Helicity Coding: Programmed Molecular Self-Organization of Achiral Nonbiological Strands into Multiturn Helical Superstructures: Synthesis and Characterization of Alternating Pyridine-Pyrimidine Oligomers
作者:Masakazu Ohkita、Jean-Marie Lehn、Gerhard Baum、Dieter Fenske
DOI:10.1002/(sici)1521-3765(19991203)5:12<3471::aid-chem3471>3.0.co;2-5
日期:1999.12.3
Alternating pyridine-pyrimidine oligomers 1, 2, and 3, composed of nineteen, twenty one, and twenty seven heterocycles, respectively, have been synthesized in stepwise fashion and characterized. Examination of their H-1 NMR spectra revealed that these achiral nonbiological oligomers organize spontaneously into multiturn helical structures 1A-3A in solution, as indicated by marked upfield shifts of the aromatic protons coupled with distinct NOE effects. In view of their potential electron-acceptor properties compounds 1-3 also represent coiled wires of nanometric lengths, up to about 90 Angstrom for 3 in its extended form. The helical structure has been confirmed for 1 in the solid state by X-ray crystallography; a chiral channel arrangement involving only one helical enantiomer was found despite of the lack of chiral center in the strand. The oligomers exhibit a broad structureless fluorescence with a large Stokes shift, attributable to intramolecular pyridine excimer emission resulting from the helical ordering. Variable-temperature H-1 NMR experiments revealed that the oligomers exist as equilibrating mixtures of right-handed and left-handed helices. The barrier for helical handedness reversal was found to be independent on the length of the strand; two- (6), three- (1), and four-turn (3) helices showed comparable free energies of activation. Based on these observations, a stepwise folding mechanism involving two perpendicularly twisted pyridine-pyrimidine units in the transition states may be proposed for the helicity inversion processes. The present results together with earlier work indicate that the pyridine-pyrimidine sequence may be considered as a helicity codon, enforcing the formation of helical structures on the basis of: intramolecular structural information.