• Bulletin of the Korean Chemical Society
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• 제29권4호
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• pp.772-776
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• 2008

Second-order rate constants ($k_N$) have been measured spectrophotometrically for reactions of 3,4-dintrophenyl 2-furoate (2) with a series of secondary alicyclic amines in 80 mol % $H_2O$/20 mol % dimethyl sulfoxide (DMSO) at 25.0 ${^{\circ}C}$. The Bronsted-type plot exhibits a downward curvature for the aminolysis of 2, which is similar to that reported for the corresponding reactions of 2,4-dintrophenyl 2-furoate (1). Substrate 2 is less reactive than 1 toward all the amines studied but the reactivity difference becomes smaller as the amine basicity increases. Dissection of the second-order rate constants into the microscopic rate constants has revealed that the reaction of 2 results in a smaller $k_2/k_{-1}$ ratio but slightly larger $k_1$ value than that of 1. Steric hindrance has been suggested to be responsible for the smaller $k_1$ value found for the reactions of 1, since the ortho-substituent of 1 would inhibit the attack of amines (i.e., the $k_1$ process).

• Bulletin of the Korean Chemical Society
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• 제34권10호
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• pp.2877-2882
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• 2013

Second-order rate constants ($k_{Ox^-}$) have been measured spectrophotometrically for nucleophilic substitution reactions of 4-nitrophenyl X-substituted-cinnamates (7a-7e) and Y-substituted-phenyl cinnamates (8a-8e) with butane-2,3-dione monoximate ($Ox^-$) in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. The Hammett plot for the reactions of 7a-7e consists of two intersecting straight lines while the Yukawa-Tsuno plot exhibits an excellent linearity with ${\rho}_X$=0.85 and r=0.58, indicating that the nonlinear Hammett plot is not due to a change in the rate-determining step but is caused by resonance stabilization of the ground state (GS) of the substrate possessing an electron-donating group (EDG). The Br${\o}$nsted-type plot for the reactions of Y-substituted-phenyl cinnamates (8a-8e) is linear with ${\beta}_{lg}$ = -0.64, which is typical of reactions reported previously to proceed through a concerted mechanism. The ${\alpha}$-nucleophile ($Ox^-$) is more reactive than the reference normal-nucleophile ($4-ClPhO^-$). The magnitude of the ${\alpha}$-effect (i.e., the $k_{Ox^-}/k_{4-ClPhO^-}$ ratio) is independent of the electronic nature of the substituent X in the nonleaving group but increases linearly as the substituent Y in the leaving group becomes a weaker electron-withdrawing group (EWG). It has been concluded that the difference in solvation energy between $Ox^-$ and $4-ClPhO^-$ (i.e., GS effect) is not solely responsible for the ${\alpha}$-effect but stabilization of transition state (TS) through a cyclic TS structure contributes also to the Y-dependent ${\alpha}$-effect trend (i.e., TS effect).

• Bulletin of the Korean Chemical Society
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• 제34권1호
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• pp.49-53
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• 2013

Second-order rate constants have been measured spectrophotometrically for the reactions of phenyl Y-substituted-phenyl carbonates 7a-g with butane-2,3-dione monoximate ($Ox^-$) in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. The ${\alpha}$-nucleophile $Ox^-$ is 53-95 times more reactive than the corresponding normal-nucleophile 4-$ClPhO^-$ toward 7a-g, indicating that the ${\alpha}$-effect is operative. The magnitude of the ${\alpha}$-effect (e.g., the $k_{Ox^-}/k_{4-ClPhO^-}$ ratio) is independent of the electronic nature of the substituent Y. The cause of the ${\alpha}$-effect for the reactions of 7a-g has been suggested to be ground-state (GS) effect rather than transition-state (TS) stabilization through a six-membered cyclic TS, in which $Ox^-$ behaves a general acid/base catalyst. This idea is further supported by the result that $OH^-$ exhibits negative deviation from the linear Br${\o}$nsted-type plot composed of a series of aryloxides, while $Ox^-$ deviates positively from the linearity. Differential solvation of the GS of $Ox^-$ and 4-$ClPhO^-$ has been suggested to be responsible for the ${\alpha}$-effect exerted by $Ox^-$.

