Spectroscopic studies of the effects of salt addition in the protonation and deprotonation of emeraldine films

Emeraldine base films were treated with HClO₄ solutions of various concentrations, with and without the addition of NaClO₄. The protonation of the films was studied using angle-resolved x-ray photoelectron spectroscopy (XPS), Fourier transform infra-red (FTIR) absorption spectroscopy, and ultraviolet (UV)-visible absorption spectroscopy. The protonation level can be substantially increased by the presence of salt if sufficient time is allowed for the anions to migrate into the bulk of the film. Conversely, in the partial deprotonation of emeraldine salt films, a higher retention of anions is observed in the presence of salt, even though the effect is not as pronounced as in the protonation process. © 1995 John Wiley & Sons, Inc.     

Determination of microscopic acid–base parameters from NMR–pH titrations

The theory and practice of proton microspeciation based on NMR–pH titrations are surveyed. Principles of bi-, tri-, tetra-, and n-protic microequilibrium systems are discussed. Evaluation methods are exemplified by case studies on bi- and tetraprotic biomolecules. Selection criteria and properties of reporter NMR nuclei are described. Literature data on complete microspeciations of small ligands and site-specific basicity characterizations of peptides and proteins are critically reviewed.     

Thermodynamics of Dipeptides in Water. Calorimetric determination of enthalpy change values related to proton transfer processes of a series of dipeptides in water

Calorimetric measurements on a series of dipeptides having methionine as first common term enabled us to determine the thermodynamic quantities related to the proton transfer processes at the free α-amino and α-carboxyl groups of the dipeptides using methionyl-methionine (MetMet) as reference compound. The drawing forces were found to be the solvation processes of zwitterions, ions and undissociated molecules of dipeptides, hydrophobic effects of side-chains playing an important role. The results were interpreted in terms of the reciprocal influence of the dipeptides components. Relative and absolute scales related to the enthalpic contribution to the acidity and basicity of the dipeptides studied are supplied. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure and bonding in cyclic isomers of BAl2Hnm (n=3-6, m=-2 to +1): preference for planar tetracoordination, pyramidal tricoordination, and divalency

The structure and energetics of cyclic BAl2Hnm (n=3-6, m=-2 to +1), calculated at the B3LYP/6-311+G** and QCISD(T)/6-311++G** levels, are compared with their corresponding homocyclic boron and aluminium analogues. Structures in which the boron and aluminium atoms have coordination numbers of up to six are found to be minima. There is a parallel between structure and bonding in isomers of BAl2H(3)2- and BSi2H3. The number of structures that contain hydrogens out of the BAl2 ring plane is found to increase from BAl2H3(2-) to BAl2H6+. Double bridging at one bond is common in BAl2H5 and BAl2H6+. Similarly, species with lone pairs on the divalent boron and aluminium atoms are found to be minima on the potential energy surface of BAl2H(3)2-. BAl2H4- (2 b) is the first example of a structure with planar tetracoordinate boron and aluminium atoms in the same structure. Bridging hydrogen atoms on the B--Al bond prefer not to be in the BAl2 plane so that the pi MO is stabilised by pi-sigma mixing. This stabilisation increases with increasing number of bridging hydrogen atoms. The order of stability of the individual structures is decided by optimising the preference for lower coordination at aluminium, a higher coordination at boron and more bridging hydrogen atoms between B--Al bonds. The relative stabilisation energy (RSE) for the minimum energy structures of BAl2Hnm that contain pi-delocalisation are compared with the corresponding homocyclic aluminium and boron analogues.     

Sites of Protonation and Unimolecular Fragmenta tion of Protonated N-Hydroxyphthalimide in the Gas Phase

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The First Conjugated Allene Polymer

A novel conjugated polymer with allene moieties in the polymer main chain, poly(4,4′‐biphenylylene‐1,3‐diphenylpropadien‐1,3‐ylene), was synthesized from 1,3‐bis(4‐bromophenyl)‐1,3‐diphenylpropadiene by dehalogenative polycondensation using Ni(cod)₂. The polymer shows good solubility in common organic solvents, good processability for making thin films, intense blue fluorescence in solution and acid sensitivity resulting in coloration. The conductivity increased by an order of 10⁴ upon exposure to trifluoroacetic acid vapor.     

Long-Distance Protonation-Conformation Coupling in Phytochrome Species

Phytochromes are biological red/far-red light sensors found in many organisms. The connection between photoconversion and the cellular output signal involves light-mediated global structural changes in the interaction between the photosensory module (PAS-GAF-PHY, PGP) and the C-terminal transmitter (output) module. We recently showed a direct correlation of chromophore deprotonation with pH-dependent conformational changes in the various domains of the prototypical phytochrome Cph1 PGP. These results suggested that the transient phycocyanobilin (PCB) chromophore deprotonation is closely associated with a higher protein mobility both in proximal and distal protein sites, implying a causal relationship that might be important for the global large-scale protein rearrangements. Here, we investigate the prototypical biliverdin (BV)-binding phytochrome Agp1. The structural changes at various positions in Agp1 PGP were investigated as a function of pH using picosecond time-resolved fluorescence anisotropy and site-directed fluorescence labeling of cysteine variants of Agp1 PGP. We show that the direct correlation of chromophore deprotonation with pH-dependent conformational changes does not occur in Agp1. Together with the absence of long-range effects between the PHY domain and chromophore pK_a, in contrast to the findings in Cph1, our results imply phytochrome species-specific correlations between transient chromophore deprotonation and intramolecular signal transduction.     

Multinuclear Magnetic Resonance Study of Some Mesoionic 1,2,3-Triazoles and Related Compounds

¹H, ¹³C, ¹⁴N and ¹⁵N NMR results are reported for some mesoionic 1,2,3-triazoles and related compounds. It is demonstrated that ¹⁴N NMR linewidths are dependent on the charge present on the nitrogen atom. In addition, it is shown that nitrogen NMR is of great value in the determination of the site of protonation, the ability to distinguish readily between mesoionic cyclic and non-cyclic structures and to follow the transformation between such structures.