Due to the development of two-dimensional Fourier transformation techniques (for reviews see Bax, 1982; Ernst et al. 1987), NMR spectroscopy has become a powerful tool for determining the 3D structures of small proteins (MW ≤ 10 kDa); for reviews see Wüthrich, 1986; Clore & Gronenborn, 1987. For larger molecules, however, the amount of detailed structural information that can be obtained using homonuclear 2D NMR techniques is limited because of the vast number of overlapping signals. In order to extend the capabilities of NMR to the study of larger systems, new approaches are required.

Structural studies of DNA-binding proteins and their complexes with DNA have proceeded at an accelerating pace in recent years due to important technical advances in molecular genetics, DNA synthesis, protein crystallography and nuclear magnetic resonance. The last major review on this subject by Pabo & Sauer (1984) summarized the structural and functional studies of the three sequence-specific DNA-binding proteins whose crystal structures were then known, the E. coli catabolite gene activator protein (CAP) (McKay & Steitz, 1981; McKay et al. 1982; Weber & Steitz, 1987), a cro repressor from phage λ (Anderson et al. 1981), and the DNA-binding proteolytic fragment of λcI repressor protein (Pabo & Lewis, 1982) Although crystallographic studies of the E. coli lac repressor protein were initiated as early as 1971 when it was the only regulatory protein available in sufficient quantities for structural studies (Steitz et al. 1974), little was established about the structural aspects of DNA-binding proteins until the structure of CAP was determined in 1980 followed shortly thereafter by the structure of λcro repressor and subsequently that of the λ repressor fragment. There are now determined at high resolution the crystal structures of seven prokaryotic gene regulatory proteins or fragments [CAP, λcro, λcI repressor fragment, 434 repressor fragment (Anderson et al. 1987), 434 cro repressor (Wolberger et al. 1988), E. coli trp repressor (Schevitz et al. 1985), E. coli met repressor (Rafferty et al. 1989)], EcoR I restriction endonuclease (McClarin et al. 1986), DNAse I (Suck & Ofner, 1986), the catalytic domain of γδ resolvase (Hatfull et al. 1989) and two sequence-independent double-stranded DNA-binding proteins [the Klenow fragment of E. coli DNA polymerase I (Ollis et al. 1985) and the E. coli Hu protein (Tanaka et al., 1984)].

The use of isotopic substitution is a time-honoured method for simplifying the nuclear magnetic resonance spectra of biological macromolecules. For example, the biosynthetic incorporation of a heteronucleus such as 15N or 13C into a specific amino acid residue in a protein followed by direct observation of the 15N or 13C NMR spectrum could provide a means to specifically observe a given amino acid type in that protein. By observation of the chemical shift or relaxation properties as a function of pH, ligand concentration, etc. a number of important conclusions concerning the pKa values of specific residues, the affinity of the protein for various ligands, or dynamic properties of the protein can be deduced. (See Henry et al. 1986 a, b; 1987 for an elegant modern example). In such situations, direct observation of the heteronucleus is a powerful means to observe environmental changes (Niu et al. 1979) but often these measurements are not readily interpretable in terms of alterations of protein structure. Although proton-proton dipolar interactions (NOEs) typically provide the richest source of such structural information, these interactions are not monitored in most experiments which directly observe the heteronucleus.

This article tries to be a compact summary of some recent research on cytochrome c oxidase (EC 1.9.3.1), an important enzyme in membrane bioenergetics. Cytochrome oxidase is the terminal catalyst of the mitochondrial respiratory chain. It uses the electrons flowing through the chain to reduce oxygen molecules to water. Four electrons and four protons are consumed in the reduction of O2 to two molecules of water (Fig. 1). Cytochrome oxidase contains four redoxactive metal centres. Two of these are copper atoms, two haem A groups. These four centres are employed in the dioxygen-binding site and in the electron-transferring pathways from cytochrome c. The enzyme is also called cytochrome aa3, because the protein-bound haems are functionally and spectroscopically different.

