is Xanthosine Xanthine plus ribose? I'm so confused. I used to think Xanthosine is Xanthine with a ribose instead of H but it appears to me as if it isn't. If you look at the picture in top-right of its wikipedia page you will see that the Imidazole is reversed from the picture in here also the picture in the bottom of this page in additional images section is like the second one. I'm sorry if didn't use the right chemical terms! I'm no chemist. Also I while I was searching I saw different images for Adenine and Guanine where the Imidazole loop was reversed in some by reversed I mean like the position of NH and N was reversed with respect to the Benzene loop! P.S. It would be great if you could also teach me the correct way to ask this question by means of the chemical terms I should use. P.S.S I hope it is not a wikipedia problem!

The two structures are tautomers of one another:

![enter image description here](

Tautomers are often considered the same compound even though they are really constitutional isomers. Technically they are called 7H-xanthine and 9H-xanthine, respectively (thanks to gilleain for the links).