NMR Coupling versus NMR Chemical Shift Information in Metallobiochemistry. High-Resolution One-Bond 1H−13C Coupling Constants Obtained by a Sensitive Reverse Detection Method

Abstract

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.