Using HMBC and ADEQUATE NMR data to define and differentiate long-range coupling pathways: is the Crews rule obsolete?

It is well known that as molecules become progressively more proton-deficient, structure elucidation becomes correspondingly more challenging. When the ratio of (1)H to (13)C and the sum of other heavy atoms falls below 2, an axiom that has been dubbed the "Crews rule" comes into play. The general premise of the Crews rule is that highly proton-deficient molecules may have structures that are difficult, and in some cases impossible, to elucidate using conventional suites of NMR experiments that include proton and carbon reference spectra, COSY, multiplicity-edited HSQC, and HMBC (both (1)H-(13)C and (1)H-(15)N). However, with access to modern cryogenic probes and microcyroprobes, experiments that have been less commonly utilized in the past and new experiments such as inverted (1)J(CC) 1,n-ADEQUATE are feasible with modest sized samples. In this light, it may well be time to consider revising the Crews rule. The complex, highly proton-deficient alkaloid staurosporine (1) is used as a model proton-deficient compound for this investigation to highlight the combination of inverted (1)J(CC) 1,n-ADEQUATE with 1.7 mm cryoprobe technology.

NMR-based approaches for lead discovery.

NMR methods have long been used for studying molecular interactions. In the last few years, various NMR approaches have been developed to aid lead discovery. These involve different NMR screening methods to identify initial compounds, which often bind only weakly (in the micro- to millimolar range) to the drug target. Intelligent and focused follow-up strategies enable the development of these compounds into potent, submicromolar drug-like inhibitors for use as leads in drug discovery projects. NMR can be used as both a remarkably reliable screening tool and a structural tool; thus, this technique has unique opportunities for lead discovery.

Non-peptidic small-molecule inhibitors of the single-chain hepatitis C virus NS3 protease/NS4A cofactor complex discovered by structure-based NMR screening.

NMR-based screening of a customized fragment library identified 16 small-molecule hits that bind weakly (K(D) approximately 100 microM to 10 mM) to substrate binding sites of the NS4A-bound NS3 protease of the hepatitis C virus (HCV). Analogues for five classes of NMR hits were evaluated by a combination of NMR and biochemical data yielding SAR and, in most cases, optimized hits with improved potencies (K(D) approximately K(I) approximately 40 microM to 1 mM). NMR chemical shift perturbation data were used to establish the binding location and orientation of the active site directed scaffolds in these five analogue series. Two of these scaffolds, which bind the enzyme at the proximal S1-S3 and S2' substrate binding sites, were linked together producing competitive inhibitors of the HCV NS3 protease with potencies in the micromolar range. This example illustrates that the low molecular weight scaffolds discovered from structure-based NMR screening can be optimized with focused structure-guided chemistry to produce potent nonpeptidic small-molecule inhibitors of the HCV NS3 protease.

Application of fragment-based NMR screening, X-ray crystallography, structure-based design, and focused chemical library design to identify novel microM leads for the development of nM BACE-1 (beta-site APP cleaving enzyme 1) inhibitors.

Fragment-based NMR screening, X-ray crystallography, structure-based design, and focused chemical library design were used to identify novel inhibitors for BACE-1. A rapid optimization of an initial NMR hit was achieved by a combination of NMR and a functional assay, resulting in the identification of an isothiourea hit with a K(d) of 15 microM for BACE-1. NMR data and the crystal structure revealed that this hit makes H-bond interactions with the two catalytic aspartates, occupies the nonprime side region of the active site of BACE-1, and extends toward the S3 subpocket (S3sp). A focused NMR-based search for heterocyclic isothiourea isosteres resulted in several distinct classes of BACE-1 active site directed compounds with improved chemical stability and physicochemical properties. The strategy for optimization of the 2-aminopyridine lead series to potent inhibitors of BACE-1 was demonstrated. The structure-based design of a cyclic acylguanidine lead series and its optimization into nanomolar BACE-1 inhibitors are the subject of the companion paper

Unambiguous structural characterization of hydantoin reaction products using 2D HMBC NMR spectroscopy.