• Applied Biological Chemistry
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• 제36권5호
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• pp.364-369
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• 1993

널리 이용되는 검량방법중의 하나인 NBT 환원방법은 계면활성제인 Triton X-100의 존재하에서 그 측정감도가 크게 증폭된다. 본 연구는 이러한 Triton X-100 효과의 물리화학적 본질을 이해하기 위하여 수행하였으며, 이를 위하여 이 계면활성제가 NBT의 환원생성물인 불용성 formazan 콜로이드를 안정화시키는 효과와 수용액 중에서 동시에 진행되는 두 반응 $(NBT-O^-_{\.{^.2}}$ 산화환원 반응과 $O^-_{\.{^.2}}$의 disproportionation반응) 간의 경쟁관계에 미치는 속도론적 영향에 촛점을 맞추었다. Triton X-100 유무 조건하에서 $KO_2-DMSO$ 용액과 $K-PO_4$ 완충용액에 녹인 여러가지 농도의 NBT 용액을 빠르게 혼합하고, 이때 생성된 formazan과 $H_2O_2$를 각각 정량한 결과, 이 계면활성제는 formazan 콜로이드를 안정화 시킴으로써 결과적으로 formazan의 흡수도를 상승시키는 효과를 보일뿐만 아니라 $O^-_{\.{^.2}}$의 자발적인 dismutation을 억제하는 결과를 초래함을 확인하였다. 또한 종합적인 Triton X-100의 효과에 대한 콜로이드 안정화 효과와 속도론적 효과의 상대적 기여도는 실험조건에 따라 바뀌는데 특히 반응계의 $NBT/O^-_{\.{^.2}}$ 농도비율에 크게 좌우되는 것으로 밝혀졌다.

• Archives of Pharmacal Research
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• 제29권8호
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• pp.712-719
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• 2006

In this study, we prepared core-shell type nanoparticles of a poly(DL-lactide-co-glycolide) (PLGA) grafted-dextran (DexLG) copolymer with varying graft ratio of PLGA. The synthesis of the DexLG copolymer was confirmed by $^1H$ nuclear magnetic resonance (NMR) spectroscopy. The DexLG copolymer was able to form nanoparticles in water by self-aggregating process, and their particle size was around $50\;nm{\sim}300\;nm$ according to the graft ratio of PLGA. Morphological observations using a transmission electron microscope (TEM) showed that the nanoparticles of the DexLG copolymer have uniformly spherical shapes. From fluorescence probe study using pyrene as a hydrophobic probe, critical association concentration (CAC) values determined from the fluorescence excitation spectra were increased as increase of DS of PLGA. $^1H-NMR$ spectroscopy using $D_2O$ and DMSO approved that DexLG nanoparticles have core-shell structure, i.e. hydrophobic block PLGA consisted inner-core as a drug-incorporating domain and dextran consisted as a hydrated outershell. Drug release rate from DexLG nano-particles became faster in the presence of dextranase in spite of the release rate not being significantly changed at high graft ratio of PLGA. Core-shell type nanoparticles of DexLG copolymer can be used as a colonic drug carrier. In conclusion, size, morphology, and molecular structure of DexLG nanoparticles are available to consider as an oral drug targeting nanoparticles.

• Bulletin of the Korean Chemical Society
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• 제29권10호
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• pp.1915-1919
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• 2008

Second-order rate constants ($k_N$) have been measured spectrophotometrically for reactions of 3,4-dinitrophenyl benzoates (5b) with a series of alicyclic secondary amines in 80 mol % $H_2O$/20 mol % DMSO at 25.0 ${\pm}$ 0.1 ${^{\circ}C}$. The kinetic data have been compared with the data reported previously for the corresponding reactions of 2,4- dinitrophenyl benzoates (5a) to investigate the effect of changing the nucleofuge from 2,4-dinitrophenoxide to 3,4-dinitrophenoxide on reactivity and mechanism. The kinetic results show that aminolyses of 5a and 5b proceed through the same mechanism, i.e., a zwitterionic tetrahedral intermediate ($T^{\pm}$) with a change in the rate-determining step (RDS). Substrate 5a is more reactive than 5b when breakdown of $T^{\pm}$ is the RDS but less reactive when formation of $T^{\pm}$ is the RDS. Dissection of kN values into the microscopic rate constants (e.g., $k_1$ and $k_2/k_{-1 }$ ratio) has revealed that 5a results in larger $k_2/k_{-1}$ ratios but smaller k1 values than 5b for all the amines studied. Since 2,4-dinitrophenoxide is less basic and a better nucleofuge than 3,4-dinitrophenoxide, the larger $k_2/k_{-1}$ ratios determined for the reactions of 5a than for those of 5b are as expected. The steric hindrance exerted by the ortho-nitro group on 5a contributes to the smaller k1 values found for the reactions of 5a than for those of 5b.