Comparison of the metal ion coordinating properties of tubercidin 5'-monophosphate (7-deaza-AMP) with those of adenosine 5'-monophosphate (AMP) and 1,N6-ethenoadenosine 5'-monophosphate (.epsilon.-AMP). Definite evidence for metal ion-base-backbinding to N-7 and extent of macrochelate formation in M(AMP) and M(.epsilon.-AMP)