Macromolecular design, nucleic acid junctions, and crystal formation.

The simplest form of macromolecular design involves the ligation of nucleic acids. Recent results on the concatenation of nucleic acid junctions show that these molecules can act as fairly rigid macromolecular valence clusters on the nanometer scale. These clusters can be joined to form closed stick figures in which each edge is double helical DNA or RNA and each vertex is a nucleic acid junction. The geometrical criteria for forming discrete-closed and periodic structures from these components are established. The helicity of each edge limits the possible structures that can be formed. The formation of a periodic array from nucleic acid junction building blocks is compared with the crystallization of molecular systems. This comparison leads to a new interpretation of the nature of order in the solid state for molecular crystals. The suggestion is made that the structure of a solid molecular system described by the fewest unique orthogonal (Fourier) components is the one which will be entropically favored, since it contains the least information. This is the crystalline state, with a small number of molecules per asymmetric unit. The free energy from the proposed entropic driving force responsible for this behavior is available, in principle, to correct small deviations from ideality in forming covalent crystals from nucleic acid junction components, as well as in non-bonded molecular systems. Nucleic acid junction periodic arrays provide an appropriate vehicle with which to test this interpretation.