Hydrogen/Deuterium Exchange Reaction Rate of Cytochrome <i>c</i> Determined by Droplet Collision Atmospheric Pressure Infrared\nLaser Ablation Mass Spectrometry

The hydrogen/deuterium (H/D) exchange\nrate is an optimal measure\nfor studying the structures and dynamics of hydrogen bonding systems,\nas it reflects the molecular contact environment and the strength\nof the hydrogen bonds. A method for rapid measurement of the H/D exchange\nreaction rates is required to examine the intermolecular environments\nof molecules in solutions. We developed a droplet collision atmospheric\npressure infrared laser ablation mass spectrometry technique for this\npurpose. We obtained the H/D exchange reaction rate of cytochrome <i>c</i> in a methanol/H<sub>2</sub>O·D<sub>2</sub>O solution.\nWe revealed that the first hydration shell of the cytochrome <i>c</i> molecule hinders the penetration of D<sub>2</sub>O to\nthe surface of the molecule from the rates, which provides a novel\nmethod to investigate solution structures by a mass-spectrometric\nmethod. The droplet-collision mass spectrometry method developed in\nthe present study can be extended to research on the molecular interactions\nin solutions, such as the mutual interactions of protein molecules,\nwhich are of importance in living cells.