Data from two-dimensional (2D) NMR experiments were used to identify the reaction products resulting from the opening of pyroglutamates with isocyanates or thioisocyanates. The reaction has the potential to produce compounds that would have very similar one-dimensional proton ((1)H) or carbon-13 ((13)C) NMR spectra. Careful analysis of (1)H--(1)H COSY, (1)H--(1)H NOESY, and HMBC data, including chemical shifts and coupling constants, were used to distinguish correctly between carbamoyl-2-pyrrolidinone, hydantoin, and perhydro-1,3-diazepine-2,4-dione type structures that could result from this reaction. This work describes their preparation and subsequent identification using 2D NMR spectroscopy, and includes complete (13)C assignments of the reaction products. The 2D NMR techniques and analysis described here can be applied successfully to other synthetic reactions with the potential to produce isomeric products.

Screening of protein kinases by ATP-STD NMR spectroscopy.

ATP-STD NMR takes advantage of Mg2+ binding to ATP to adjust the ATP affinity for protein kinases permitting a wide range of Ki's to be determined for ATP competitive ligands. Substituting Mn2+ for Mg2+ creates a paramagnetic probe (MnATP) from which the proximity of non-ATP competitive ligands can be inferred. Internal standards and references are used to reduce false positives due to protein or compound degradation. Use of the natural ATP ligand confers active site-specificity that is not available a priori from other ligand binding experiments.

Characterization of autocatalytic conversion of precursor BACE1 by heteronuclear NMR spectroscopy.

Accumulation of the cytotoxic 40- to 42-residue beta-amyloid peptide represents the primary pathological process in Alzheimer's disease (AD). BACE1 (beta-site APP cleaving enzyme 1) is responsible for the initial required step in the neuronal amyloidogenic processing of beta-amyloid precursor protein and is a major drug target for the therapeutic intervention of AD. In the present study, BACE1 is initially synthesized as an immature precursor protein containing part of the pre domain and the entire pro domain, and undergoes autocatalytic conversion to yield the well-folded mature BACE1 enzyme. To understand the mechanism of the conversion and the role of the pro domain, we monitored the autocatalytic conversion of BACE1 by heteronuclear NMR spectroscopy and used chemical shift perturbations as a probe to study the structural changes accompanying the autocatalytic conversion. NMR data revealed local conformational changes from a partially disordered to a well-folded conformation associated with the conversion. The conformational changes are largely concentrated in the NH(2)-terminal lobe. Conversely, the active site conformations are conserved during the autocatalytic conversion. The precursor and mature BACE1 proteins were further characterized for their ability to interact with a substrate-based transition state BACE1 peptide inhibitor. The precursor BACE1 rapidly adopted the bound conformation in the presence of the inhibitor, which is identical to the bound conformation of the mature protein. The interaction of the inhibitor with both the precursor BACE1 and the fully processed BACE1 is in slow exchange on the NMR time scale, indicating a tight binding interaction. Overall, the NMR data demonstrated that the pro domain does not hinder inhibitor binding and may assist in the proper folding of the protein. The fully processed BACE1 represents a high quality well-folded protein which is highly stable over a long period of time, and is suitable for evaluation of inhibitor binding by NMR for drug intervention.

Flexible lid to the p53-binding domain of human Mdm2: implications for p53 regulation.

The stabilization of p53 against Mdm2-mediated degradation is an important event in DNA damage response. Initial models of p53 stabilization focused on posttranslational modification of p53 that would disrupt the p53-Mdm2 interaction. The N-terminal regions of both p53 and Mdm2 are modified in vivo in response to cellular stress, suggesting that modifications to Mdm2 also may affect the p53-Mdm2 interaction. Our NMR studies of apo-Mdm2 have found that, in addition to Mdm2 residues 25-109 that form the well ordered p53-binding domain that was observed in the p52-Mdm2 complex, Mdm2 residues 16-24 form a lid that closes over the p53-binding site. The Mdm2 lid, which is strictly conserved in mammals, may help to stabilize apo-Mdm2. It also competes weakly with peptidic and nonpeptidic antagonists. Modifications to the Mdm2 lid may disrupt p53-Mdm2 binding leading to p53 stabilization. Mdm2 and Mdm4 possess nearly identical p53-binding domains but different lids suggesting that lid modifications may select for p53 binding.