Zwitterions - Introduction

    Amino acids, peptides, and proteins typically contain both acidic and basic functional groups such as carboxyl and amino groups. Carboxylic acids with a pKa of ~5 can easily protonate an amine (pKb ~ 4) in aqueous solution and therefore molecules containing both carboxyl and amino groups are found to be zwitterions (both cations and anions) under near-neutral pH conditions. The reaction

occurring in water is exothermic by ΔH° = -13 kcal/mol.[1] In the absence of solvent, in the gas phase, the same reaction (1) is extremely endothermic by +144 kcal/mol.[2] The glycine zwitterion has been calculated to be unstable (no minimum on potential surface) by ~18 kcal/mol.[3] However, solvating the glycine zwitterion with water molecules makes it more stable than the solvated neutral form by ~11 kcal/mol.[4] Adding counter ions to zwitterions has a similarly stabilizing effect. For instance, adding a sodium ion to glycine makes the zwitterion a stable minimum on the potential energy surface, but it is still ~3 kcal/mol less stable than the sodiated neutral glycine.[5] The effect of adding alkali ions to glycine has been extensively studied by Hoyau and Ohanession,[6] who showed that the zwitterion structure is increasingly less stable with increasing alkali ion size (Na+ through Cs+). (The lithiated zwitterion does not follow this trend and is unusually unstable.) In the most stable structures, Na+ is solvated by the nitrogen and the carbonyl oxygen of neutral glycine (charge solvation structure CS1), whereas the larger alkali ions Rb+ and Cs+ are bound to the two oxygens of neutral glycine (CS2). For K+ both structures CS1 and CS2 are about equally stable. Hence, in glycine-like systems sodium is a good choice for stabilizing the zwitterion and only a few kcal/mol are missing, which could potentially be gained by increasing the proton affinity (PA) of the amine. Indeed it is found that for glycine-like systems there is a fairly linear relationship between PA and the zwitterion stability.[7] Increasing the PA by >5 kcal/mol over that of glycine makes the zwitterion the most stable structure and hence, alpha-aminoisobutyric acid is found by theory to be a zwitterion when sodiated. The linearity does not hold, however, for N-amidino glycine, where the zwitterion is not as stable as expected on the basis of the PA values, because the protonated guanidinium group does not line up as perfectly with the dipole of the
-COO-···M+ salt bridge as the protonated amine in glycine does.[8] Hence, charge-dipole alignment is an important factor as well for stabilizing cationized zwitterions. Similarly, the sodiated diglycine zwitterion is extremely unstable by ~14 kcal/mol and the sodiated tri- and tetraglycine zwitterions by ~13 kcal/mol.[5] In addition to the charge-dipole alignment factor in these larger systems, charge solvation becomes more effective and correspondingly charge solvation structures win out. The same effect is observed for sodiated triproline, where the zwitterion and the charge solvation structure are energetically about equal, compared to proNa+, where the zwitterion is more stable by ~5 kcal/mol.[9] As systems get larger and metal ion/proton charge solvation becomes "complete", it is expected that the zwitterion will start to benefit from further increasing system size until all three charges of the zwitterion are well solvated. After that neither charge solvation nor zwitterion structures benefit much by a further increase in system size with respect to self-solvation. Most of the data presented so far have been theoretical in nature. The question is: "Is there experimental evidence for the existence of zwitterions in the gas phase?" Williams and coworkers have presented a fair amount of data that is consistent with the presence of zwitterions in the gas phase. For instance, the thermal dissociation kinetics of protonated bradykinin (BK), a nonapeptide containing two arginine residues, is very different from that of protonated BK methyl ester. BK loses NH3 with an activation barrier (Ea) of 1.3 eV, whereas the methyl ester loses MeOH with an Ea of 0.6 eV.[10] The methyl ester cannot form a zwitterion because the only acidic site in BK is methylated. Similarly, in SORI-CAT and BIRD experiments, argCs+ loses NH3, whereas argLi+ loses H2O and arg-OMeCs+ loses MeOH, suggesting that argCs+ is a zwitterion.[11] Another example is hydrated valine. The rate constant for water loss from (val + Li + 2 H2O)+ is nearly identical to that of the corresponding ala-OEt system but very different from that of betaine. However, the kinetics for (val + Li + 3 H2O)+ are neither close to ala-OEt nor betaine suggesting a different structure than either reference system. Theoretical work indicates that the best candidate for such a different structure is a zwitterion.[12] Wesdemiotis and coworkers investigated the structures of amino acids by the kinetic method. They found that clusters of an alkali ion (X+) with an amino acid (AA) and an amino acid methyl ester (AA + AA-OMe + X)+ tend to lose AA-OMe if AA is a zwitterion.[13] For instance, they conclude that proNa+ and argCs+ are zwitterions.

Bowers group members who have worked on these projects include: