• Macromolecular Research
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• 제11권5호
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• pp.375-381
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• 2003

Silver salt complex membranes with glassy poly(methyl methacrylate) (PMMA) unexpectedly showed higher propylene permeance than those with rubbery poly(butyl methacrylate) (PBMA) where as neat PMMA is much less permeable to propylene than that of neat PBMA. Such unusual facilitated olefin transport has been systematically investigated by changing the side chain length of polymethacrylates (PMAs) from methyl, ethyl to butyl. The ab initio calculation showed almost the same electron densities of the carbonyl oxygens in the three PMAs, expecting very similar intensity of the interaction between carbonyl oxygen and silver ion. However, the interaction intensity decreases with the length of the alkyl side chain: PMMA > PEMA > PBMA according to wide angle X-ray scattering and FT-Raman spectroscopy. The difference in the interaction intensity may arise from the difference in the hydrophilicity of the three PMAs, as confirmed by the contact angle of water, which determines the concentrations of the ionic constituents of silver salts: free ion, contact ion pair and higher order ionic aggregate. However, propylene solubilities and facilitated propylene transport vary with the side chain length significantly even at the same concentration of the free ion, the most active olefin carrier, suggesting possible difference in the prohibition of the molecular access of propylene to silver ion by the side chains: the steric hindrance. Therefore, it may be concluded that both the hydrophilicity and the steric hindrance associated with the side chain length in the three PMAs are of pivotal importance in determining facilitated olefin transport through polymer/silver salt complex membranes.

• Bulletin of the Korean Chemical Society
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• 제10권3호
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• pp.254-258
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• 1989

Adsorption of 1,3-propanedithiol, 1,4-butanedithiol, 1,5-pentanedithiol, and 1,6-hexanedithiol on silver surface has been investigated by surface-enhanced Raman spectroscopy. It has been found that the conformations of the adsorbates were mainly affected by steric interaction of the adsorbates with the surface. As the alkyl chain length separating the thiol groups increased, surface stacking efficiency became increasingly important in determining conformation of the adsorbate on the surface.

• 멤브레인
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• 제12권4호
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• pp.207-215
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• 2002

한정된 미세공간에서의 제한확산(hindered diffusion)은 멤브레인 기공(pore)에서 입자들의 운동에 의해 결정되는 여과 메카니즘을 매우 미세한 수준에서 이해하는데 중요한 현상이다. 구형(spherical) 콜로이드 입자에 비해 보다 복잡한 형태(conformation)인 고분자사슬 구조를 갖는 다가전해질(polyelectrolyte)의 제한확산 거동에는 다양한 인자들이 관련되어 있기 때문에, 이론 접근은 물론 실험적 접근도 한층 어려운 것이 사실이다. 본 연구에서는, 슬릿형 미세기공에 한정되어 있는 단일한 다가전해질(single polyelectrolyte)에 coarse-grained bead spring model과 먼거리(long-range) 정전상호작용(electrostatic interaction)인 Debye-Huckel potential을 적용하여 분자시뮬레이션 기법인 브라운 동력학 모사를 수행하였다. 기공과 다가전해질 사슬(Polyelectrolyte chain)의 주어진 크기에서, 용액의 전해질 이온농도가 감소함에 따른 사슬의 신장(extension)효과는 제한확산계수를 감소시켰고, 기공 벽면의 하전성은 제한확산계수를 더욱 감소시켰다. 이는, 다가전해질 사슬(polyelectrolyte chain)의 입체적 장애(steric hindrance)와 함께 정전반발력이 미세기공에서의 확산이동을 억제함을 의미한다.

• Bulletin of the Korean Chemical Society
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• 제35권7호
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• pp.2081-2085
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• 2014

Second-order rate constants ($k_N$) have been measured spectrophotometrically for the reactions of phenyl 2- pyridyl carbonate (6) with a series of cyclic secondary amines in MeCN at $25.0{\pm}0.1^{\circ}C$. The Br${\o}$nsted-type plot for the reaction of 6 is linear with ${\beta}_{nuc}$ = 0.54, which is typical for reactions reported previously to proceed through a concerted mechanism. Substrate 6 is over $10^3$ times more reactive than 2-pyridyl benzoate (5), although the reactions of 6 and 5 proceed through the same mechanism. A combination of steric hindrance, inductive effect and resonance contribution is responsible for the kinetic results. The reactions of 6 and 5 proceed through a cyclic transition state (TS) in which H-bonding interactions increase the nucleofugality of the leaving group (i.e., 2-pyridiniumoxide). The enhanced nucleofugality forces the reactions of 6 and 5 to proceed through a concerted mechanism. In contrast, the corresponding reaction of 4-nitrophenyl 2-pyridyl carbonate (7) proceeds through a stepwise mechanism with quantitative liberation of 4-nitrophenoxide ion as the leaving group, indicating that replacement of the 4-nitrophenoxy group in 7 by the PhO group in 6 changes the reaction mechanism (i.e., from a stepwise mechanism to a concerted pathway) as well as the leaving group (i.e., from 4-nitrophenoxide to 2-pyridiniumoxide). The strong electron-withdrawing ability of the 4-nitrophenoxy group in 7 inhibits formation of a H-bonded cyclic TS. The presence or absence of a H-bonded cyclic TS governs the reaction mechanism (i.e., a concerted or stepwise mechanism) as well as the leaving group (i.e., 2-pyridiniumoxide or 4-nitrophenoxide).

