• 한국자기공명학회논문지
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• 제8권2호
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• pp.127-138
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• 2004

We obtained the chemical shifts of amide protons (NHs) in helical peptides at various temperatures and trifluoroethanol (TFE) concentrations using 2-dimensional NMR spectroscopy. These NH chemical shifts and their temperature dependence exhibited characteristic periodicity of 3-4 residues per cycle along the helix, where downfield shifted NHs showed larger temperature dependence. In an attempt to understand these observations, we focused on hydrogen bonding changes in the peptides and examined the validity of two possible explanations: (1) changes in intermolecular hydrogen bonding caused by differential solvation of backbone carbonyl groups by TFE, and (2) changes in intramolecular hydrogen bonding due to disproportionate variations in the hydrogen bonding within the peptide helix. Interestingly, the slowly exchanging NHs, which were on the hydrophobic side of the helix, showed consistently larger temperature dependences. This could not be explained by the differential solvation assumption, because the slowly exchanging NHs would become more labile if the preceding carbonyl groups were preferentially solvated by TFE. We suggest that the disproportionate changes in intramolecular hydrogen bonding better explain both the temperature dependence and the exchange behavior observed in this study.

• Journal of Photoscience
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• 제1권1호
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• pp.53-59
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• 1994

Molecular orientation and polarity of solubilization site of dipolar azobenzenes solubilized in micellar solutions are discussed. The polarity of solubilization was estimated by using Taft $\pi$$^*$ scale with linear solvation energy relationship, $\Delta$E=$\Delta$E$_0$ + S($\pi$$^*$ + d$\delta$)+a$\alpha$ + b$\beta$. Hydrogen bonding effects were taken into account for the estimation of micropolarity. The polarity that azobenzenes experienced in the miceliar solutions was close to water which represented that the azobenzenes were mostly solubilized at the interface. For the orientations of azobenzenes were concerned, the nitro group of NPNOH faced the interface and the hydroxy group of NPNO$^-$ located at the interfacial area.

• Bulletin of the Korean Chemical Society
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• 제22권6호
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• pp.597-604
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• 2001

The reaction of [(2-N,N-dimethylaminomethyl) pheny] methylvinychlorosiane with t-BuLi in hexane solvent gave dimers, five isomeric 1,3-disilacyslobutanes which were weparated and charaterized. In trapping experiments with various trapping agents, no corresponding silene-trapping aduct was observed. We suggest that more important species for the formation of five isomeric dimers might be the zwitterionic species generated by virtue of intramolecular donor atom rather than the silene.

• 대한화학회지
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• 제53권2호
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• pp.133-136
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• 2009

4-아미노피리딘 (4AP)–HCl와 –HBr 복합체에 대한 양자화학적 계산을 수행하였다. 전하분리된 (즈비 터이온) 형태의 [4AP$H^+-Cl^-$]는 안정하지 않으나, [4AP$H^+-Br^-$]은 안정하여 저온기체상에서 관찰될 것으로 예측되었다. 이것은 HCl에 비하여 HBr의 해리에너지가 작다는 사실과, 4AP의 아미노기가 HBr의 “용매”로 작용 한 결과로 해석되었다.

• Bulletin of the Korean Chemical Society
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• 제34권8호
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• pp.2251-2255
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• 2013

Second-order rate constants ($k_{HOO^-}$) for the nucleophilic substitution reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with $HOO^-$ in $H_2O$ have been measured spectrophotometrically. The ${\alpha}$-nucleophile $HOO^-$ is 10-70 times more reactive than the reference nucleophile $OH^-$ although the former is ca. $4pK_a$ units less basic than the latter, indicating the ${\alpha}$-effect is operative. The Bronsted-type plot for the reactions of 4a-4i with $HOO^-$ is linear with ${\beta}_{lg}=-0.51$, a typical ${\beta}_{lg}$ value for reactions which were reported to proceed through a concerted mechanism. The Yukawa-Tsuno plot is also linear with ${\rho}=1.40$ and r = 0.47, indicating that a negative charge develops partially on the O atom of the leaving group, which can be delocalized to the substituent Y through resonance interactions. Thus, the reactions have been proposed to proceed through a concerted mechanism. The magnitude of the ${\alpha}$-effect (i.e., the $k_{HOO^-}/k_{HO^-}$ ratio) decreases linearly as the leaving-group basicity increases. It has been concluded that solvation effect is not solely responsible for the ${\alpha}$-effect found in this study but the transition-state stabilization through an intramolecular H-bonding interaction is also responsible for the ${\alpha}$-effect.