## 4 Glycol and Butyl Nucleic Acids (GNA and BuNA)
After such irradiation, the reference TpT dinucleotide lost 50% of its absorbance at 270 nm. Several reports indicate a potential application of GNA oligomers in nanotechnology. Since GNA provides easy access to both left- and right-handed helical geometries, these molecules can potentially be used to construct nanostructures with unique topologies. [PERSON] and co-workers published a report on the synthesis of two 4-helix junctions (4 HJ) with mirror-image symmetry using (\(R\))- or (\(S\))-GNA monomers [47]. The CD spectra for both GNA 4 HJ nanostructures were similar to previous CD data collected on double-stranded GNA duplexes. This indicates that short helices were present in each arm of the 4 HJ. However, the spectra differed significantly from spectra typical of natural DNA and RNA helices, suggesting that the GNA helices have a unique helical structure. Melting experiments showed that the GNA 4 HJ was significantly more stable than the corresponding DNA 4 HJ (76 vs. 37 \({}^{\circ}\)C). The aforementioned artificial hydroxypyridone nucleobase and C6-(pyridin-2-yl)purine, acting as either homo- or hetero-base pairs, were introduced into a GNA duplex 3\({}^{\prime}\)-AATATATTTTA-2\({}^{\prime}\)/2\({}^{\prime}\)-TTATAATATAAAAAT-3\({}^{\prime}\) and tested for their ability to accommodate a range of metal cations [48]. For all three combinations of base pairs, the addition of CuSO\({}_{4}\) and NiCl\({}_{2}\) resulted in a huge increase (by 18-37 \({}^{\circ}\)C) in the thermal stability of the duplexes, compared to the same GNA duplex in the absence of the transition metal salt. These duplexes were also significantly more stable than the reference duplex with X=A and Y=T. Smaller effects were found for AgNO\({}_{3}\), Co(NO\({}_{3}\))\({}_{2}\) and ZnCl\({}_{2}\), and the effects for Pd(NO\({}_{3}\))\({}_{2}\) and AuCl\({}_{3}\) were negligible. The authors suggested that such metallo-GNAs can be used for the generation of duplexes with interesting electronic and magnetic properties. Metal-dependent tuning of the thermal stability of GNA duplexes was found for GNA nucleotides with diphenylporphyrinacetylide (R, **22**, Chart 6) [49]. When the porphyrin acetylide-labeled GNA unit (R) is incorporated opposite an abasic ethylene glycol site (H), the resultant duplexes, e.g.,  
3\({}^{\prime}\)-TAAAAAATRATAATAT-2\({}^{\prime}\)  
2\({}^{\prime}\)-ATTTTTTAHTATAAA-3\({}^{\prime}\)  
or  
3\({}^{\prime}\)-TTATAAAAAATRHTAAATATAT-2\({}^{\prime}\)  
2\({}^{\prime}\)-AATATTTTAHRTATAAATTA-3\({}^{\prime}\)  
are thermally destabilized, but the overall structures of the GNA duplexes remain very little changed. Interestingly, the thermal stabilities of such R:H base pairs can be modulated by the addition of Zn\({}^{2+}\) and Ni\({}^{2+}\), as these cations decrease and increase the duplex stability, respectively. Such porphyrin-GNA conjugates can potentially be used for light collection and charge transport. A stabilizing effect (\(\Delta\)Tm = 10 \({}^{\circ}\)C) of electrostatic complementarity between the electron-deficient perylene diimide (B, **23**, Chart 6) and the relatively electron-rich porphyrin (B), which serve as the nucleobases in the middle of the 16-mer GNA duplex 3\({}^{\prime}\)-TAAAAAATHTRATAATT-2\({}^{\prime}\)/2\({}^{\prime}\)-ATTTTTABHATTAAA-3\({}^{\prime}\) (H stands for the abasic site), has been reported [50]. However, while 2, 4 or 5 consecutive chromophores (arranged in a zipper-like inter-strand alternating fashion) provided an approximately cumulative effect (\(\Delta\)Tm = 16-25 \({}^{\circ}\)C), only \(\Delta\)Tm = 3 \({}^{\circ}\)C was observed for a set HRH/BHB. Several experiments were performed with GNA units located within DNA oligomers and carrying unnatural nucleobases.