Valentine Stephen J, Clemmer David E
Beyond Genomics, Inc, Waltham, Massachusetts, USA.
J Am Soc Mass Spectrom. 2002 May;13(5):506-17. doi: 10.1016/S1044-0305(02)00372-0.
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 approximately 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 approximately 60 to 70 hydrogens. Above 335 K, the levels increase to a value of approximately 200 for the +5 state and approximately 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.
利用离子迁移技术，测量了气态细胞色素c离子（+5和+9价态）紧密构象和细长构象与D2O的同位素交换反应随温度（从300到约440K）的变化情况。高温（>400K）下发生交换的位点的速率常数比300K下发生交换的位点的速率常数小约一个数量级。尽管交换速率降低，但快速交换位点的最大交换水平随温度升高。在300K时，紧密态和细长态的交换水平分别测得为53±3和63±3。从300到335K，交换水平略有增加，达到约60到70个氢原子。在335K以上，+5价态的交换水平增加到约200，+9价态增加到约190，接近这些电荷态各自的最大可能水平200和204。为了探究交换过程，对计算截面接近实验值的结构进行了分子动力学模拟。总体而言，电荷位点和交换位点的接近程度似乎是细长+9离子和紧密+5离子交换分布的重要因素。
