ON THE SODIUM AND LITHIUM ION AFFINITIES OF SOME ALPHA-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 b e assigned to sodium ion-bound amino acids. Assuming that the most abu ndant ion in the fragment ion spectrum corresponds to the amino acid w ith the highest sodium ion affinity (SIA), the 20 common alpha-amino a cids 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 c arried out by comparison of the lithium ion affinity (LIA) of Ala with that of dimethylformamide (DMF) in a fragment ion scan of the ion-bou nd dimer Ala-Li+-DMF. LIA(Ala) was calculated from LIA(Ala) = LIA(DMF) -(1/C)ln[I(AlaLa+)/I(DMF-Li+), where the constant C was estimated fro m measurements of proton-bound amine-amino acid clusters. From fragmen t ion analysis of nine other Li+-bound alpha-amino acid dimers, the fo llowing lithium ion affinities were obtained: Gly 51.0, Ala 52.6, Sar 53.5, alpha-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 alpha-amino acid dimers t he following sodium ion affinities were obtained: Gly 37.9, Ala 39.4, alpha-aminobutyric acid 40.3, Val 41.0, glycine methylster 41.0 and Sa r 41.2.