pH dependence of tailing in reversed-phase chromatography of a cationic dye: measurement of the strong adsorption site surface density

A question that has interested Dr. J.J. Kirkland is addressed: what is the nature of the silanols that cause tailing to persist at low pH in reversed-phase chromatography? Chromatograms for a cationic dye, 1,1'-didodecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (DiI), were studied at varying pH using an Agilent SB-C8 column and 80% ACN/water for six DiI concentrations ranging from 0.9 to 316 microM. The chromatograms showed increased retention and tailing from pH 1 to 5, as expected. Simulations of the chromatograms agreed well with experiment for a bi-Langmuir isotherm with weak (C8) and strong (silica) adsorption sites. The simulation parameters revealed that the number of strong adsorption sites decreases by 40% from pH 1 to 5, which indicates that the silanols causing tailing are in the SiOH, not the SiO-, form. This seems paradoxical because tailing increases with increasing pH. The simulation parameters reveal that this increased tailing from pH 1 to 5 owes to doubling of the partition coefficient for DiI to the strong adsorption site, which more than compensates for the decreasing number of sites. We attribute this increased partition coefficient to increased long-range coulombic interactions with the increasingly abundant SiO- groups at higher pH, which boosts DiI's partition coefficient for both the C8 and SiOH sites. The picture thus emerges that for DiI, higher pH causes increased tailing because the SiO- groups exacerbate tailing that actually originates from adsorption to SiOH groups.     

A comparison of crystalline- and graft polymer-based chemosensors

Herein, we report a highly sensitive luminescent thin film chemosensor constructed out of a small-molecule donor/acceptor system. Two types of films were compared: one using a small-molecule crystalline donor/acceptor pair and the other using a donor-graft polymer/small-molecule acceptor pair. The acceptor selected for this proof of concept responds to acid, causing its absorption and emission bands to red-shift, which increases spectral overlap with the donor. This increase in overlap greatly enhances energy transfer from the acceptor to the donor. Signal amplification was ascertained by measuring the ratio of acceptor fluorescence when the donor was excited versus direct excitation of the acceptor. Both types of films exhibited large amplification. For the polymeric system, the mechanism of energy migration was investigated by the use of steady-state fluorescence spectroscopy. The mechanism was determined to be dominated by an exciton-hopping process.     

Hydrogen Atoms Are Produced When Tryptophan within a Protein Is Irradiated with Ultraviolet Light

Abstract— The UV photolysis of the aromatic amino acid, tryptophan (Trp), in the Ca²⁺-binding protein, cod paralbumin, type III, was studied using electron paramagnetic resonance (EPR) spectroscopy in the temperature range 4–80 K. For the Ca²⁺-bound protein, irradiation with UV light (250–400 nm) resulted in the generation of atomic hydrogen with a hyperfine splitting of 50.9 mT, whereas in the Ca²⁺-free form, where the Trp is exposed to solvent, the trapped atomic hydrogen was not in evidence. In the same spectra, the radical signal in the g = 2.00 region could be detected. The line shape of the Ca²⁺-bound form is similar to the EPR line shape obtained for Trp in micellar systems. In contrast, the EPR line shape for the Ca²⁺-free form is essentially featureless up to 80 K. The EPR spectra of the photoproducts of Trp and the nature of the photoreactions are therefore sensitive to the environment of Trp within the protein.     

Selection of the cis and trans phosph(III)azane macrocycles [{P(mu-NtBu)}2(1-Y-2-NH-C6H4)]2(Y=O, S)

The 1 : 1 reactions of [ClP(mu-NtBu)]2 with the difunctional aromatic amines 1,2-1-YH-2-NH2-C6H4 in the presence of Et3N give the dimeric phosph(III)azane macrocycles [{P(mu-NtBu)2(1-Y-2-HN-C6H4)]2, predominantly as the cis isomer in the case of Y=O (1.cis) and as the trans isomer for Y=S (2.trans). Model M.O. calculations suggest that the selection of the cis and trans isomers is not thermodynamically controlled. The alternative isomers 1.trans and 2.cis are generated exclusively by the deprotonation of the model intermediates [(1-Y-2-NH2-C6H4)P(mu-NtBu)]2[Y=O (3), S (4)] with nBuLi followed by cyclisation with [ClP(mu-NtBu)]2. The solid-state structures of 1.cis/trans(50 : 50), 2.cis, 3 and 4 are reported.     

