SELF-RECOGNITION AND SELF-ASSEMBLY OF FOLIC-ACID SALTS - COLUMNAR LIQUID-CRYSTALLINE POLYMORPHISM AND THE COLUMN GROWTH-PROCESS

Alkaline folates in water, with or without added NaCl, form columnar mesophases; the columns are composed of a stacked array of folate tetramers held together by Hoogsteen-type hydrogen bonds and stacking interactions. There is a considerable difference in both the liquid crystalline polymorphism and the columnar characteristics in pure water and those in 1 mol L(-1) NaCl solutions. In pure water, the tetramer-tetramer interaction is weak, the columnar length changes slowly with the folate concentration, and only the hexagonal mesophase is observed. In 1 mol L(-1) NaCl, the tetramer-tetramer interaction is stronger, a cholesteric mesophase is observed, and, even at relatively low folate concentration, the length of the columns is significant (ca. 20 disks at the isotropic-to-cholesteric transition). In both cases, the columns are finite at all concentrations investigated. The formation of a cholesteric mesophase with a pitch of ca. 14 mu m indicates that the tetramers do not pile up in register but are rotated one with respect to the other to give a chiral column similar both to the four-stranded helix of poly(G) and to the columnar aggregates formed by homoguanylic oligodeoxynucleotides.