The DNA fragment d(CpGpCpGpCpG) crystallises as a left-handed double helical molecule with Watson-Crick base pairs and an antiparallel organisation of the sugar phosphate chains. The helix has two nucleotides in the asymmetric unit and contains twelve base pairs per turn. It differs significantly from right-handed B-DNA.

Molecular structure of a left-handed double helical DNA fragment at atomic resolution

The structures of the three major classes of biopolymers-polynucleotides, polypeptides and polysaccharides-are surveyed in the light of the most recent results from X-ray diffraction, electron microscopy and other studies. The review contains an introductory section setting out the basic features of each class of biopolymer: the natures of the 'building blocks', the linkages between them, conformational principles, types of repeating (secondary) structures and so on. The main section on each class is deliberately selective, because of the large amount of structural information now available. The section on polynucleotides concentrates on DNA, reviewing the classical double-helical models and the evidence provided by X-ray fibre diffraction, but also paying special attention to the rapid growth of new information on such features as the variations in helix geometry and in base-stacking, those data being derived primarily from studies of small self-complementary oligonucleotides. Alternative models and the question of right-handed versus left-handed structures are also considered. Among polypeptide structures, both globular and fibrous proteins are described. Globular proteins are treated as a group, with no detailed description of any one of them; this section concentrates on the organisation of globular proteins via folding units and domains, some general features (protein homology, catalysis, flexibility, interaction with water), membrane proteins, immunoglobulins and viruses.

REVIEW ARTICLE: Biopolymers

Here we review studies that provided important information about conformational properties of DNA using circular dichroic (CD) spectroscopy. The conformational properties include the B-family of structures, A-form, Z-form, guanine quadruplexes, cytosine quadruplexes, triplexes and other less characterized structures. CD spectroscopy is extremely sensitive and relatively inexpensive. This fast and simple method can be used at low- as well as high-DNA concentrations and with short- as well as long-DNA molecules. The samples can easily be titrated with various agents to cause conformational isomerizations of DNA. The course of detected CD spectral changes makes possible to distinguish between gradual changes within a single DNA conformation and cooperative isomerizations between discrete structural states. It enables measuring kinetics of the appearance of particular conformers and determination of their thermodynamic parameters. In careful hands, CD spectroscopy is a valuable tool for mapping conformational properties of particular DNA molecules. Due to its numerous advantages, CD spectroscopy significantly participated in all basic conformational findings on DNA.

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Circular dichroism and conformational polymorphism of DNA

Salt-induced co-operative conformational change of a synthetic DNA: Equilibrium and kinetic studies with poly(dG-dC)☆

Conformation of two-stranded DNA in H2O–methanol, H2O–ethanol, H2O–isopropanol, and H2O–dioxane solutions at different concentrations of alkaline ions has been studied with the aid of circular dichroism. The following conclusions are drawn:

The conformation of DNA in H2O and H2O–methanol belongs to a family of B forms (B, C, T forms are the representatives of the family). The magnitude of the winding angle between adjacent base pairs (θ) is determined by the concentration and type of the cations. In H2O the cation action is nonspecific and leads to an increase in θ value. In 80% methanol the ions act specifically, Cs+ being to stabilize a form with a greater θ value, and Li+ being with a lesser one. The total θ change is likely within the limits of 33° ⩽ θ ⩽ 45°.

At high content of ethanol, isopropanol, or dioxane (∼80%), but not with methanol, and in low ionic strength the conformation of DNA belongs to a family of A forms (A form is one of the members of the family) and is specified by the concentration and type of cation involved. The two-stranded regions of RNA in H2O are also of A type and winds with the rise of cation concentration. The range of θ variation is not narrower than 30° ⩽ θ 33°.

The conformational transitions within the families (induced by ions) are of non-cooperative pattern, wheras the transitions between the families (induced by nonpolar component) are of cooperative pattern. The effect of cations, when specific, is discussed on the basis of steric correspondence between the width of DNA narrow groove and the size of a hydrated cation.

Different conformations of double‐stranded nucleic acid in solution as revealed by circular dichroism

X-ray diffraction and circular dichroism studies indicate that poly d(I–C).poly d(I–C) exists in a new polynucleotide configuration—an eight-fold helix, possibly left-handed.

Physical and enzymatic studies on poly d(I-C)-poly d(I-C), an unusual double-helical DNA.

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Physical and enzymatic studies on poly d(I-C)-poly d(I-C), an unusual double-helical DNA.