Conformational analysis of oligomers of (R)-3-hydroxybutanoic acid in solutions by 1H NMR spectroscopy

Oligomers of (R)-3-hydroxybutanoic acid (3-HB) with different end groups and chain lengths have been prepared as model compounds of bacterial poly[(R)-3-hydroxybutanoate] [P(3-HB)]. They were studied in terms of their conformational behavior generated by rotation about CH2-CH bond of 3-HB units in solutions. The conformational behaviors of 3-HB oligomers were investigated in various solvents by analysis of vicinal coupling in the 500-MHz H-1 NMR spectra. For all 3-HB oligomers studied in this work, 3-HB units without hydroxy group were found, in every solvent, to adopt similar conformational distributions to those of P(3-HB) polymer backbone in solutions, in which trans and gauche conformers are preferable, while the other gauche conformer is strongly disfavored. On the other hand, 3-HB units adjacent to hydroxy terminal group in 3-HB oligomers show different conformational behaviors relative to those of other 3-HB units. In nonpolar organic solvents, only gauche conformer is predominant, due to the formation of an intramolecular hydrogen bond between hydroxy and carbonyl groups, while in polar organic solvents, the conformer distributions are similar to those of 3-HB units without hydroxy groups. In aqueous solution, for 3-HB units adjacent to hydroxy terminal group, the fractions of gauche conformer are still higher than those of trans conformer due to the formation of an intramolecular hydrogen bond.