COMPLEXES OF THE TRIOLIDE FROM (R)-3-HYDROXYBUTANOIC ACID WITH SODIUM, POTASSIUM, AND BARIUM SALTS - CRYSTAL-STRUCTURES, ESTER CHELATES AND ESTER CROWNS, CRYSTAL PACKING, BONDING, AND ELECTRON-LOCALIZATION FUNCTIONS

The triolide of(R)-3-hydroxybutanoic acid ((R,R,R)-3,7,11-trimethyl-2,6,10-trioxadodecane-1,5,9-trione; 1), readily available from the corresponding biopolymer P(3-HB) in one step, forms crystalline complexes with alkali and alkaline earth salts. The X-ray crystal structures of three such complexes, (3 NaSCN) . 4 1 (2), (2 KSCN) . 2 1 . H2O (3), and (2 Ba(SCN)2) . 2 1.2 H2O . THF (4), have been determined and are compared. The triolide is found in these structures i) as a free molecule, making no contacts with a cation (clathrate-type inclusion), ii) as a monodentate ligand coordinated to a single ion with one carbonyl O-atom only, iii) as a chelator, forming an eight-membered ring, with two carbonyl O-atoms attached to the same ion, iv) as a linker, using two carbonyl O-atoms to bind to the two metals of an ion-X-ion unit (ten-membered ring), and v), in a crown-ester complex, in which an ion is sitting on the three unidirectional C=O groups of a triolide molecule (Figs. 1-3). The crystal packing is such that there are columns along certain axes in the centers of which the cations are surrounded by counterions and triolide molecules, with the non-polar parts of 1 on the outside (Fig. 4). In the complexes 2-4, the triolide assumes conformations which are slightly distorted, with the carbonyl 0-atoms moved closer together, as compared to the 'free' triolide 1 (Fig.5). These observed features are compatible with the view that oligo (3-HB) may be involved in the formation of Ca polyphosphate ion channels through cell membranes. A comparison is also made between the triolide structure in 1-4 and in enterobactin, a super Fe chelator (Fig. 5). To better understand the binding between the Na ion and the triolide carbonyl O-atoms in the crown-ester complex, we have applied electron-localization function (ELF) calculations with the data set of structure 2, and we have produced ELF representations of ethane, ethene, and methyl acetate (Figs. 6-9). It turns out that this theoretical method leads to electron-localization patterns which are in astounding agreement with qualitative bonding models of organic chemists, such as the 'double bond character of the CO-OR single bond' or the 'hyperconjugative n-->sigma* interactions between lone pairs on the O-atoms and neighbouring sigma-bonds' in ester groups (Fig.8). The noncovalent, dipole/pole-type character of bonding between Na+ and the triolide carbonyl O-atoms in the crown-ester complex (the Na-O=C plane is roughly perpendicular to the O-C=O plane) is confirmed by the ELF calculation; other bonding features such as the C=N bond in the NaSCN complex 2 are also included in the discussion (Fig. 9).