Please look at the following below links which may help you in your analysis:

https://pubs.acs.org/doi/abs/10.1021/om200580t?src=recsys&journalCode=orgnd7 https://pubs.acs.org/doi/10.1021/ed069p375

Thanks

binding of metal ion causes, in general, a change in the local electron density distribution of the binding site, so the protons attached to/in proximity of the binding site experience a different electronic environment, which results in a different effective Zeeman field at these protons in presence of the binding effect and hence altered chemical shifts, depending on the extent of distortion of the local electronic distribution brought about by the binding process.

It sepends, as a binding of metal ion changes the local environment, or local spin distribution if you will. Such change could be either an upfielf or a downfield. So it's really case dependent. But generally as you emntioned, if there is a change in ppm, there should be something happening. It can be either a binding site or an allosteric/conformational change site.

Great! I understand.

Thank you all kind explanation!

Isam Eldin Hussein Elgailani

Debopam Ghosh

Xiaoran Liu