Abstract
The strong chiral preferences of some magic clusters of amino acids have attracted continually increasing interests due to their unique structures, properties and possible roles in homochirogenesis. However, how chirality can influence the generation and stability of cluster ions in a wild range of cluster sizes is still unknown for most amino acids. In this study, the preference for threonine clusters to form homochiral and heterochiral complex ions has been investigated by electrospray ionization (ESI) mass spectrometry. Abundant cluster [Thrn+mH]m+ ions (7 ≤ n ≤ 78, 1 ≤ m ≤ 5) have been observed for both samples of enantiopure (100% L) and racemic (50:50 L:D) threonine solutions. Further analyses of the spectra show that the [Thr14+2H]2+ ion is characterized by its most outstanding homochiral preference, and [Thr7+H]+ and [Thr8+H]+ ions also clearly exhibit their homochiral preferences. Although most of the triply charged clusters (20 ≤ n ≤ 36) are characterized by heterochiral preferences, the quadruply charged [Thrn+4H]4+ ions (40 ≤ n ≤ 59) have no obvious chiral preference in general. On the other hand, a weak homochiral preference exists for most of the quintuply charged ions observed in the experiment.
Highlights
Clusters of amino acids can exhibit strong chiral preferences [1,2,3,4,5,6]
Substitution reactions in the serine octamer were observed for other amino acids, in which one or two units can be replaced by other L/D amino acids with homochiral preference [8,26,27,28]
We aimed to study how chirality can influence the cluster ions of L/D-threonine generated in the electrospray ionization (ESI) process, especially for large-sized and multiply charged clusters
Summary
The most famous example is the protonated serine octamer, which is characterized by its high abundance in electrospray ionization (ESI) mass spectra and pronounced homochirality preference [1,2,3,4]. Substitution reactions in the serine octamer were observed for other amino acids, in which one or two units can be replaced by other L/D amino acids with homochiral preference [8,26,27,28] These results have led to the suggestion that this cluster might have played a role in homochirogenesis [7,8,16]
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