Nucleation 0-0 bonding interactions

On the other hand, the group of No wick [29] developed polyurea templates with the general structure 35 in order to nucleate parallel /1-sheet structures. They reported that in a model compound (36) the urethane carbonyls are hydrogen bonding to the adjacent NH and that the orientation of the bonding interactions is controlled by the size of the end substituent, in this case a phenyl group. They also recently prepared 37 and showed by NMR that it adopts mostly the proposed parallel /1-sheet conformation in chloroform [30]. 

MC) and HPMC could significantly inhibit the crystallization of supersaturated hydrocortisone acetate (HA). The mechanism of nucleation retardation was believed to be due to the hydrogen bonding interactions between HA and the polymers. As to the... 

Fig. 5. Protein folding. The unfolded polypeptide chain coUapses and assembles to form simple stmctural motifs such as -sheets and a-hehces by nucleation-condensation mechanisms involving the formation of hydrogen bonds and van der Waal s interactions. Small proteins (eg, chymotrypsin inhibitor 2) attain their final (tertiary) stmcture in this way. Larger proteins and multiple protein assembhes aggregate by recognition and docking of multiple domains (eg, -barrels, a-helix bundles), often displaying positive cooperativity. Many noncovalent interactions, including hydrogen bonding, van der Waal s and electrostatic interactions, and the hydrophobic effect are exploited to create the final, compact protein assembly. Further stmctural...

Eagland et al. ( 3) propose a different scheme. The helix formation is compared to a first order reaction concerning only individual chains. In the first step the helices are nucleated and stabilized by the solvent. Next, the chains slowly fold back and the helical sequences associate by hydrogen bonds. Van der Waals interactions or entanglements between the folded chains are responsible for the gel gormation (see Figure 1-b). 

The simplest j3 structure is the hairpin (Chapter 1, section C3 and Figure 1.12). The /3 hairpin requires the pairing of hydrophobic side chains to stabilize it. The hairpin has to nucleate at its central turn, unlike the helix, which can nucleate at any residue whose >C=0 can form a hydrogen bond with residue i + 4. The formation of j3 structure is inherently slower than the formation of a helixes because of the fewer nucleation sites and the requirements to make side-chain interactions.5 Depending on precise structure, helixes form with half-lives of a few hundred nanoseconds, and /3 structures form with half-lives 10 times longer. 

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