HS-SPME as an efficient tool for discriminating chemotypes of Lippia alba (Mill.) N. E. Brown

Abstract

Recebido em 15/04/2016; aceito em 15/07/2016; publicado na web em 26/08/2016 HS-SPME AS AN EFFICIENT TOOL FOR DISCRIMINATING CHEMOTYPES OF Lippia alba (Mill.) N. E. Brown. Lippia alba (Mill.) N. E. Brown (Verbenaceae) is a medicinal plant for which several biological activities are reported, such as sedative, anxiolytic, anti-ulcer, antifungal, antimicrobial, antioxidant, antispasmodic, anti-nociceptive and anti-inflammatory. It is characterized by the production of essential oils which have been used to classify the plant in different chemotypes. In the Northeast region of Brazil, the presence of three chemotypes are reported: myrcene-citral (chemotype I), limonene-citral (chemotype II) and carvone-limonene (chemotype III). In this work, headspace-solid phase microextraction (HS-SPME) was used on the analysis of the volatile organic compounds (VOCs) of three chemotypes of L. alba from the Northeast region of Brazil, and compared to the essential oils of the plants extracted by hydrodistillation. Volatile compounds from each chemotype were more effectively differentiated when extracted by HS-SPME than by hydrodistillation.

The definitive challenge of forming uncommon pseudo‐π···H–F and C···H–F hydrogen bonds on cyclic and cubic nonpolar hydrocarbons

“…Sure, these are the foundations of the quantum theory of atoms in molecules (QTAIM), [95] which has been used in routine studies of intermolecular systems, mainly those weakly bound ones. [96][97][98][99] Historically, the FÁÁÁH, OÁÁÁH, and NÁÁÁH hydrogen bonds [22,23,35,100] with energies of 30 KJ.mol −1 as well as the πÁÁÁH and pseudo-πÁÁÁH as the weaker ones, all of them have been characterized on the basis of the QTAIM formalism. The modeling of the hydrogen bonds in the C 3 H 6 , C 4 H 8 , C 6 H 6 , and C 8 H 8 alkanes summarizes the cornerstone of this current work if, specially, beyond an atom, a chemical bond, or even a triangular or square faces of the rings, can function as nucleophiles to interact with hydrogen donors.…”

“…Our results show that the hydrogen bond formed in the cyclopropane reproduces exactly the well-known profile of pseudo-πÁÁÁH. [19,22,23,37] In the complex of prismane, the hydrogen bond should be preferably formed in one of the two three-member faces because these small rings are highly strained and contain the higher electronic concentration for interacting with hydrogen fluoride. In cyclobutane, even though it may be cogitated that the hydrogen bond occurs precisely on the carbon atom, which is a proton acceptor, on the structural viewpoint, the interaction is aligned exactly in the middle of the carbon-carbon bond.…”

“…In addition to the I complex, maybe III might be formed by the pseudo-πÁÁÁH hydrogen bond, and in opposition to this, in the II and IV complexes may be considered the contact of the hydrogen fluoride towards the carbon atom to form the CÁÁÁH interaction. [22,23,111] The interaction occurring in the cubane refers to the projection IV 0 , wherein the coordinate for measuring the intermolecular distance of 2.5025 Å is the central point of the four-member face. Differently of a pseudo-π bond or even if the aromaticity is taken into account, [125] but the more resembled interactions with all that has been demonstrated for the "cubane-hydrogen fluoride" complex could be those ones in T-shaped intermolecular systems formed by benzene as proton acceptor with the πÁÁÁH hydrogen bond aligned precisely in the central point of the aromatic-ring.…”

“…[13] Regardless, the capability of the hydrocarbons to form intermolecular interaction must be increasingly explored, although this theme has been discussed in studies published along the last decade, [14][15][16][17][18][19][20][21][22] by which the existence of hydrogen bonds with properties never seen before has been revealed. [23,24] In a brief comment, it should be emphasized that the hydrogen bond model framed as YÁÁÁH-X is universal, [25][26][27][28][29] even so some restrictions must be considered on the basis of elementary criteria; eg, the electronegativity values of Y and X should be higher in comparison to the hydrogen. [21] As a matter of fact, on account of the drastic difference between the electronegativity values of Y and X, the charge transfer between these species is overestimated, resulting into a strong hydrogen bond.…”

“…[55] These cyclic and cubic alkanes represent a high-level challenge in the field of intermolecular interactions formed with standard molecular acids. [56] By assuming that C 3 H 6 and C 4 H 8 can be suitable structures to form hydrogen bonds, such as the pseudo-πÁÁÁH-X [37] instead of the πÁÁÁH-X type, [22,23] and in this insight, the great goal of this current research is definitively established.…”