xDNA and yDNA are new classes of synthetic nucleic acids characterized by having base-pairs with one of the bases larger than the natural congeners. Here these larger bases are called x- and y-bases. We recently investigated and reported the structural and electronic properties of the x-bases (Fuentes-Cabrera et al. J. Phys. Chem. B 2005, 109, 21135-21139). Here we extend this study by investigating the structure and electronic properties of the y-bases. These studies are framed within our interest that xDNA and yDNA could function as nanowires, for they could have smaller HOMO-LUMO gaps than natural DNA. The limited amount of experimental structural data in these synthetic duplexes makes it necessary to first understand smaller models and, subsequently, to use that information to build larger models. In this paper, we report the results on the chemical and electronic structure of the y-bases. In particular, we predict that the y-bases have smaller HOMO-LUMO gaps than their natural congeners, which is an encouraging result for it indicates that yDNA could have a smaller HOMO-LUMO gap than natural DNA. Also, we predict that the y-bases are less planar than the natural ones. Particularly interesting are our results corresponding to yG. Our studies show that yG is unstable because it is less aromatic and has a Coulombic repulsion that involves the amino group, as compared with a more stable tautomer. However, yG has a very small HOMO-LUMO gap, the smallest of all the size-expanded bases we have considered. The results of this study provide useful information that may allow the synthesis of an yG-mimic that is stable and has a small HOMO-LUMO gap.