xDNA (also known as expanded DNA or benzo-homologated DNA) is a size-expanded nucleotide system synthesized from the fusion of a benzene ring and one of the four natural bases: adenine, guanine, cytosine, and thymine. This size expansion produces an 8 letter alphabet which has a larger information storage capacity than natural DNA's (often referred to as B-DNA in literature) 4 letter alphabet. As with normal base-pairing, A pairs with xT, C pairs with xG, G pairs with xC, and T pairs with xA. The double helix is thus 2.4Å wider than a natural double helix. While similar in structure to B-DNA, xDNA has unique absorption, fluorescence, and stacking properties.
Initially synthesized as an enzyme probe by Nelson J. Leonards group, benzo-homologated adenine was the first base synthesized. Later, Eric T. Kools group finished synthesizing the remaining three expanded bases, eventually followed by yDNA ("wide" DNA), another benzo-homologated nucleotide system, and naphtho-homologated xxDNA and yyDNA. xDNA is more stable when compared to regular DNA when subjected to higher temperature, and while entire strands of xDNA, yDNA, xxDNA and yyDNA exist, they are currently difficult to synthesize and maintain. Experiments with xDNA provide new insight into the behavior of natural B-DNA. The extended bases xA, xC, xG, and xT are naturally fluorescent, and single strands composed of only extended bases can recognize and bind to single strands of natural DNA, making them useful tools for studying biological systems. xDNA is most commonly formed with base pairs between a natural and expanded nucleobase, however x-nucleobases can also be paired together. Current research supports xDNA as a viable genetic encoding system in the near future.