Moore Scott J., Iwamoto Marian, Marzilli Luigi G.
Department of Chemistry, Emory University, Atlanta, Georgia 30322.
Inorg Chem. 1998 Mar 23;37(6):1169-1174. doi: 10.1021/ic971299y.
Imidazole rings are involved in acid/base chemistry, catalysis, H-bonding, and metal complexation throughout biochemistry; these rings are frequently targets for anticancer drugs and carcinogens. However, interpreting the changes in (13)C NMR shifts of these rings is often difficult. We explore the use of high-resolution one-bond (1)H-(13)C coupling constants ((1)J(CH)) for the identification of electronic changes within imidazole rings of samples containing (13)C in natural abundance. The reverse detection method used, called J-coupled heteronuclear multiple quantum coherence (JHMQC) spectroscopy, employs a modified HMQC pulse sequence. The method was evaluated with B(12) models of the type Me(3)BzmCo(DH)(2)(R or X), where Me(3)Bzm = 1,5,6-trimethylbenzimidazole and DH = the monoanion of dimethylglyoxime. (1)J(CH) values of Me(3)BzmCo(DH)(2)CH(3) obtained from both JHMQC and standard coupled 1D (13)C NMR spectra led to similar values, but the JHMQC method gave better resolution and much higher signal-to-noise ratios. The (1)J(CH) values for the endocyclic carbons and the N-methyl group of the Me(3)Bzm in five models fell between those of the free and protonated Me(3)Bzm ligands. Thus, donation of electron density from Me(3)Bzm to the Co center typically increases (1)J(CH) values with respect to the free ligand. Values of (1)J(CH) for several (13)C NMR signals correlated with both EP, a reported measure of electron-donating ability of R or X, and Co-N bond lengths from X-ray structures. For the assessment of electronic properties of the metal center, the (1)J(CH) values appear to be more reliable parameters than the traditionally used (13)C shifts, especially for C's close to the metal. Moreover, (1)J(CH) values for the (13)C signals for Co-(13)C were observed in several models; the (13)C signals for these carbons attached to the quadrupolar cobalt are too broad for (1)J(CH) determination by the traditional 1D method. The JHMQC method developed here is thus very versatile and can provide information on any type of molecule showing resolved CH signals.
咪唑环在整个生物化学过程中都参与酸碱化学、催化、氢键形成和金属络合作用；这些环常常是抗癌药物和致癌物的作用靶点。然而，解读这些环的（13）C NMR化学位移变化往往很困难。我们探索利用高分辨率一键（1）H - （13）C耦合常数（（1）J（CH））来识别天然丰度含（13）C样品中咪唑环内的电子变化。所采用的反向检测方法，即J耦合异核多量子相干（JHMQC）光谱法，采用了一种改进的HMQC脉冲序列。该方法用Me（3）BzmCo（DH）（2）（R或X）类型的B（12）模型进行评估，其中Me（3）Bzm = 1,5,6 - 三甲基苯并咪唑，DH = 二甲基乙二肟单阴离子。从JHMQC和标准耦合一维（13）C NMR光谱获得的Me（3）BzmCo（DH）（2）CH（3）的（1）J（CH）值相近，但JHMQC方法具有更好的分辨率和更高的信噪比。五个模型中Me（3）Bzm的环内碳和N - 甲基的（1）J（CH）值介于游离和质子化Me（3）Bzm配体的（1）J（CH）值之间。因此，从Me（3）Bzm到Co中心的电子密度捐赠相对于游离配体通常会增加（1）J（CH）值。几个（13）C NMR信号的（1）J（CH）值与EP（一种报道的衡量R或X给电子能力的指标）以及X射线结构中的Co - N键长都相关。对于评估金属中心的电子性质，（1）J（CH）值似乎是比传统使用的（13）C化学位移更可靠的参数，特别是对于靠近金属的C。此外，在几个模型中观察到了Co - （13）C的（13）C信号的（1）J（CH）值；与四极钴相连的这些碳的（13）C信号对于通过传统一维方法测定（1）J（CH）来说太宽了。因此，这里开发的JHMQC方法非常通用，可以为任何显示出分辨出的CH信号的分子提供信息。
