H-bonding sites

Fig. 13. Newkome s dendritic host with internal H-bonding sites...

Okino and co-workers produced composites from acetylated rubberwood (Okino etal., 2001) and acetylated cypress (Okino etal., 2004), bonded using a UF resin. The composites exhibited inferior mechanical properties compared to those produced from unmodified wood. The reduction in mechanical properties is undoubtedly due to poor wetting and weak interaction of the surface of the modified wood with the UF resin due to a reduction of H-bonding sites on the acetylated wood surface. 

Chart 10.3 Peptidomimetic /8-sheet modules with varying numbers of H bonding sites. 

It is also possible to affect H-bond strength electrochemically without the H-bonding site being an integral part of the redox couple. Good examples of this are found in the many redox-dependent receptors that utilize metallocenes, primarily ferrocene and cobaltocenium, as the redox couple. These are primarily used in ion receptors, but examples of metallocene receptors that show a significant redox dependence with neutral guests are the cobaltocenium and ferrocene diamides,... 

The template used for generating P-sheet structures described in this section is based on the structure of gramicidin S (1, Scheme 1). Gramicidin S is a head-to-tail cyclic decapeptide discovered over 50 years ago and has the sequence c[-Val-Om-Leu-D-Phe-Pro-]2. 13 The tertiary structure of gramicidin S has since been elucidated and found to exist in a P-sheet/p-turn conformation. 14,15 As shown in Scheme 1, two antiparallel P-strands containing the Val-Om-Leu sequence are held in place by two type II P-tums defined by the D-Phe-Pro sequence. Val and Leu residues occupy H-bonded sites while Orn residues are located in non-H-bonded sites. 

The so-called Janus molecules, which have the H-bonding sites of one face mirrored on the other face, were synthesized by the route shown in Scheme 4 (94TL397). 

Fig. 6.22. Schematic model showing the distribution of hydrogen binding energies. The more distorted Si—Si weak bonds trap hydrogen in deeper states. The dispersive hydrogen diffusion corresponds to the trapping and release from the weak bonds and Si—H bond sites (Street el at. 1988a).

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