What is the difference between xDNA and yDNA?

Question

Wikipedia states that:

xDNA contains expanded bases, in which a benzene ring has been added, which may pair with canon bases, resulting in four possible base-pairs (8 bases:xA-T,xT-A,xC-G,xG-C, 16 bases if the unused arrangements are used). Another form of benzene added bases is yDNA, in which the base is widened by the benzene.

In this context, does widened and expanded refer to the same thing? If so, what would be the exact different between yDNA and xDNA.

NOTE: I am not referring to the X and Y chromosomes.

Answer 1

yDNA came first:

Lee & Kool (2005) A New Four-Base Genetic Helix, yDNA, Composed of Widened Benzopyrimidine−Purine Pairs. J Am Chem. Soc. 127: 3332-3338

As seen in this Figure from the paper it is a DNA consisting of T and C substituted with yT and yC, widened analogues.

xDNA came next (also from the Kool lab.):

Lynch et al. (2006) Toward a Designed, Functioning Genetic System with Expanded-Size Base Pairs: Solution Structure of the Eight-Base xDNA Double Helix. J Am Chem. Soc. 128: 14704-14711

As seen in this Figure from the paper, xDNA consists of an 8 base system A,C,G,T,xA,xC,xG,xT.

This field is reviewed in:

The XNA world: progress towards replication and evolution of synthetic genetic polymers Pinheiro & Holliger (2012) The XNA world: progress towards replication and evolution of synthetic genetic polymers. Current Opinion in Chemical Biology 16:245-252