3D NMR method

D-NMR methods have been extensively exploited to tackle problems in polymer science, however, the use of 3D-NMR techniques in polymer science has been rare. Several factors have discouraged the exploitation of these techniques in polymer science, including the scarcity and expense of isotopically labeled monomers and the relatively small amount of funded research compared to that performed in the biological area. Nevertheless, 3D-NMR method can be extremely useful for studying problems in polymer structure, mechanism and properties. In this review, we will summarize some of our work in this area in the hope that it will inspire others to use this methodology. 

In this chapter, a small sample of the 3D NMR methods available to identify structures and obtain resonance assignments from synthetic polymers will be presented. Once assignments are obtained, they can be used together with standard quantitative ID NMR to measure the compositions of structures present and to learn about the polymerization chemistry. 

Chai et al employed triple-resonance H/ C/ Si 3D-NMR methods to unambiguously assign the resonances in Cl (90) and G2 polycarbosilane dendrimers (91), using signals from one-bond and two-bond connectives among atoms coupled to C and Si. 

Another approach to obtain spatially selective chemical shift information is, instead of obtaining the entire image, to select only the voxel of interest of the sample and record a spectrum. This method called Volume Selective spectroscopY (VOSY) is a ID NMR method and is accordingly fast compared with a 3D sequence such as the CSI method displayed in Figure 1.25(a). In Figure 1.25(b), a VOSY sequence based on a stimulated echo sequence is displayed, where three slice selective pulses excite coherences only inside the voxel of interest. The offset frequency of the slice selective pulse defines the location of the voxel. Along the receiver axis (rx) all echoes created by a stimulated echo sequence are displayed. The echoes V2, VI, L2 and L3 can be utilized, where such multiple echoes can be employed for signal accumulation. 

The use of NMR continues to improve existing methods, and to develop new concepts. By cleverly combining existing pulse-sequences, new sequences are formed with improved properties. An example is the combination of the COSY and DOSY sequence to a new 3D-NMR COSY-IDOSY sequence with improved sensitivity, a 32-fold decrease in experiment time, and an improved resolution resulting in better data analysis [34]. 

The simplest experiments are those that focus just on the ligand. These are typically used to determine solution conformations or 3D structures of ligands. Homonuclear1H II ) or 21) NMR experiments are used mainly here. At the other end of the scale, experiments to study the macromolecular binding partner often require labeled protein and multidimensional NMR methods, as indicated on the right-hand side of Fig. 1. Finally, many NMR experiments provide information... 

In Fig. 1 we have highlighted with a dark background the different types of NMR methods that are used in drug-discovery projects. These include basic ID and 2D methods that are used to confirm the identity of peptides, determine their conformation, or derive restraint information used in 3D structure calculations (left side of Fig. 1). Methods to study binding interactions (middle section of Fig. 1) can be broadly categorized as being based on NOEs, diffusion, relaxation, or chemical shift changes. NOE-based methods include the transferred NOE... 

The method today is mostly applicable to relatively small molecules of up to 15,000 Daltons. With higher field NMR spectrometers and with practical 3D NMR, 25-30,000 Daltons will generally approach the limit (early- to mid-1990s). Figure 10.2 shows the 2D NMR spectrum of a double helical nucleic acid (MW 7000) while a stereo pair of the structure is shown in Figure 10.3. 

NMR methods have also been used extensively to determine the configuration and conformation of both moderate-size molecules and synthetic polymers, whose primary molecular structure is already known. During the past decade high resolution NMR, particularly employing 2D and 3D methods, has become one of only two methods (x-ray crystallography is the other) that can be used to determine precise three-dimensional structures of biopolymers—proteins, nucleic acids, and their cocomplexes—and NMR alone provides the structure in solution, rather than in the solid state. 

Stractnral conhrmation and complete NMR assignments were accomplished by 3D PFG NOESY-HMQC experiments nsing a C-emiched sample (Satake et al. 1995). Althongh the 3D NMR techniqne has become a rontine method for studies of protein and nucleotide, apphcations to natural products are rate. Unlike proteins, it is very difficult to enrich marine natural products with more than 90% abnndance. Moreover, the stractural elucidation of most natural products can be accomplished by 2D NMR experiments. MTX needed 3D NMR experiments becanse more than 200 proton signals give rise to over 2000 cross peaks in the 2D NOESY spectra. 

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