On the structure of poly(3-hydroxybutanoic acid) in solution and in phospholipid bilayers. Circular dichroism and fluorescence spectroscopy with oligo(3-hydroxybutanoic acid) derivatives

Oligomers of (R)-3-hydroxybutanoic acid (OHBs) consisting of 8, 16, and 32 HB units have been investigated by CD spectroscopy in solution and in a racemic phospholipid bilayer. A Freudenberg plot (⊖/n vs (n - 2)/n) of the data obtained in solution can be interpreted as an indication for the presence of a chiral secondary structure of the oligoester chain-on the very short time scale of UV spectroscopy! The OHBs were also combined with fluorescent groups, including donor/acceptor pairs (coumarin, rhodamine, cyanine, oxazine). This enabled their detection by fluorescent microscopy and fluorescence-resonance-energy-transfer (FRET) measurements. With these techniques it was possible to calculate the distance between the fluorescent groups, thus providing information about the conformation of the OHB chain. With a calculated Förster distance R₀ of 60 Å the distance (in 10^-6 M CHCl₃) between the termini of HB 8-, 16-, and 32-mer were calculated to be 41, 38, and 49 Å, respectively. This result is compatible with conclusions drawn from other observations, indicating that longer HB chains fold in a hairpin-type fashion. Incorporation of fluorescent 16- and 32-mers in liposomes shows that the latter tend to aggregate while the former do not: The investigation described here did not lead to a decision as to whether an OHB chain, and thus also the poly(hydroxybutanoate) (PHB) chain, attains a 2₁- or a 3₁-helical arrangement in solution.