Ring opened analogues of adenine nucleosides substituted at the ninth adenine positions, with and without esterification (compounds I and II), have been studied kinetically and thermodynamically at various temperatures in order to determine the sites of ring opened analogues. These are deemed to be important for binding to the adenosine deaminase. Adenosine deaminase is found to bind more strongly to compound I than to compound II, therefore compound I is a stronger inhibitor than II, because the position of (5') OH on the ribose moiety decreases the inhibitory strength on the ring opened analogue.