Structural codons: linearity/helicity interconversion by pyridine/pyrimidine exchange in molecular strandsElectronic supplementary information (ESI) available: spectroscopic (1H,13C-NMR, mass) data for compounds 6a,b; 7a,b and 8a,b. See http://www.rsc.org/suppdata/cc/b3/b311045a/