A theoretical study on the structure of poly((R)-3-hydroxybutanoic acid)

Conformational features of oligomers of 3-(R)-butanoic acid have been studied using quantum mechanics methods. Conformational search of Ac-OCH(CH3)-CH2-COOCH3 indicates that the compound is quite flexible with several conformations similar in stability. Study of Ac-[OCH(CH3)-CH2-CO](n)OCH2CH3, n = 1-8, using a repeating unit approach for 2(1)-helix, 3(1)-helix, 4(1)-helix, 5(1)-helix, and pleated strand structure indicates that only the 31-helix has a cooperative effect and is also most stable. Crystal orbital calculations on the crystal packing energies of the 2(1)-, 3(1)-, and 4(1)-helices have been performed. The 2(1)-helix is found to have much stronger crystal packing stabilization than the 3(1)- and 4(1)-helices. This explains why the 2(1)-helix is found in crystal structures of poly((R)-3-hydroxybutanoic acid) (PHB) despite the fact that the 3(1)-helix is the most stable single helix. The stabilization of the 2(1)-helix in the crystal structure is mainly from the dipole interaction between adjacent parallel helices but not from adjacent antiparallel helices. The study also provides useful information for the study of ion channel structures of PHB