Temperature-dependent H/D exchange of compact and elongated cytochrome c ions in the gas phase
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Isotopic exchange reactions of compact and elongated conformations of gaseous cytochrome c ions (+5 and +9 states) with D2O have been measured as a function of temperature (from 300 to ∼440 K) using ion mobility techniques. Rate constants for those sites that exchange at high temperatures (>400 K) are about an order of magnitude smaller than rate constants for sites that exchange at 300 K. Although the exchange rates decrease, the maximum exchange levels for rapidly exchanging sites increase with temperature. At 300 K, exchange levels of 53 ± 3 and 63 ± 3 are measured for the compact and elongated states, respectively. From 300 to 335 K, the exchange levels increase slightly to ∼60 to 70 hydrogens. Above 335 K, the levels increase to a value of ∼200 for the +5 state and ∼190 for the +9 state, near the maximum possible levels, 200 and 204 for these respective charge states. Molecular dynamics simulations have been carried out on structures having calculated cross sections that are near the experimental values in order to explore the exchange process. Overall, it appears that charge site and exchange site proximities are important factors in the exchange profiles for the elongated +9 ion and the compact +5 ion.