General, Non-Base-Pairing Hydrogen Bonds

Where cyclic hydrogen-bond configurations occur, they are referred to as base pairs. They can be of the homo type, if the bases self-associate, or of the hetero type, if different bases interact with each other. The hydrogen-bonded base pairs can form isolated dimers and in many cases this dimerization is extended to two-dimensional ribbon patterns or to three-dimensional networks of hydrogen bonds (see Chap. 16). 

There is a considerable variety of hydrogen bonds in the crystal structures of purine and pyrimidine bases, far more than in the carbohydrates and amino acids. 

The percentage occurrence of different hydrogen bonds in a sample of 463 hydrogen bonds from 185 crystal structures is given in Tkble 7.14 their metrical characteristics are described in Part IB, Chapter 7. 

The proportion of three-center bonds in the pyrimidine, purine, and barbiturate crystal structures of about 30% is comparable to the number observed in the carbohydrate crystal structures (Thble 2.3). There is also a small proportion, about 1 %, of four-center bonds. A particular feature of the three-center bonds in these crystal structures is the occurrence of chelation, where both acceptor atoms are covalently bonded to the same atom(s), as in Fig. 15.7. 

These three-center bonds are invariably unsymmetrical with one strong bond (H 0, 1.9 to 2.4 A N-ft A, 145-170°) and one weak bond (H 0, 2.4 to 2.8 A N-ft A, 105-135 °). These unsymmetrical chelated structures are more a property of the molecular configuration and crystal packing than of the hydrogen bonds. In the absence of crystal packing forces, i.e., in solution, we doubt whether such chelation would be energetically favorable. 

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