Chemical Properties
Hoogsteen pairs have quite different properties from Watson-Crick base pairs. The angle between the two glycosylic bonds (ca. 80° in the A• T pair) is larger and the C1′–C1′ distance (ca. 860 pm or 8.6 Å) is smaller than in the regular geometry. In some cases, called reversed Hoogsteen base pairs, one base is rotated 180° with respect to the other. In some DNA sequences, especially CA and TA dinucleotides, Hoogsteen base pairs exist as transient entities that are present in thermal equilibrium with standard Watson–Crick base pairs. The detection of the transient species required the use of NMR techniques that have only recently been applied to macromolecules.
Hoogsteen base pairs have been observed in protein–DNA complexes. Some proteins have evolved to recognize only one base-pair type, and use intermolecular interactions to shift the equilibrium between the two geometries. DNA has many features that allow its sequence-specific recognition by proteins. This recognition was originally thought to primarily involve specific hydrogen-bonding interactions between amino-acid side chains and bases. But it soon became clear that there was no identifiable one-to-one correspondence — that is, there was no simple code to be read. Part of the problem is that DNA can undergo conformational changes that distort the classical double helix. The resulting variations in the way that DNA bases are presented to proteins can thus affect the recognition mechanism. It has become evident that distortions in the double helix are themselves dependent on base sequence. This enables proteins to recognize DNA shape in a manner reminiscent of the way that they recognize other proteins and small ligand molecules. For example, stretches of A and T bases can narrow the minor groove of DNA (the narrower of the two grooves in the double helix), thus enhancing local negative electrostatic potentials and creating binding sites for appropriately placed, positively charged arginine amino-acid residues.
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