Hydrogen bonding half-live

The first experimental data for a reaction involving proton transfer from a hydrogen-bonded acid to a series of bases which were chosen to give ApK-values each side of ApK=0 are given in Fig. 15 (Hibbert and Awwal, 1976, 1978 Hibbert, 1981). The results were obtained for proton transfer from 4-(3-nitrophenylazo)salicylate ion to a series of tertiary aliphatic amines in aqueous solution, as in (64) with R = 3-nitrophenylazo. Kinetic measurements were made using the temperature-jump technique with spectrophoto-metric detection to follow reactions with half-lives down to 5 x 10"6s. The reciprocal relaxation time (t ), which is the time constant of the exponential... 

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. 

Drewes et al. have found that the half-lives can be reduced significantly if the methyl acrylate is reacted with strong electrophilic aldehydes by using 3-hydroxyquinuclidine (3-HQD), instead of DABCO, as catalyst. It was speculated that the hydrogen-bonding stabilization of the catalyst-acrylate adduct by means of the hydroxyl group led to a rate-enhancement effect over DABCO.Subsequently, they further confirmed this hypothesis with the fact that 3-hydroxyquinuclidine enhanced the rate of the MBH reaction whereas its acetylated derivative was a poor catalyst. Moreover, a comparative study showed that methyl vinyl ketone was more reactive than methyl acrylate in the MBH reaction (Scheme 2.29). 

Chang et al. (2009) reported the viability of using a consensus 3D QSAR pharmacophore model to predict the stereospecific rat serum carboxylesterase (CEs) hydrolysis rates and half lives for the pyrethroids. Utilizing a stereoselective in vitro metabolism dataset from Sprague-Dawley rat serum CES, and 27 Type I and II pyrethroids, a ligand-derived pharmacophore model was developed to filter catalytically competent and incompetent ligands, that were based on four pharmacophore features (i.e., two aromatic, and two projected hydrogen bond... 

---