Intramolecular Interactions versus Hydration Effects on <i>p</i>‑Guanidinoethyl-phenol Structure and p<i>K</i>_a Values

We analyze the structure, hydration, and p<i>K</i>_a values of <i>p</i>-guanidinoethyl-phenol through a combined experimental and theoretical study. These issues are relevant to understand the mechanism of action of the tetrameric form, the antibacterial compound tetra-<i>p</i>-guanidinoethyl-calix­[4]­arene (Cx1). The investigated system can also be useful to model other pharmaceutical drugs bearing a guanidine function in the vicinity of an ionizable group and the effect of arginine on the p<i>K</i>_a of vicinal ionizable residues (in particular tyrosine) in peptides. The <i>p</i>-guanidinoethyl-phenol monomer (mCx1) has two ionizable groups. One important particularity of this system is that it exhibits high molecular flexibility that potentially leads to enhanced stabilization in folded structures by direct, strong Coulombic interactions between the ionizable groups. The first p<i>K</i>_a corresponding to ionization of the −OH group has experimentally been shown to be only slightly different from usual values in substituted phenols. However, because of short-range Coulombic interactions, the role of intramolecular interactions and solvation effects on the acidities of this compound is expected to be important and it has been analyzed here on the basis of theoretical calculations. We use a discrete-continuum solvation model together with quantum-mechanical calculations at the B3LYP level of theory and the extended 6-311+G­(2df,2p) basis set. Both intra- and intermolecular effects are very large (∼70 kcal/mol) but exhibit an almost perfect compensation, thus explaining that the actual p<i>K</i>_a of mCx1 is close to free phenol. The same compensation of environmental effects applies to the second p<i>K</i>_a that concerns the guanidinium group. Such a p<i>K</i>_a could not be determined experimentally with standard titration techniques and in fact the theoretical study predicts a value of 14.2, that is, one unit above the p<i>K</i>_a of the parent ethyl-guanidinium molecule

Development and validation of a multi-residue method for pesticide determination in honey using on-column liquid-liquid extraction and liquid chromatography-tandem mass spectrometry

We report on the development and validation under ISO 17025 criteria of a multi-residue confirmatory method to identify and quantify 17 widely chemically different pesticides (insecticides: Carbofuran, Methiocarb, Pirimicarb, Dimethoate, Fipronil, Imidacloprid; herbicides: Amidosulfuron, Rimsulfuron, Atrazine, Simazine, Chloroturon, Linuron, Isoxaflutole, Metosulam; fungicides: Diethofencarb) and 2 metabolites (Methiocarb sulfoxide and 2-Hydroxytertbutylazine) in honey. This method is based on an on-column liquid liquid extraction (OCLLE) using diatomaceous earth as inert solid support and liquid chromatography (LC) coupled to mass spectrometry (MS) operating in tandem mode (MS/MS). Method specificity is ensured by checking retention time and theoretical ratio between two transitions from a single precursor ion. Linearity is demonstrated all along the range of concentration that was investigated, from 0.1 to 20 ng g(-1) raw honey, with correlation coefficients ranging from 0.921 to 0.999, depending on chemicals. Recovery rates obtained on home-made quality control samples are between 71 and 90%, well above the range defined by the EC/657/2002 document, but in the range we had fixed to ensure proper quantification, as levels found in real samples could not be corrected for recovery rates. Reproducibility is found to be between 8 and 27%. Calculated CC alpha and CC beta (0.0002-0.943 mg g(-1) for CC alpha, and 0.0002-1.232 ng g(-1) for CCP) show the good sensitivity attained by this rnulti-residue analytical method. The robustness of the method has been tested in analyzing more than 100 raw honey samples collected from different areas in Belgium, as well as some wax and bee samples, with a slightly adapted procedure. (C) 2007 Elsevier B.V. All rights reserved

LR12-peptide quantitation in whole blood by RP-HPLC and intrinsic fluorescence detection: Validation and pharmacokinetic study

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