On the solution structure of PHB: Preparation and NMR analysis of isotopically labeled Oligo[(R)-3-hydroxybutanoic acids] (OHBs)

While the chain conformation of poly- and oligo[(R)-3-hydroxybutanoate] (PH B, OHB) is known to be 2(1)- and 3(1)-helical in stretched fibers and in th e crystalline state, respectively (Fig. 2). the structure in solution is un known. To be able to determine the NMR-solution structure, specifically lab eled linear oligomers have been prepared: a 16-mer consisting of alternatin g pairs of fully C-13-labeled and non-labeled residues (1) and a 20-mer con taining an O-(CH)-C-13((CH2D)-C-13)-(CHDS1)-C-13-(CO)-C-13 residue in posit ion 9 (from the O-terminus) and a fully C-13-labeled residue in position 12 (2), both with (t-Bu)Ph2Si protection at the O- and Bn protection at the C -terminus. The labeled (R)-3-hydroxybutanoic acid building blocks were prep ared by Noyori hydrogenation of the ethyl ester of fully C-13-labeled aceto acetic acid, and the D-atoms were incorporated by D-2/Pd-C reduction of a p reviously reported dibromo-1,3-dioxinone 8 (Scheme 1), The oligomers were o btained by a series of fragment couplings (Schemes-1 and 3). 600-MHz NMR CO SY. HSQC. ROESY, and cross-correlated relaxation measurements (Figs. 4-6. 9 , and 12, and Tables 1-3) at different temperatures and interpretations the reof led to assignments of all resonances, including those from the diaster eotopic C(2)H-2 protons. and to determination of the conformationally avera ged dihedral angles phi (2) and phi (3) (Figs. 2, 7, and 8) in the chain of the oligoester. The conclusions are: all but five or six terminal residues adopt the same conformations the 2(1) helix is not the predominant seconda ry structure: the structure of the HB chain is averaged. even at -30. Our i nvestigation confirms the high flexibility of the polyester chain, a proper ty that has been deduced previously from biological studies of PHB in membr anes. in ion channels, and as appendage of proteins.