Hydrogen-bonded chain formation comparison

Fig. 30 Comparison of hydrogen-bonded chain formation in F4BIm (flat ribbons) and F4BImNN (piled stack motif essentially the same as that in Fig. 29).

Helical columns of bifunctional ureidotriazines have also been created in water.40 In this solvent the aromatic cores of compound 39 stack and create a hydrophobic environment that favors the formation of intermolecular hydrogen bonds. The chiral side chains can express their chirality within the columnar polymer because of the helicity generated by the backbone. In contrast, for monofunctional 68 water interferes with the hydrogen bonding and 68 does not stack to form a column. As a consequence the chiral side chain does not express its chirality in the aromatic system. For 39, the bifunctional nature allows for a high local concentration of stacking units. A comparison might be made here to the individual DNA bases that also do not dimerize and stack in water, unless they are connected to a polymer backbone. 

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

They clearly indicate that the pf is not the same as solvent hydrogen bond basicity, /ii, because the pf value treats the solvent as a solute in the chemical interactions, and that the pf and fli scales are relatively collinear but not interchangeable . (The latter scale is based on the comparison of the indicators p-nitroaniline and p-nitro-Af, N-dimethylaniline.) Neither is the value connected to solute proton-transfer basicity . They have established that this pf value is relatively constant for homologous series of solvents, and that substituents on the parent structure of the solvent do not overly influence the pf value in terms of inductive or polar effects, unless the substituent is halogenated, in which case the will decrease. Chain branching of the parent also has little effect on the pf value. This makes it possible to predict average P2 values for solutes whose ATg values are not known. Correlations of kinetic data with pf are not always accurate because the pf parameter does not take into consideration solvent size, which can lead to steric hindrance of hydrogen bond formation . ... 

As mentioned above, Glu 61 NpH hydrogen is relatively isolated from solvent by the side-chains of Lys 60, Glu 61, and Glu 62. The conformation of Glu 61 side-chain is fixed by the formation of a hydrogen bond between the Glu 61 Of atom and N H hydrogen of Lys 99. Lys 60, Glu 61 and Glu 62 in horse cyt-c are replaced by Asn 60, Asn 61 and Asp 62, respectively, in tuna protein.Asn 61 NpH proton signal in tuna protein is not resolved below 10 ppm. Comparison of the crystal structures between the two proteins indicates that the chemical environment of Asn 61 NpH hydrogen in tuna protein is more exposed to the solvent than Glu 61 NpH hydrogen in horse cyt-... 

The abbreviations mean amino-, epoxy-, carboxyl- and non-functional end-capped polybutadiene-acrylonitrile copolymers, respectively, of the same molar mass and 18% AN content. The polarity of rubbers changes in the same order except for CTBN. Lower values of CTBN miscibility in comparison with ATBN and ETBN can possibly be caused by the great self-association effect of carboxyl groups through hydrogen bonding with the formation of high molar mass chains. 

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