Temperature-Resolved Proton Transfer Reactions of Biomolecular Ions

Abstract

Temperature-resolved proton transfer reactions of multiply-protonated angiotensin I, disulfide-intact and -reduced lysozyme, and ubiquitin ions to primary, secondary and aromatic amines were examined in the gas phase. Absolute reaction rate constants for the proton transfer were determined from the intensities of the parent and product ions in mass spectra. Dramatic changes were observed in the distribution of product ions and the reaction rate constants. In particular, the rate constants for disulfide-intact lysozyme ions changed more drastically with the change in charge state and temperature compared to the corresponding values for disulfide-reduced ions. Proton transfer reactions were enhanced or suppressed as the result of the formation of complexes between the ions with gaseous molecules, which is related to changes in their conformation with changing.

Keywords: peptide ions; protein ions; proton transfer reaction; temperature dependence.

Fig. 1. (a) Time-of-flight mass spectra of [Ang+2H]²⁺ reacted with 1-Bu at various temperatures. (A) Ions of a specific charge state, [Ang+2H]²⁺, were selected with QMASS as a reactant. (B)–(H) Mass spectra of [Ang+2H]²⁺ reacted with 1-Bu at various temperatures. (b) In the reaction of [Ang+2H]²⁺ with 1-Bu, the branching ratios of parent and product ions are plotted as a function of temperature in the gas cell. (Reproduced with permission from ref .)

Fig. 2. Reaction rate constants of proton transfer from [Ang+2H]²⁺, to 1-Bu, Dpr, 35Dmpy, and 26Dmpy are plotted as a function of temperature in the gas cell. (Adapted with permission from ref .)

Fig. 3. Time-of-flight mass spectra of disulfide-intact lysozyme ions for an 11+ charge state, [Lys_i+11H]¹¹⁺ reacted with 26Dmpy at various temperature. (A) Mass spectrum of all ions produced with ESI, where DC voltage in QMASS was put off. (B) Ions of a specific charge state, [Lys_i+11H]¹¹⁺, were selected with QMASS as a reactant. (C)–(G) Mass spectra of [Lys_i+11H]¹¹⁺ reacted with 26Dmpy at various temperatures. Temperature of gas cell was (C) 460, (D) 410, (E) 360, (F) 310, and (G) 290 K, respectively. (Reproduced with permission from ref .)

Fig. 4. The reaction rate constants for disulfide-intact and -reduced lysozyme ions, [Lys_i+zH]^z+ and [Lys_r+zH]^z+ respectively, are plotted as a function of temperature in the gas cell, for comparing the same charge states. The rate constants for 7+, 8+, 9+, 10+, 11+, and 12+ charge states are presented in (a), (b), (c), (d), (e), and (f), respectively. In the figures, letters represented “I” indicate the rate constants of [Lys_i+zH]^z+, whereas letters represented “R” indicate those of [Lys_r+zH]^z+. (Reproduced with permission from ref .)

Fig. 5. Absolute reaction rate constants for proton transfer for [Ubi+zH]^z+ (z=6–12) are plotted as a function of temperature in the gas cell. (Adapted with permission from ref .)