Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
J Magn Reson. 2010 Jan;202(1):57-63. doi: 10.1016/j.jmr.2009.09.018. Epub 2009 Sep 30.
In solids spinning at the magic angle, the indirect detection of single-quantum (SQ) and double-quantum (DQ) (14)N spectra (I=1) via spy nuclei S=1/2 such as protons can be achieved in the manner of heteronuclear single- or multiple-quantum correlation (HSQC or HMQC) spectroscopy. The HMQC method relies on the excitation of two-spin coherences of the type T(11)(I)T(11)(S) and T(21)(I)T(11)(S) at the beginning of the evolution interval t(1). The spectra obtained by Fourier transformation from t(1) to omega(1) may be broadened by the homogenous decay of the transverse terms of the spy nuclei S. This broadening is mostly due to homonuclear dipolar S-S' interactions between the proton spy nuclei. In this work we have investigated the possibility of inserting rotor-synchronized symmetry-based C or R sequences and decoupling schemes such as Phase-Modulated Lee-Goldburg (PMLG) sequences in the evolution period. These schemes reduce the homonuclear proton-proton interactions and lead to an enhancement of the resolution of both SQ and DQ proton-detected (14)N HMQC spectra. In addition, we have investigated the combination of HSQC with symmetry-based sequences and PMLG and shown that the highest resolution in the (14)N dimension is achieved by using HSQC in combination with symmetry-based sequences of the R-type. We show improvements in resolution in samples of l-alanine and the tripeptide ala-ala-gly (AAG). In particular, for l-alanine the width of the (14)N SQ peak is reduced from 2 to 1.2 kHz, in agreement with simulations. We report accurate measurements of quadrupolar coupling constants and asymmetry parameters for amide (14)N in AAG peptide bonds.
在魔角旋转的固体中，可以通过像质子这样的自旋为 1/2 的间谍核 S 来实现间接检测单量子（SQ）和双量子（DQ）（14）N 谱（I=1），其方式类似于异核单或多量子相关（HSQC 或 HMQC）光谱学。HMQC 方法依赖于在演化间隔 t(1) 开始时激发类型为 T(11)(I)T(11)(S)和 T(21)(I)T(11)(S)的两自旋相干。通过从 t(1)到 omega(1)的傅里叶变换获得的谱可能由于间谍核 S 的横向项的均匀衰减而变宽。这种展宽主要是由于质子间谍核之间的同核偶极 S-S'相互作用。在这项工作中，我们研究了在演化期间插入基于转子同步的对称 C 或 R 序列和去耦方案（如相位调制 Lee-Goldburg（PMLG）序列）的可能性。这些方案减少了同核质子-质子相互作用，并导致 SQ 和 DQ 质子检测（14）N HMQC 谱的分辨率提高。此外，我们研究了 HSQC 与基于对称序列和 PMLG 的组合，并表明在（14）N 维中实现最高分辨率的方法是使用与基于对称序列的 HSQC 相结合，其中 R 型序列。我们在 l-丙氨酸和三肽 ala-ala-gly（AAG）的样品中展示了分辨率的提高。特别是对于 l-丙氨酸，（14）N SQ 峰的宽度从 2 降低到 1.2 kHz，与模拟结果一致。我们报告了 AAG 肽键中酰胺（14）N 的四极耦合常数和非对称参数的精确测量结果。
