Final answer:
The heterogeneous base attached to IMP is deprotonated N1.
Step-by-step explanation:
The heterogeneous base attached to IMP is not specifically mentioned in the provided information. However, based on the information given, it can be inferred that the binding site for IMP is deprotonated N1, as the H2 and H8 signals of IMP undergo a downfield shift. This suggests that the binding site is deprotonated N1, since N7 binding to a purine base usually shifts the H8 signal downfield by approximately 0.5 ppm, leaving the H2 shift almost unchanged.