Base pairs hydrogen-bonded structures

Base-pair hydrogen bonding of the Watson-Crick type is fundamental in all biological processes where nucleic acids are involved. These processes, which are chiefly DNA replication and protein biosynthesis [650, 651], were understood only at the molecular level when Watson and Crick discovered the three-dimensional structure of DNA [27, 527J. This structure consists of two polynucleotide chains running in opposite directions (antiparallel), and twisted into a right-handed double helix. The hydrophobic purine and pyrimidine bases are stacked in the center... 

At higher concentrations they also interact with the nucleic acid bases so that they can even destroy the base pair hydrogen bonding, and compromise the structural integrity of the nucleic acid polymers [90-92]. 

Abstract IR spectroscopy of nucleobases in the gas phase reflects simultaneous advances in both experimental and computatitMial techniques. Important properties, such as excited state dynamics, depend in subtle ways on structure variations, which can be followed by their infrared signatures. Isomer specific spectroscopy is a particularly powerful tool for studying the effects of nucleobase tautomeric form and base pair hydrogen-bonding patterns. 

In the previous Sections, the properties of acids and bases in macrocycles and other concave structures have been compared. A number of factors have been recognized which influence the acidity or basicity of an acid or base (i) hydrogen bonds, (ii) hindered solvation (exclusion of solvent), (iii) formation of tight ion pairs (high microacidity but low overall acidity), and (iv) Coulomb forces when poly anions are formed. A fifth influence, (v) steric hindrance, still has to be discussed. 

It is reasonable to assume that the complementary units form the expected triply hydrogen bonded pairs, so that the entirely different behaviour of the pure compounds and of the 1 1 mixtures may be attributed to the spontaneous association of the complementary components into a polymolecular entity based on hydrogen bonding. The overall process may then be described as the self-assembly of a supramolecular liquid-crystalline polymer based on molecular recognition (Figure 40). The resulting species (TP2, TU2) is represented schematically by structure 174. 

Fig. 20.22. Detail of Fig. 20.21, showing the amino acid stem of tRNAoln. The side-chain Leul36 opens the U1-A72 base pair, and the structure of the C74-C75-A76 loop is stabilized by several hydrogen bonds. From [715]... 

The key to DNA s functioning is its double-helical structure with complementary bases on the two strands. The bases form hydrogen bonds to each other, as shown in Fig. 22.38. Note that the structures of cytosine and guanine make them perfect partners for hydrogen bonding, and they are always found as pairs on the two strands of DNA. Thymine and adenine form similar hydrogen-bonding pairs. 

Which base will hydrogen-bond with uracil within an RNA molecule Draw the structure of this base pair. 

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