solutions. Accurate simulations show astonishingly rich and unexpected dynamics in SABRE/X-SABRE, which we explain with a combination of perturbation theory and average Hamiltonian approaches. This theoretical picture predicts simple pulse sequences with field values far from LACs (both instantaneously and on average) using different terms in the effective Hamiltonian to strategically control evolution and improve polarization transfer. Substantial signal enhancements under such highly nonintuitive conditions are verified experimentally.
通过可逆交换实现的信号放大（SABRE）利用过渡金属催化剂和仲氢产生“超极化”（大自旋磁化），解决了磁共振的灵敏度限制问题。SABRE及其异核变体X-SABRE简单、快速且通用，但迄今为止尚未产生与更成熟方法一样高的极化水平。我们在此表明，这些应用中常用的理论框架侧重于避免交叉（也称为能级反交叉或LACs），这使得当前的SABRE和X-SABRE实验偏离了最优解。精确模拟显示SABRE/X-SABRE中存在惊人丰富且意想不到的动力学，我们用微扰理论和平均哈密顿方法相结合来解释这一现象。这一理论图景预测了简单的脉冲序列，其场值远离LACs（即时和平均而言），利用有效哈密顿量中的不同项来策略性地控制演化并改善极化转移。在这种高度非直观的条件下，大量的信号增强通过实验得到了验证。
