Metal Cation Dependence of Interactions with Amino Acids: Bond Energies of Rb⁺ to Gly, Ser, Thr, and Pro

Abstract

The interactions of rubidium cations with the four amino acids (AA), glycine (Gly), serine (Ser), threonine (Thr), and proline (Pro), are examined in detail. Experimentally, the bond energies are determined using threshold collision-induced dissociation of the Rb(+)(AA) complexes with xenon in a guided ion beam tandem mass spectrometer. Analyses of the energy dependent cross sections include consideration of unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. 0 K bond energies of 108.9 +/- 7.0, 115.7 +/- 4.9, 122.1 +/- 4.6, and 125.2 +/- 4.5 kJ/mol are determined for complexes of Rb(+) with Gly, Ser, Thr, and Pro, respectively. Quantum chemical calculations are conducted at the B3LYP, B3P86, and MP2(full) levels of theory with geometries and zero point energies calculated at the B3LYP level using both HW*/6-311+G(2d,2p) and Def2TZVP basis sets. Results obtained using the former basis sets are systematically low compared to the experimental bond energies, whereas the latter basis sets show good agreement. For Rb(+)(Gly), the ground state conformer has the rubidium ion binding to the carbonyl group of the carboxylic acid, and a similar geometry is found for Rb(+)(Pro) except the secondary nitrogen accepts the carboxylic acid hydrogen to form the zwitterionic structure. Both Rb(+)(Ser) and Rb(+)(Thr) are found to have tridentate binding at the B3LYP and MP2(full) levels, whereas the B3P86 slightly prefers binding Rb(+) at the carboxylic acid. Comparison of these results to those for the lighter alkali ions provides insight into the trends in binding affinities and structures associated with metal cation variations.

Compounds of Amino Acids as Anions

“…3.8 left) and cis-bis (glycinato)-platinum (Pozhidaev et al 1975;Fig. 3 In addition to the salts listed in Table 3.1, Bowman et al (2010) reported a rubidium salt of glycine (along with serine, threonine, and proline analogs) by mass spectrometry, although no crystals suitable for structure analysis were obtained.…”

“…Among these were the sodium and potassium salts of phenylalanine, tyrosine, and tryptophan (Ruan and Rodgers 2004), whose molecular structures were assessed via mass spectroscopy. Likewise, lithium, sodium, and potassium prolinates as well as rubidium prolinate (Bowman et al 2010) were characterized by the same method.…”

Thermochemistry of Non-Covalent Ion–Molecule Interactions

“…Relative measurements of the interactions of lithium [72][73][74] and sodium 73,75) cations with most amino acids have been performed using the kinetic method, 76,77) with absolute thermochemistry obtained using GIBMS for all the alkali metal cations with several amino acids: glycine (Gly), [78][79][80][81][82] proline (Pro), [81][82][83] serine (Ser), 81,82,84) threonine (Thr), 81,82,84) cysteine (Cys), 82,85) methionine (Met), 86) aspartic acid (Asp), asparagine (Asn), glutamic acid (Glu), glutamine (Gln), 25,87) phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp). 88) In all cases, the TCID experiments yield quantitative BDEs that are generally consistent with quantum chemical values.…”

“…As for any other ligands, 5) the BDEs increase as the size of the metal cation decreases. 81,82) This is shown in Fig. 3 for Na + and K + , the alkali metals for which the most data are available, largely because these are the most relevant biologically.…”

Experimental and Theoretical Investigation of the Proton-Bound Dimer of Lysine

“…Naturally occurring amino acids are well known to exist as zwitterions in the solid state and aqueous solution within a wide range of pH values, whereas in the gas phase the isolated amino acids are exclusively in the canonical form. However, interactions with other molecules or ions can play a key role in stabilizing the zwitterionic form of amino acids, as demonstrated by many previous studies [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].…”