• Bulletin of the Korean Chemical Society
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• 제14권1호
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• pp.101-107
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• 1993

Ab initio calculations are carried out at the 6-311G$^{**}$ level for the $C_{2v}$ interactions of Be and Li atoms with acetylene molecule. The main contribution to the deep minima on the $^3B_2\;BeC_2H_2\;and\;^2B_2 LiC_2H_2$ potential energy curves is the b_2\;(2p(3b_2)-l{\pi}_g^*(4b_2))$ interaction, the $a_1\;(2s(6a_1)-I{\pi}_u(5a_1))$ interaction playing a relatively minor role. The exo deflection of the C-H bonds is basically favored, as in the $b_2$ interaction, due to steric crowding between the metal and H atoms, but the strong in-phase orbital interaction, or mixing, of the $a_1$ symmetry hydrogen orbital with the $5a'_1,\;6a'_1,\;and\;7a'_1$ orbitals can cause a small endo deflection in the repulsive complexes. The Be complex is more stable than the Li complex due to the double occupancy of the 2s orbital in Be. The stability and structure of the $MC_2H_2$ complexes are in general determined by the occupancy of the singly occupied frontier orbitals.

• 분석과학
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• 제5권3호
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• pp.239-247
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• 1992

메틸기가 한 개 또는 두 개 치환된 10가지 메틸아닐리늄 이온과 18-크라운-6와의 착물 형성 및 선택성을 메탄올에서 적하수은 전극에 의해 조사하였다. 메틸아닐리늄 이온과 18-크라운-6와의 착물의 안정도 상수는 메틸기의 위치와 개수에 따른 입체장애 효과에 의해 큰 차이를 나타내었다. 또한 반파전위의 차가 매우 작아 일반적인 방법으로는 분석이 불가능한 메틸아닐리늄 이성질체 혼합물 등에 보조 착화제로 18-크라운-6를 첨가하여 입체장애에 의한 착물반응의 선택성을 이용하여 분석을 시도하였다. 그 결과 18-크라운-6와 두 게스트 이온의 안정도 상수의 차, ${\Delta}log\;K$가 대략 0.7~1.3인 경우 이성분 혼합물의 확인이 정성적으로 가능하였으며, 1.6 이상인 경우에는 정량적인 개별분석도 가능하였다. 이는 18-크라운-6가 아닐리늄의 메틸치환기의 위치에 따라 입체장애의 정도를 선별적으로 인식하여 큰 착물형성 선택성을 나타낸 결과로 각 환원파의 음전위 이동 정도가 크게 달라지기 때문이다.

• BMB Reports
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• 제31권5호
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• pp.459-467
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• 1998

A new set of functional group interaction energy descriptors relevant to the ACE (Angiotensin-Converting Enzyme) inhibitory peptide, QSAR (Quantitative Structure Activity Relationships), is presented. The functional group interaction energies approximate the charged interactions and distances between functional groups in molecules. The effective energies of the computationally derived geometries are useful parameters for deriving 3D-QSAR models, especially in the absence of experimentally known active site conformation. ACE is a regulatory zinc protease in the renin-angiotensin system. Therapeutic inhibition of this enzyme has proven to be a very effective treatment for the management of hypertension. The non bond interaction energy values among functional groups of six-feature of ACE inhibitory peptides were used as descriptor terms and analyzed for multivariate correlation with ACE inhibition activity. The functional group interaction energy descriptors used in the regression analysis were obtained by a series of inhibitor structures derived from molecular mechanics and semi-empirical calculations. The descriptors calculated using electrostatic and steric fields from the precisely defined functional group were sufficient to explain the biological activity of inhibitor. Application of the descriptors to the inhibition of ACE indicates that the derived QSAR has good predicting ability and provides insight into the mechanism of enzyme inhibition. The method, functional group interaction energy analysis, is expected to be applicable to predict enzyme inhibitory activity of the rationally designed inhibitors.

