Infrared spectroscopy of cationized arginine in the gas phase: direct evidence for the transition from nonzwitterionic to zwitterionic structure

Abstract

The gas-phase structures of protonated and alkali metal cationized arginine (Arg) and arginine methyl ester (ArgOMe) are investigated with infrared spectroscopy and ab initio calculations. Infrared spectra, measured in the hydrogen-stretch region, provide compelling evidence that arginine changes from its nonzwitterionic to zwitterionic form with increasing metal ion size, with the transition in structure occurring between lithium and sodium. For sodiated arginine, evidence for both forms is obtained from spectral deconvolution, although the zwitterionic form is predominant. Comparisons of the photodissociation spectra with spectra calculated for low-energy candidate structures provide additional insights into the detailed structures of these ions. Arg*Li+, ArgOMe*Li+, and ArgOMe*Na+ exist in nonzwitterionic forms in which the metal ion is tricoordinated with the amino acid, whereas Arg*Na+ and Arg*K+ predominately exist in a zwitterionic form where the protonated side chain donates one hydrogen bond to the N terminus of the amino acid and the metal ion is bicoordinated with the carboxylate group. Arg*H+ and ArgOMe*H+ have protonated side chains that form the same interaction with the N terminus as zwitterionic, alkali metal cationized arginine, yet both are unambiguously determined to be nonzwitterionic. Calculations indicate that for clusters with protonated side chains, structures with two strong hydrogen bonds are lowest in energy, in disagreement with these experimental results. This study provides new detailed structural assignments and interpretations of previously observed fragmentation patterns for these ions.

Figure 1

Schematic diagram of the Berkeley external nanospray ion source 2.75 T Fourier-transform ion cyclotron resonance mass spectrometer (side view) and tunable infrared laser system (top view). MP^a and MP^b are 8 and 24 L/s mechanical pumps, respectively, and CPs are 1500 L/s cryopumps. Detailed descriptions of the nanospray interface and ion cell are available elsewhere.

Figure 2

Experimental infrared photodissociation spectra of Arg•M⁺, M = H, Li, Na, and K, and ArgOMe•M⁺, M = H, Li, and Na. Copper jacket temperatures are marked on the respective experimental spectra. All spectra are corrected for background blackbody infrared radiative dissociation and laser power.

Figure 3

Blackbody and photofragmentation kinetics of Arg•Li⁺ and Arg•Na⁺ obtained at a copper jacket temperature of 403 K. Kinetic data were obtained with both species of interest present in the ion cell simultaneously and without isolation of the precursor ions.

Figure 4

Energy minimized structures of the nonzwitterionic and zwitterionic forms of Arg•Na⁺ from B3LYP/LACVP++** calculations. Zero-point energy corrected 0 K energies (left of slash) and free energies at the temperatures of the relevant experiments (right of slash) are in kJ/mol and reported relative to the lowest-energy structures for Arg•Na⁺ and analogous Arg•M⁺, M = H, Li, and K, ions with similar structures. Arg•H⁺ structures, have protonated side chains and despite being exclusively nonzwitterionic, are compared to the zwitterionic, metal-cationized structures with similar hydrogen bonds.

Figure 5

B3LYP/LACVP++** 0 K (zero-point energy corrected) energies of Arg•M⁺, M = Li, Na, and K, relative to that of the lowest-energy nonzwitterionic structure (A). Free energies at 404 K of Arg•M⁺, M = Li, Na, and K, relative to that of the lowest-energy nonzwitterionic structure (A) are shown in Supplemental Figure 1 (see Supporting Information).

Figure 6

Infrared photodissociation spectrum of Arg•Li⁺ obtained with a copper jacket temperature of 404 K and B3LYP/LACVP++** calculated vibrational spectra for the three lowest-energy Arg•Li⁺ nonzwitterionic structures (A–C) and three low-energy zwitterionic structures (D, E, and G); 0 K energies (left of slash) and free energies at 404 K (right of slash) are reported relative to the lowest-energy structure (A). Calculated vibrational spectra for all structures are shown in Supplemental Figure 2 (see Supporting Information).

Figure 7

Infrared photodissociation spectra of Arg•Li⁺, ArgOMe•Li⁺, and ArgOMe•Na⁺ and B3LYP/LACVP++** calculated vibrational spectra for structures A and C of ArgOMe•Li⁺ and structure A of ArgOMe•Na⁺. The spectrum for Arg•Li⁺ has been expanded by a factor of 5 to show the lower intensity bands. Copper jacket temperatures are marked on the respective experimental spectra; 0 K energies (left of slash) and free energies at 477 K (right of slash) of ArgOMe•Li⁺ are reported relative to the lowest-energy structure (A).

Figure 8

Infrared photodissociation spectrum of ArgOMe•H⁺ obtained with a copper jacket temperature of 477 K and B3LYP/LACVP++** calculated vibrational spectra for the three lowest-energy ArgOMe•H⁺ structures (D–F); 0 K energies (left of slash) and free energies at 477 K (right of slash) are reported relative to the lowest-energy structure (D).

Figure 9

Infrared photodissociation spectrum of Arg•H⁺ obtained with a copper jacket temperature of 471 K and B3LYP/LACVP++** calculated vibrational spectra for all low-energy Arg•H⁺ structures (D–G and I). Zwitterionic structures of Arg•H⁺ analogous to structures G and I were not calculated to be stable; 0 K energies (left of slash) and free energies at 471 K (right of slash) are reported relative to the lowest-energy structure (D).

Figure 10

Infrared photodissociation spectrum of Arg•Na⁺ obtained with a copper jacket temperature of 404 K and B3LYP/LACVP++** calculated vibrational spectra for two low-energy Arg•Na⁺ nonzwitterionic structures (A and C) and three low-energy zwitterionic structures (D, E, and H); 0 K energies (left of slash) and free energies at 404 K (right of slash) are reported relative to the lowest-energy structure (D). Calculated vibrational spectra for all structures are shown in Supplemental Figure 3 (see Supporting Information).

Figure 11

Photodissociation spectra for Arg•Li⁺ and ArgOMe•H⁺, which serve as reference spectra for the nonzwitterionic and zwitterionic forms of Arg•Na⁺, respectively, are added to form a composite spectrum that closely resembles the experimental spectrum of Arg•Na⁺ in this region.

Figure 12

Infrared photodissociation spectrum of Arg•K⁺ obtained with a copper jacket temperature of 404 K and B3LYP/LACVP++** calculated vibrational spectra for the lowest-energy nonzwitterionic structure (A) and four low-energy zwitterionic structures (D–F, and H) of Arg•K⁺; 0 K energies (left of slash) and free energies at 404 K (right of slash) are reported relative to the lowest-energy structure (D). Calculated vibrational spectra for all structures are shown in Supplemental Figure 4 (see Supporting Information).

Figure 13

Conceptualized potential energy surfaces for structural isomerization between the nonzwitterionic (NZ) and zwitterionic (Z) forms of Arg•M⁺, M = Li, Na, and K, and the loss of neutral molecules from the two forms of these ions. The surface for Arg•Na⁺ illustrates how structural isomerization and water loss can be the major reaction pathway even thought the zwitterionic form is the predominate structure.