Characterization of low molecular weight poly(β-hydroxybutyrate)s from alkaline and acid hydrolysis

Abstract

In the present paper, low molecular weight poly(β-hydroxybutyrates) (PHBs), which were obtained by acidic and basic hydrolysis of bacterially produced PHB, were characterized by gel permeation chromotography (GPC), 1H, 13C and 31P NMR. The assignments of the resonances of 13C NMR spectra from unsaturated end and hydroxy end groups were made using heteronuclear multiple quantum coherence (HMQC) NMR technique. Unsaturation contents and number average molecular weights of the PHBs were determined mainly by 1H and 31P NMR. Direct measurement of the molecular weights of PHB has also been carried out by matrix assisted laser desorption ionization (MALDI). Based on the characterization of these low molecular weight PHBs, the heterogeneous hydrolysis mechanism is discussed and the scission of PHB chains is proposed to occur at the surfaces and interfaces of lamellar crystals. The unsaturated end group is formed by dehydration of the chain ends by β-elimination after ester hydrolysis, which is different from the mechanistic steps of the formation of unsaturated ends in thermal decomposition of PHB. The correlation of thickness of crystal lamellae and the melting points has also been discussed. The plot of the melting point against reciprocal lamellar thickness for low molecular weight PHB indicates the pronounced effect of the surface energy of these crystals, which is caused by the large proportion of end groups.