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Why Pentose and Not Hexose Nucleic Acids? Part I . Introduction to the Problem, Conformational Analysis of Oligonucleotide Single Strands Containing 2′,3′‐Dideoxyglucopyranosyl Building Blocks (‘Homo‐DNA’), and Reflections on the Conformation of A‐ and B‐DNA Summary in collaboration with Prof. Dr. C.E. Wintner , Haverford College, Haverford, PA 19041‐1392; academic guest, ETH, March and June/July, 1991. Chemical rationalization of the structure of a biomolecule can be sought through consideration of two criteria: first, the relationship between the structure and its biological function; and second, the structure's potential for constitutional self assembly. The latter criterion convers the judgment, by chemical reasoning, of the chance of its preformation, that is, a synthetic event which must have been undergone by any molecular structure in order to have been selected (or to have selected itself) to become a biomolecule. One way to further the task of rationalizing a biomolecule's structure by experimental means is the design, synthesis, and study of structural alternatives which might have become biomolecules on the basis of either criterion, but which do not, in fact, appear in Nature today. In the formation of sugar phosphates from glycolaldehyde phosphate under basic conditions, straightforward and selective formation of rac ‐hexose 2,4,6‐triphosphates is observed in the absence of formaldehyde, while rac ‐pentose 2,4‐diphosphates are dominant, when (0.5 equiv.) formaldehyde is present 1. This and other observations indicate that hexose sugars should be regarded to have had a chance of preformation comparable with that of pentose sugars. Why, then, did Nature choose pentoses and not hexoses as the sugar building blocks of nucleic acids? The reason must be functional; it must be the case that pentose nucleic acids are biologically superior to potential hexose alternatives. To the extent that biological function is a consequence of molecular structure and reactivity, the origin of this superiority should be decipherable through chemical experiment, that is, through synthesis of hexose nucleic acids, systematic study of their chemical properties, and comparison of these properties with those of their natural counterparts. This has been the object of the present investigation, initiated in 1986. The paper introduces a series of papers which will describe the results of a model study, namely, the synthesis, pairing properties and structure of homo‐DNA oligonucleotides.
Eschenmoser et al. (Wed,) studied this question.