Syntheses and properties of zinc and calcium complexes of valinate and isovalinate: Metal alpha-amino acidates as possible constituents of the early earth's chemical inventory

We have studied the ligand behavior of racemic isovalinate (iva(-)) and val inate (val(-)) towards zinc(II) and calcium(II). The following solid metal amino acidates were obtained from aqueous solutions: Zn3C2(iva)(4) (1), Zn3 Cl2(val)(4) (2), Zn(val)(2) (3), Zn(iva)(2) . 2H(2)O (4), Zn(iva)(2) . 3.25 H2O (5), Zn(iva)(2) (6), Ca(iva)(2) . xH(2)O (7), and Ca(val)(2) . H2O (8) . Except for complex 3, these were hitherto unknown compounds. The conditio ns under which they formed, together with current ideas of the conditions o n early Earth, support the assumption that alpha -amino acidate complexes o f zinc and calcium might have belonged to early Earth's prebiotic chemical inventory. The zinc isovalinates 1, 4, and 5 were characterized by X-ray cr ystal structure analyses. Complex 1 forms a layer structure containing four - and five-coordinate metal atoms, whereas the zinc atoms in 4 and 5 are fi ve-coordinate. Compound 5 possesses an unprecedented nonpolymeric structure built from cyclic [Zn-6(iva)(12)] complexes, which are separated by water molecules. The thermolyses of solids 1, 3, and 8 at 320 degreesC in an N-2 atmosphere yielded numerous organic products, including the cyclic dipeptid e of valine from 3 and 8. Condensation, C-C bond breaking and bond formatio n, aromatization, decarboxylation, and deamination reactions occurred durin g the thermolyses. Such reactions of metal-bound alpha -amino acidates that are abiotically formed could already have contributed to an organic-geoche mical diversity before life appeared on Earth.