Probing the molecular and electronic structure of norhipposudoric and hipposudoric acids from the red sweat of hippopotamus amphibius: A DFT investigation

Vinicio Galasso, Fabio Pichierri

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14 Citations (Scopus)

Abstract

Molecular structure and tautomeric/conformational preferences of norhipposudoric and hipposudoric acids, the recently isolated pigments of the Hippopotamus amphibius' sweat, were investigated using the density functional theory (DFT) PBEO formalism. Among a large variety of possible structures, two similar keto-enol tautomer/conformers are nearly isoenergetic and markedly more stable than the others both in the gas phase and aqueous solution. The bulk solvent effect was accounted for with the polarizable continuum model (PCM). A distinctive structural feature is the strong intramolecular hydrogen bonding in the keto-enol O-H⋯O bridge, as shown by analysis of the atoms-in-molecules topological properties of the electron density. To elucidate the claimed strong acidity of these pigments, the site-specific microscopic dissociation constants were also calculated using the cluster-continuum model, a hybrid approach based on inclusion of explicit solvent molecules and solvation of the cluster by the dielectric continuum. Notably, the first deprotonation should occur predominantly from the enolic group with a remarkably low pK i value. This factor could play an important role in the potent antibiotic activity of the pigments. The absorption spectra of the undissociated and dissociated compounds in aqueous solution were interpreted with time-dependent DFT/PCM calculations. The π-π* diquinoid excitations, mainly occurring in the fluorenoid nucleus, are the major contributors to the color and strong absorption bands in the UVA and UVB regions, which are closely related to the efficient sunscreen activity exerted by the pigments.

Original languageEnglish
Pages (from-to)2534-2543
Number of pages10
JournalJournal of Physical Chemistry A
Volume113
Issue number11
DOIs
Publication statusPublished - 2009 Mar 19

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

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