Solid-State Hydrogen-Deuterium Exchange Mass Spectrometry (ssHDX-MS) of Lyophilized Poly-d,l-Alanine.

Solid-state hydrogen-deuterium exchange mass spectrometry (ssHDX-MS) has been developed to study proteins in amorphous solids, but the relative contributions of protein structure and protein-matrix interactions to exchange are not known. In this work, short unstructured poly-d,l-alanine (PDLA) peptides were colyophilized with sucrose, trehalose, mannitol, sodium chloride, or guanidine hydrochloride to quantify the contributions of protein-matrix interactions to deuterium uptake in ssHDX-MS in the absence of a higher order structure. Deuterium incorporation differed with the excipient type and relative humidity (RH) in D2O(g), effects that were not observed in solution controls and are not described by the Linderstrom-Lang model for solution HDX. A reversible pseudo first-order kinetic model for deuterium uptake in ssHDX-MS is proposed. The model agrees with the experimentally observed dependences of the apparent deuteration rate and plateau value on RH in ssHDX-MS of PDLA and reduces to the Linderstrom-Lang limit when the forward rate of exchange is much greater than the reverse rate.