• Bulletin of the Korean Chemical Society
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• 제25권12호
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• pp.1917-1923
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• 2004

Although the proteolytic activity of the Cu(II) complex of cyclen (Cyc) is greatly enhanced upon attachment to a cross-linked polystyrene (PS), the Cu(II)Cyc-containing PS derivatives reported previously hydrolyzed only a very limited number of proteins. The PS-based artificial metalloproteases can overcome thermal, mechanical, and chemical instabilities of natural proteases, but the narrow substrate selectivity of the artificial metalloproteases limits their industrial application. In the present study, artificial metalloproteases exhibiting broad substrate selectivity were synthesized by attaching Cu(II)Cyc to a PS derivative using linkers with various structures in an attempt to facilitate the interaction of various protein substrates with the PS surface. The new artificial metalloproteases hydrolyzed all of the four protein substrates (albumin, myoglobin, ${\gamma}$-globulin, and lysozyme) examined, manifesting $k_{cat}/K_m$ values of 28-1500 $h_{-1}M_{-1}$ at 50 $^{\circ}C$. The improvement in substrate selectivity is attributed to steric and/or polar interaction between the bound protein and the PS surface as well as the hydrophobicity of the microenvironment of the catalytic centers.

• Bulletin of the Korean Chemical Society
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• 제23권2호
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• pp.201-204
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• 2002

The rates of the aminolysis of S-phenyl substituted-acetate series $(RC(=O)SC_6H_4Z$, with R=Me, Et, i-Pr, t-Bu and Bn) with benzylamines $(XC_6H_4CH_2NH_2)$ are not correlated simply with the Taft's polar $({\sigma}^{\ast})$ and/or steric effect constants $(E_s)$ of the substituents due to abnormally enhanced rate of the substrate with R=Et. Furthermore, the cross-interaction constant, ${\rho}x_z$ , is the largest with R=Et. These anomalous behaviors can only be explained by invoking the vicinal bond $({\sigma})$-antibond $({\sigma}^{\ast})$ charge transfer interaction between C-$C{\alpha}$ and C-S bonds. In the tetrahedral zwitterionic intermediate, $T^{\pm}$ , formed with R=Et the vicinal ${\sigma}_{c-c}-{\sigma}^{\ast}_{c-s}$ delocalization is the strongest with an optimum antiperiplanar arrangement and a narrow energy gap, ${\Delta}{\varepsilon}={\varepsilon}_{{\sigma}^{\ast}}-{\varepsilon}_{\sigma}$. Due to this charge transfer interaction, the stability of the intermediate increases (with the concomitant increase in the equilibrium constant K (= $k_a/k_{-a}$)) and also the leaving ability of the thiophenolate leaving group increases (and hence $k_b$ increases) so that the overall rate, $k_n\;=\;Kk_b$, is strongly enhanced. Theoretical support is provided by the natural bond orbital (NBO) analyses at the B3LYP/6-31+$G^{\ast}$ level. The anomaly exhibited by R=Et attests to the stepwise reaction mechanism in which the leaving group departure is rate limiting.

• Bulletin of the Korean Chemical Society
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• 제29권3호
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• pp.647-655
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• 2008

Microsomal prostaglandin E2 synthase (mPGES-1) is a potent target for pain and inflammation. Various QSAR (quantitative structure activity relationship) analyses used to understand the factors affecting inhibitory potency for a series of MK886 analogues. We derived four QSAR models utilizing various quantum mechanical (QM) descriptors. These QM models indicate that steric, electrostatic and hydrophobic interaction can be important factors. Common pharmacophore hypotheses (CPHs) also have studied. The QSAR model derived by best-fitted CPHs considering hydrophobic, negative group and ring effect gave a reasonable result (q2 = 0.77, r2 = 0.97 and Rtestset = 0.90). The pharmacophore-derived molecular alignment subsequently used for 3D-QSAR. The CoMFA (Comparative Molecular Field Analysis) and CoMSIA (Comparative Molecular Similarity Indices Analysis) techniques employed on same series of mPGES-1 inhibitors which gives a statistically reasonable result (CoMFA; q2 = 0.90, r2 = 0.99. CoMSIA; q2 = 0.93, r2 = 1.00). All modeling results (QM-based QSAR, pharmacophore modeling and 3D-QSAR) imply steric, electrostatic and hydrophobic contribution to the inhibitory activity. CoMFA and CoMSIA models suggest the introduction of bulky group around ring B may enhance the inhibitory activity.