In order to study proteins that do not occur in two- or three-dimensionally ordered form, one may take two different approaches: either search for conditions that induce the formation of crystals, and proceed with the established methods of X-ray or electron crystallography, or attempt to study the molecules in the form of single particles with the EM. Although many proteins have been successfully crystallized, and some general recipes for inducing ordered arrangement have been found (Mannella, 1984; Uzgiris & Kornberg, 1983), there exists a large number of proteins and protein assemblies that have resisted such attempts for a long time. Furthermore, there are macromolecular assemblies, associated with membranes and engaged in switching or gating, whose function is tied to their occurrence in isolated form, and hence are best studied without extraction from the membrane. For these reasons, the single-particle approach to the study of macromolecular structure (Frank et al. 1978, 1981; Radermacher et al. 1987a, b; for recent reviews, see Frank et al. 1985, 1988e; Frank, 1989) has found numerous applications after initial technical and conceptual hurdles were overcome. Although atomic resolution cannot be achieved with this approach for a variety of reasons, a quantitative description of architecture on the quaternary level is nevertheless possible, as exemplified in the 3D studies of ribosomal particles (overviews, see Frank et al. 1988a) and, most recently, the junctional channel complex (Wagenknecht et al. 1989a).

Gram-negative bacteria such as Escherichia coli (E. coli) and Salmonella typhimurium (S. typhimurium) have two layers of membranes in the cellular envelope – the cytoplasmic membrane and the outer membrane (Fig. I). Between these membranes is a periplasmic space in which there is a peptidoglycan layer that provides the cells with mechanical rigidity. In this periplasmic space, there are also a variety of hydrolases and binding proteins. The composition of the outer membrane is somewhat unusual. This membrane bilayer is asymmetric, having an inner (periplasmic) leaflet composed of phospholipids and an outer (extracellular) leaflet formed by lipopolysaccharide (LPS). Unlike phospholipids having two acyl chains, LPS has six or seven saturated fatty acid chains (see reviews, Lugtenberg & Van Alphen, 1983; Nikaido & Vaara, 1985; Nakae, 1986). The head groups of LPS have a strong affinity for divalent cations such as Ca2+, and given a sufficient concentration of these ions the outer membrane can form quite a formidable permeability barrier through this head group/salt bridge network (Nikaido & Vaara, 1985). The function of the outer membrane is to serve as a protective envelope against hostile environments such as those in the intestinal tract of animals where harmful and toxic substances - for example, bile salts and various enzymes - are often found. The outer membrane itself would be impermeable to most hydrophilic solutes were it not for the presence of membrane channels. The presence of a large number of pore-forming proteins provides both specific and nonspecific diffusion pathways across the outer membrane for solutes such as nutrients and waste products to diffuse into or out of the cell.

In the last few years since the early NMR structural studies of small proteins such as glucagon (Braun et al. 1983) and lac represser headpiece (Zuiderweg et al. 1984) the quality of the structure determinations have improved considerably. Of major importance has been the introduction of phase sensitive detection in the Tl dimension (States et al. 1982; Marion & Wüthrich, 1983) which has allowed for absorption presentation of 2D data with the resulting enhancement in resolution, accuracy of coupling constant measurements and accuracy of peak volume integrations. Introduction of new pulse sequences, advances in instrumentation and further developments in the structure calculation algorithms have also helped improve the quality of NMR structural analyses of proteins.

The use of heteronuclear filters enables the editing of complex 1H nuclear magnetic resonance (NMR) spectra into simplified subspectra containing a lesser number of resonance lines, which are then more easily amenable to detailed spectral analysis. This editing is based on the creation of heteronuclear two-spin or multiple-spin coherence and discrimination between protons that do or do not participate in these heteronuclear coherences. In principle, heteronuclear editing can be used in conjunction with one-dimensional or multidimensional 1H-NMR experiments for studies of a wide variety of low-molecular-weight compounds or macromolecular systems, and is implicitely applied in a wide range of heteronuclear NMR experiments with proton detection (e.g. Bax et al. 1983; Griffey & Redfield, 1987). In the present article we shall focus on the use of heteronuclear filters in two-dimensional (2D) [1H, 1H]-NMR experiments. The selection of the material covered was primarily motivated by its impact on the practice of protein structure determination in solution, and on NMR studies of intermolecular interactions with biological macromolecules. Section 2 surveys potential applications of heteronuclear filters in this area. The remainder of the article is devoted to an introduction of the theoretical principles used in heteronuclear filters, and to a detailed description of the experimental implementation of these measurements. In writing the review we tried to minimize redundancy with the recent article in Quarterly Review of Biophysics by Griffey & Redfield (1987) and to concentrate on experiments that were introduced during the period 1986–9.

Antibody molecules secreted by B-lymphocytes play a central role in the immune defense systems of higher organisms. The major function of the antibody molecule is to bind specifically to foreign molecules (antigens) and to effect their inactivation and/or removal. Antibody molecules exist in millions of different forms, each with a unique amino acid sequence and combining site structure. Collectively called immunoglobulins (abbreviated as Ig), they form one of the major classes of proteins present in the blood, constituting 20% of the total plasma protein by weight.