Hydrogen/deuterium exchange mass spectrometry was employed to probe the conformational changes in lysozyme (Lyz) during the course of formation of a protein-protected atomically precise Au8 cluster. MALDI MS showed the protein, Lyz, to be present in a denatured state in the cluster. Detailed ESI MS analysis of the Au-attached Lyz adducts, an intermediate of cluster formation, confirmed that these conformational changes are brought about by Au-S bond formation and a similar conformation is retained in the final cluster. These results were supported by computational results, which showed an increase in solvent accessible surface area upon the formation of the adducts. Infrared spectroscopy established that change in the rate as well as the extent of the hydrogen/deuterium exchange observed in the cluster was due to the change in the amide II region of the encapsulating protein. Hydrogen/deuterium exchange ESI MS of Cu adducts of Lyz showed a lower degree of denaturation than their Au counterparts. XPS analysis revealed that Cu binds differently to Lyz than it does with Au, which is likely because of the stronger soft-soft Au-S interaction. Alkali metal ion binding, on the other hand, does not affect the protein conformation because such ions do not affect the disulfide bonds. © 2019 American Chemical Society.