On the sodium and lithium ion affinities of some α‐amino acids

The decomposition of 59 different cluster ions (generated by fast atom bombardment) consisting of two different amino acids and a sodium ion was analysed. The only fragment ions of significant abundance could be assigned to sodium ion‐bound amino acids. Assuming that the most abundant ion in the fragment ion spectrum corresponds to the amino acid with the highest sodium ion affinity (SIA), the 20 common α‐amino acids could be ordered with increasing sodium ion affinity as follows: Gly, Ala, Cys, Val, (Leu, Ile), Ser, Met, Thr, (Phe, Pro), Asp, Tyr, (Glu, Lys), Trp, Asn, Gln, His, Arg. Quantitative determinations were carried out by comparison of the lithium ion affinity (LIA) of Ala with that of dimethylformamide (DMF) in a fragment ion scan of the ion‐bound dimer Ala—Li⁺—DMF. LIA(Ala) was calculated from LIA(Ala) = LIA(DMF) – (1/C)ln[I(AlaLi⁺)/I(DMF—Li⁺)], where the constant C was estimated from measurements of proton‐bound amine–amino acid clusters. From fragment ion analysis of nine other Li⁺‐bound α‐amino acid dimers, the following lithium ion affinities were obtained: Gly 51.0, Ala 52.6, Sar 53.5, α‐aminobutyric acid 53.7, glycine methyl ester 54.7 and Val 54.8. SIA(Ala) was estimated to be 75% of the lithium ion affinity and from fragment ion analysis of ten Na⁺‐bound α‐amino acid dimers the following sodium ion affinities were obtained: Gly 37.9, Ala 39.4, α‐aminobutyric acid 40.3, Val 41.0, glycine methylster 41.0 and Sar 41.2.