J. Phys. Chem. A 2008, 112, 10823–10830
10823
Interactions of Mono- and Divalent Metal Ions with Aspartic and Glutamic Acid Investigated with IR Photodissociation Spectroscopy and Theory Jeremy T. O’Brien,† James S. Prell,† Jeff D. Steill,‡ Jos Oomens,‡ and Evan R. Williams*,† Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460, and FOM Institute for Plasma Physics “Rijnhuizen”, Edisonbaan 14, 3439 MN Nieuwegein, The Netherlands ReceiVed: July 1, 2008; ReVised Manuscript ReceiVed: August 8, 2008
The interaction of metal ions with aspartic (Asp) and glutamic (Glu) acid and the role of gas-phase acidity on zwitterionic stability were investigated using infrared photodissociation spectroscopy in the spectral range 950-1900 cm-1 and by hybrid density functional theory. Lithium ions interact with both carbonyl oxygen atoms and the amine nitrogen for both amino acids, whereas cesium interacts with both of the oxygen atoms of the C-terminus and the carbonyl oxygen of the side chain for Asp. For Glu, this structure is competitive, but a structure in which the cesium ion interacts with just the carbonyl oxygen atoms is favored and the calculated spectrum for this structure is more consistent with the experimentally measured spectrum. In complexes with either of these metal ions, both amino acids are non-zwitterionic. In contrast, Glu•Ca2+ and Glu•Ba2+ both adopt structures in which Glu is zwitterionic and the metal ion interacts with both oxygens of the C-terminal carboxylate and the carbonyl oxygen in the side chain. Assignment of the zwitterionic form of Glu is strengthened by comparisons to the spectrum of the protonated form, which indicate spectral features associated with a protonated amino nitrogen. Comparisons with results for glutamine, which adopts nearly the same structures with these metal ions, indicate that the lower ∆Hacid of Asp and Glu relative to other amino acids does not result in greater relative stability of the zwitterionic form, a result that is directly attributed to effects of the metal ions which disrupt the strong interaction between the carboxylic acid groups in the isolated, deprotonated forms of these amino acids. Introduction Metal ions and solvent molecules play important roles in the structure and reactivity of biomolecules.1,2 Many proteins require metal ions to fold and become active. Ion-protein interactions are involved in ion transport through cell membranes and in enzymatic catalysis of red-ox reactions. Competitive effects of solvent influence the interactions of metal ions with biomolecules. In the late 1800s, it was recognized that ions varied in their effects on fundamental properties of ionic solutions, such as their ability to denature proteins. Hofmeister ordered ions in their tendency to precipitate proteins or denature proteins,2 and even today, the origin of this Hofmeister effect is still debated.3-5 With mass spectrometry, it is possible to isolate and investigate the structures of cationized biomolecules6-50 and investigate how ions and water molecules competitively interact with biomolecules.18,22,47-49 Such experiments make possible a detailed understanding of competitive effects of ion and solvent interactions with biomolecules. Of particular interest to many groups has been the interaction of metal ions with amino acids and the effects of metal ions on the stability of the zwitterionic form of the amino acid.10-32,35,46-56 In isolation, all naturally occurring amino acids are non-zwitterionic, even the most basic amino acid, arginine, for which the zwitterionic structure is only 12-16 kJ/mol higher in energy.57-59 The zwitterionic form of an amino acid can be stabilized when a metal ion is attached.11-32,50-56 The zwitterionic form of arginine, in which * Address correspondence to this author. E-mail: williams@ cchem.berkeley.edu, FAX: (510) 642-7714. † University of California, Berkeley. ‡ FOM Institute for Plasma Physics “Rijnhuizen”.