Exo-metal coordination by a tricyclic [(P(mu-N-2-NC5H4))2(mu-O)]2 dimer in [(P(mu-N-2-NC5H4))2(mu-O)]2(CuCl x (C5H5N)2)4 (2-NC5H4 = 2-pyridyl, C5H5N = pyridine)

The in situ reaction of the phosphazane dimer [CIP(mu-N-2-NC5H4)]2 (2) with CuCl in the presence of CsH5N/H2O gives the title complex [(P(mu-N-2-NC5H4))2(mu-O)]2(CuCl x (C5H5N)2)4 (1), containing a tricyclic [(P(mu-N-2-NC5H4))2(mu-O)]2 ligand which is isoelectronic with species of the type [(P(mu-NR))2NR]2.     

Synthesis and structure of the calixarene-like phosph(III)azane macrocycle [{P(mu-N(t)Bu)}2{1,5-(NH)2C10H6}]3

The reaction of [ClP(mu-NtBu)]2 with 1,5-diamino-naphthalene [1,5-(NH2)2C10H6] in Et3N-thf gives the trimeric macrocycle [{P(mu-NtBu)}2{1,5-(NH)2C10H6}]3(1); the X-ray structure of the toluene solvate 1.3toluene reveals a cone-shaped (calixarene-like) arrangement in which toluene guest molecules are trapped within the cavity.     

Vibrational spectra and conformational behavior of benzyl fluoride

Infrared and Raman spectra have been obtained for benzyl fluoride and a vibrational assignment has been made. The spectra indicate the presence of a large number of conformers that differ only in the orientation of the C-F bond, as a result of free or nearly free internal rotation.     

Antiaromaticity in relation to 1,3,5,7-cyclooctatetraene structures

An isodesmic energy analysis has been carried out at the MP 2/6–31G *//HF /3–21G level for the nonplanar ground state ( 1 ) of 1,3,5,7-cyclooctateraene and for two planar forms, one having complete π delocalization ( 2 ) and the other having alternating single and double bonds ( 3 ). 1 is found to have a considerable degree of stabilization, which is attributed to limited π delocalization. The polyene 3 is the more stable of the two planar forms; it is a transition state in the inversion between two possible nonplanar structures. 2 is found to be a triplet at the Hartree–Fock level and is a critical point on an alternate pathway between the two possible arrangements of alternating single and double bonds in 3 . Both 2 and 3 have negative isodesmic energies, indicating the presence of stabilizing factors. Our results for 3 show that an “antiaromatic” system need not necessarily show a net destabilization. © 1994 John Wiley & Sons, Inc.     

Application of total selective solvent extraction of germanium,phosphorus and silicon to the simultaneous determination of arsenic,germanium, phosphorus and silicon

A simple rapid, accurate, and reliable method of simultaneous determination of arsenic, germanium, phosphorus, and silicon is reported. The method involves first, the determination of germanium as its phenlfluorone complex and its selective extraction with isoamyl alcohol. Phosphorus is next determined as its heteropoly blue after selective extraction of phosphomolybdic acid by isobutyl acetate at pH 1.0-0.8 and its direct reduction in this solvent. Silicon is then determined after its extraction as silicomolybdic acid by isooctyl alcohol at pH < 0.4 and direct reduction in the solvent phase. Finally, arsenic is determined in the remaining aqueous phase after reduction to its heteropoly blue.