Bioaccumulation is an important information requirement for chemicals risk assessment. The most widely used test guideline for measuring bioaccumulation in fish is the OECD 305 test guideline and, in the future, it is likely to include a dietary exposure method for substances that are difficult to test by the more usual aqueous exposure route. This new method results in a biomagnification factor (BMF), whereas for regulatory purposes a bioconcentration factor (BCF) is often required. Therefore, being able to estimate a BCF quantitatively from the data generated in the dietary study would meet an accepted regulatory need. The information generated by the dietary study includes the depuration rate constant. To use these data to estimate a BCF, an estimate of the rate constant for uptake from water is needed, allowing a kinetic BCF to be calculated. The present study considers and tests methods that are currently available for predicting uptake rate constants from water using a database of bioconcentration data. A number of methods were found to perform similarly when tested with substances with a log K(OW) range of approximately 3.5 to 8.2. The uncertainty in the estimated uptake rate constant was relatively large, however, even for the best performing methods.

In the solid state, diesters of pyridine-2,6-dicarboxylic acid assemble into edge-to-edge, front-to-back associated tapes involving a novel triple supramolecular contact consisting of one N pyridine … H-C pyridine and two CLO … H-C pyridine interactions. This intermolecular contact is robust, proving insensitive to additional interactions involving the side arms, and is also observed when the orientation of the pyridine core is reversed.

While solution 1 H and 13 C NMR spectra of the sulfate salt of m-xylylenediguanidinium dication 2 indicate a symmetrical conformation for the cation, a single-crystal X-ray structural determination of 2 reveals that this dication does not offer symmetrical tweezer-like coordination toward sulfate in the solid state (consistent with solid phase 13 C NMR spectra) but rather assembles into a complex three-dimensional H-bonded network in which the two guanidinium arms adopt different conformations, and all guanidinium N-H units plus one C-H unit of each CH 2 group are involved in H-bonds to sulfate.While the design, synthesis, and study of organic ligands for the recognition of anions is of much current interest in the field of supramolecular chemistry, 1 complexation of sulfate and sulfonate anions by organic host molecules is not yet well documented. One design motif for such a ligand is a molecular tweezer 1, in which two cationic centers, Q, are appended to a more-or-less rigid spacer, L, with a separation suitable for the host 1 to accommodate the guest sulfate or sulfonate anion. In addition to electrostatic association, complexation can be enhanced using an appropriately tailored array of H-bond donors in Q. The application of guanidinium salts as binding sites for oxoanions in macrocyclic and tweezer host ligands has been documented. 1 Indeed, Ward and co-workers 2 have successfully crystallized the guanidinium cation with various sulfonate anions to form two-and three-dimensional arrays, the latter containing cavities that incorporate aromatic guests. 2c An example of a host showing oxoanion complexation using the guanidinium cation is mxylylenediguanidinium (2), which binds phosphate and phosphonate-based anions and catalyzes phosphate ester hydrolysis and transesterification in nonaqueous and aqueous media, 3 but structural details of this complexation behavior have not yet been characterized. Our interest in this and related species has led us to carry out a detailed structural characterization of m-xylylenediguanidinium sulfate ([2]-[SO 4 ]) by means of solution and solid-phase techniques. We now report that tweezer-like complexation, as in 3, is not observed in the solid phase but rather a complicated inter-ion hydrogen-bonded array binds sulfate to dications 2.The sulfate salt of 2 was prepared as a sparingly soluble microcrystalline powder, as reported 3a following a modification of a published procedure in which mxylylenediamine was reacted with O-methylisothiourea sulfate. Solution phase 1 H and 13 C NMR spectra of [2]-[SO 4 ] in D 2 O/TFA revealed the pairwise equivalence of the two methylene groups, the two guanidinium carbon atoms, and the two sides of the aromatic ring in 2, thus indicating at least time-averaged symmetry (C s , C 2 , or C 2v ) of cation 2 in solution. However, solid phase 13 C NMR spectra indicated an unsymmetrical environment for cation 2 in the crystalline phase; the degeneracies associated with the three pairs of carbon atoms C-1/ C-3, C-4/C-6, and methylenes being clearly r...

Single crystal X-ray diffraction structures of two simple bis(guanidinium) sulfates that have previously been shown to recognise oxoanions in solution are reported. Comparison of these solid state structures with previous data suggests a relationship between increasing ligand planarity (preorganisation) and internal sulfate complexation, the greatest degree of preorganisation resulting in formation of a discrete 1 : 1 host-guest complex.