the side-chain guanidinium group is protonated and the Cterminus is deprotonated, is most stable when complexed to sodium and the larger alkali metal ions,11,17,50 and the zwitterionic forms of other amino acids have been reported for both mono-19,27 and dications.14,16,32,53-56 Either increasing the proton affinity of the proton accepting site23-25,28,46,60,61 or increasing the acidity of the proton-donating group60 can potentially stabilize the zwitterionic form of a molecule. For example, the stabilities of the zwitterionic forms of amino acids with aliphatic side chains when complexed with sodium ions are directly related to the proton affinity of the amino acid.23,24 For amino acids that have one or more heteroatoms in their side chain, such as basic or acidic amino acids, the relation can be much less direct.11-13,17,28,30,58,59 For example, the charge-solvated forms of lithiated arginine11,17 and lithiated glycine52,53 in which the amino acids are nonzwitterionic are both ∼16 kJ/mol more stable than the corresponding zwitterionic forms despite the fact that the proton affinity of arginine is 160 kJ/mol higher than that of glycine.62 Heteroatoms in the side chain can result in significant stabilization of the charge-solvated structures.11-13,17,28,30,58,59 The roles of metal ion size, charge state, and amino acid basicity on the propensity for zwitterion formation have been extensively studied.11-32,46,52-56 In contrast, the role of gas-phase acidity on amino acid zwitterionic stability is less well known. The gas-phase acidities (∆Hacid) of the 20 naturally occurring R-amino acids range from ∼1435 kJ/mol for glycine to ∼1350 kJ/mol for aspartic (Asp) and glutamic (Glu) acid (Scheme 1), the two most acidic amino acids that have side-chain carboxylic acid groups.63,64By comparison, the gas-phase acidity of propanoic acid (CH3CH2COOH) is ∼1454 kJ/mol.65 This indicates
10.1021/jp805787e CCC: $40.75 2008 American Chemical Society Published on Web 10/02/2008
10824 J. Phys. Chem. A, Vol. 112, No. 43, 2008 SCHEME 1
O’Brien et al. TABLE 1: Photodissociation Products of Protonated and Cationized Aspartic and Glutamic Acid aspartic acid cation +
H
Li+
that the intrinsic acidity of the C-terminus is higher than that of the side-chain carboxylic acid group, consistent with calculations which indicate that the C-terminus is preferentially deprotonated in the isolated amino acid.63,64 The high acidity of Asp and Glu can be attributed to the high stability of the deprotonated forms of these amino acids in which the hydrogen atom of the side-chain carboxylic acid interacts with the C-terminal carboxylate.63,64 IR action spectroscopy can provide significant information about the structure of gas-phase ions and has been recently used to elucidate the structures of many cationized amino acids11-19,28-31,39 and amino acid complexes.34,35,40-45 Here, the results of IR action spectroscopy in the frequency range 950-1900 cm-1 and density functional theory calculations on Asp•M+, Glu•M+ (M ) H, Li, and Cs), Glu•Ca2+, and Glu•Ba2+ are presented. From these results, information about the effects of mono- and divalent cations on the structures of Asp and Glu and the role of gas-phase acidity on zwitterion stability is obtained. Experimental Section IR Photodissociation Spectroscopy. All experiments were performed on a 4.7 T Fourier-transform ion cyclotron resonance mass spectrometer. The instrument66 and experimental methods31 are described in detail elsewhere. In brief, cationized Asp and Glu were formed by electrospray ionization from a 60/40 water/ methanol solution of 2 mM amino acid and 2 mM metal chloride salt infused at a rate of 15-20 µL/min. Precursor ions were mass-selected using stored waveform inverse Fourier transforms and subsequently irradiated for 2.5 to 4.5 s using tunable irradiation from the Free-Electron Laser for Infrared eXperiments (FELIX).67 The same irradiation time was used for each ion and was selected based on the ion stability as indicated by photodissociation yield. Computations. Low-energy structures of cationized Asp and Glu were generated using Monte Carlo conformational searching and the MMFFs force field as implemented in Macromodel 9.1 (Schro¨dinger, Inc. Portland, OR). In the initial searches, no constraints were placed on the geometry, and 5000 conformers were generated for both non-zwitterionic and zwitterionic forms of Asp•Li+, Glu•Li+, Glu•Ca2+, and Glu•Ba2+. The resulting low-energy structures were sorted into different groups with similar metal coordination. Structures for cesiated complexes were generated by metal ion subsititution into lithiated structures prior to geometry optimization. Representative structures from each group were geometry optimized in Q-Chem V. 3.068 using hybrid method density functional calculations (B3LYP) with the CRENBL69 effective core potential for Cs, Ca, and Ba and the 6-31++G** basis set for all other atoms. Vibrational frequencies and intensities were calculated using the double harmonic approximation and the analytical Hessian of the energy-minimized structures using Q-Chem V. 3.0.68 Most structures yielded all positive frequency vibrations, but two of structures for the cesiated species of non-zwitterionic (NZ) Asp and Glu each had two imaginary frequencies
