Infrared spectral evidence and DFT calculations of hydrogen-bonding and molecular structures of acetogenins

FTIR spectra have been measured for 31 different five component - simplex centroid design solvent mixture extracts of shaded and sun-exposed Annonaceous leaves harvested in all four seasons. The spectral frequencies are characteristic of anonnaceous acetogenins known to be a major component of these leaves. O-H stretching spectral bands in the 3100-3600 cm(-1) region provide evidence of notable intensity changes for the shaded and sun-exposed leaves. Chemometric principal component analysis involving 264 spectra show that shaded samples tend to have more intense O-H stretching bands than those grown in the sun. B3LYP density functional calculations indicate significant O-H stretching band changes in this region owing to hydrogen bond formation. Weak O-H intensity enhancements, around 40 km mol(-1), occur when an O-H group forms a hydrogen bond with the oxygen atom of an adjacent tetrahydrofuran ring oxygen atom. Much more intense enhancements, 400-500 km mol(-1), are predicted to occur for acetogenins with two tetrahydrofuran rings for which the O-H group hydrogen bonds with its fartherest removed tetrahydrofuran ring oxygen. Whereas weak or moderate H-bond stretching intensities are obtained for acetogenins with slightly bent carbon chain structures the strongest hydrogen bond intensities are calculated for molecules with a 45 degrees V-type backbone structure. These important structural modifications as well as significant changes in bond lengths and angles owing to hydrogen bonding are detailed. (C) 2016 Elsevier B.V. All rights reserved.