Electron capture by a hydrated gaseous peptide: effects of water on fragmentation and molecular survival

Abstract

The effects of water on electron capture dissociation products, molecular survival, and recombination energy are investigated for diprotonated Lys-Tyr-Lys solvated by between zero and 25 water molecules. For peptide ions with between 12 and 25 water molecules attached, electron capture results in a narrow distribution of product ions corresponding to primarily the loss of 10-12 water molecules from the reduced precursor. From these data, the recombination energy (RE) is determined to be equal to the energy that is lost by evaporating on average 10.7 water molecules, or 4.3 eV. Because water stabilizes ions, this value is a lower limit to the RE of the unsolvated ion, but it indicates that the majority of the available RE is deposited into internal modes of the peptide ion. Plotting the fragment ion abundances for ions formed from precursors with fewer than 11 water molecules as a function of hydration extent results in an energy resolved breakdown curve from which the appearance energies of the b 2 (+), y 2 (+), z 2 (+*), c 2 (+), and (KYK + H) (+) fragment ions formed from this peptide ion can be obtained; these values are 78, 88, 42, 11, and 9 kcal/mol, respectively. The propensity for H atom loss and ammonia loss from the precursor changes dramatically with the extent of hydration, and this change in reactivity can be directly attributed to a "caging" effect by the water molecules. These are the first experimental measurements of the RE and appearance energies of fragment ions due to electron capture dissociation of a multiply charged peptide. This novel ion nanocalorimetry technique can be applied more generally to other exothermic reactions that are not readily accessible to investigation by more conventional thermochemical methods.

Figure 1

Electrospray ionization mass spectra of KYK from aqueous solution showing hydrated ions of the doubly protonated peptide, (KYK + 2H)(H₂O)_n ²⁺, and a minor distribution of the singly protonated peptide, (KYK + H)(H₂O)_n ⁺, with the temperature of the copper block that surrounds the interface capillary at (a) ~95 °C and (b) ~80 °C.

Scheme 1

Fragmentation Pathways for (KYK + 2H)²⁺ upon Electron Capture

Figure 2

Representative electron capture dissociation mass spectra for (a) (KYK + 2H)(H₂O)²⁺ and (b) (KYK + 2H)(H₂O) ²⁺ Insets are ×20 and ×6 expansions of the spectral regions indicated in a) and b), respectively; asterisks (*) indicate noise peaks.

Figure 3

Fragment abundances for major ECD product ions of (KYK + 2H)(H₂O) _n ²⁺ and suspect fragment ions formed by consecutive reactions of these ions, plotted as a function of n.

Figure 4

Normalized abundances of dissociation products from intact reduced (KYK + 2H)(H₂O)n ²⁺ and reduced ions that have lost a hydrogen atom, plotted as a function of n. Each curve is the sum of the indicated major product ion and its subsequent neutral losses (Scheme 1). The internal energy deposition scale, determined from the number of water molecules lost in ECD of these ions (see text), is used to establish an energy resolved breakdown curve for fragment ion formation; note that this scale is nonlinear above 78 kcal/mol.

Figure 5

SORI-CAD spectra of (KYK + H)⁺ formed directly by nanoelectrospray ionization with two different excitation conditions corresponding to maximum translational energies in the laboratory frame, E_max trans, of 2.2 (top) and 2.6 eV (bottom).

Figure 6

ECD spectra of (a) (KYK + 2H)(H₂O) 20₂₊and (b) (KYK + 2H)(H₂O)₂₅²⁺. Numbers, n, above ECD product ion peaks indicate major products (KYK + 2H)(H₂O)_n^+•. Insets are ×7 expansions of the relative abundances of the spectral regions indicated; asterisks (*) indicate noise peaks.

Figure 7

Average number of water molecule lost (○) and retained (■) upon ECD of (KYK + 2H)(H₂ O)_n²⁺, plotted as a function of n. The line represents a least-squares fit to the water molecule retention data for n ≥ 11.

Figure 8

Thermochemistry for electron capture by (KYK + 2H)(H₂O)_n ²⁺ deduced from ion nanocalorimetry experiments, where the recombination energy is determined from the number of water molecules lost from the reduced precursor (see text). Appearance energies (kcal/mol) relative to the reduced precursor are indicated in parentheses.

Figure 9

Representative low-energy structures of (KYK + 2H)²⁺, (KYK + 2H)(H₂O)₁₀ ²⁺, and (KYK + 2H)(H₂O)₂₅²⁺ obtained from 10 000 conformer Monte Carlo searches using the MMFFs force field (similar structures were identified using the OPLS force field), with the amino groups of both lysine sidechains protonated; these groups are circled for ease of identification.

Figure 10

Normalized abundances of (KYK + 2H)^+• (including water retention) and (KYK + H)⁺ formed by EC of (KYK + 2H)(H₂O)_n²⁺ as a function precursor ion size, n. The abundances of ions formed by subsequent loss of ammonia from (KYK + 2H)^+• or water from (KYK + H)⁺ have been included in the normalized abundance of their respective precursors.