Geometry optimization results are reported on putative elements of secondary structure in decameric units of polylactic acid (PLA) analogous to those seen in protein structure-helical structures (α, π, 3 10 ) as well as a β-sheet-employing molecular mechanics, semiempirical, ab initio and density functional methods. The four possible structures of the deca-PLA are generally predicted, with all methods to be within ∼15 kcal/mol of each other, with the more stable conformation varying depending on the method employed. The highest-level method employed here (M062x/6-311+G**) predicts that the α, π and 3 10 structures have very similar energies, with π slightly favored by values within the error limits of the method; this is in contrast with results obtained with less accurate semiempirical and empirical methods, which predict larger differences and other structures as favorites. Relative energies of poly-l and poly-d,l lactic acid structures indicate the former to be energetically-favored over the latter. Three types of weak interactions appear to dictate the relative stabilities of secondary structure elements in polylactic acid structures.
ABSTRACT. Putative elements of secondary, tertiary and quaternary structure were examined for polylactic acid chains, attempting a parallel with secondary structure elements known from protein biology and also attempting an estimate, based on accurate atomic-level calculations, of interaction energies between polylactic acid chains. Spectroscopic parameters were predicted for all types of structure examined, in an attempt to aid our on-going efforts in synthesis and characterization of polylactic acid variants.
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