The hydrophobic interior of a liposome membrane was used as a platform for the organic synthesis of hydrophobic compounds in water. The 1,3-dipolar cycloaddition of benzonitrile oxide (BNO) and N-ethylmaleimide (EMI) in liposome suspensions was carried out, and an increase in the reaction rate constant was observed depending on the liposome characteristics. While the reaction rate constant in 1,4-dioxane was 1.5 times higher than that in water, the reaction rate constant in an aqueous solution of cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) liposome was 3 times higher than in water. The amount of substrate, BNO, accumulated in the DOTAP liposome was higher than that in 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP), indicating that BNO prefers to be distributed in the liposome membrane in the liquid-disordered phase. The membrane polarity, GP340, as monitored by Laurdan, varied with the presence of BNO, while EMI slightly affected the membrane properties of the liposomes. These results suggest that the pseudo-interphase afforded by the liposome membrane can promote the 1,3-dipolar cycloaddition between BNO and EMI in water.
We first report that a highly enantioselective C–C bond formation reaction was achieved with liposomes in aqueous media. Alkylation of N -(diphenylmethylene)glycine tert -butyl ester (DMGBE) with benzyl bromide was conducted in the presence of cetyltrimethylammonium bromide micelles, resulting in a high conversion of DMGBE but little enantiomeric excess ( e.e. ) of the product. The same reaction was then carried out in 1,2-dioleoyl- sn -glycero-3-phosphocholine liposome suspensions, where the e.e. values were high (at least 90 % (S)), indicating that the liposome membranes can behave as the promoter of the enantioselective reaction. Changing the type of lipid to 1,2-dipalmitoyl- sn -glycero-3-phosphocholine to form a more ordered bilayer membrane lowered the reaction conversion but still maintained high e.e. % , that is, >90 (S), regardless of lipid chirality. It is indicated that multiple interactions between the DMGBE intermediate and lipid molecules promoted the migration of the intermediate into the interior of the membrane, whose bottom side ( Si face) could be free for alkylation. These results suggest that liposomes can promote and regulate the alkylation of amino acid derivatives.
Artificial vesicles formed from sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in aqueous solution are used successfully as additives for enzymatic oligomerizations or polymerizations of aniline or the aniline dimer p-aminodiphenylamine (PADPA) under slightly acidic conditions (e.g., pH 4.3 with horseradish peroxidase and hydrogen peroxide as oxidants). In these systems, the reactions occur membrane surface-confined. Therefore, (i) the physicochemical properties of the vesicle membrane and (ii) the interaction of aniline or PADPA with the AOT membrane play crucial roles in the progress and final outcome of the reactions. For this reason, the properties of AOT vesicles with and without added aniline or PADPA were investigated by using two fluorescent membrane probes: 1,6-diphenyl-1,3,5-hexatriene (DPH) and 6-lauroyl-2-dimethylaminonaphthalene (Laurdan). DPH and Laurdan were used as "sensors" of the membrane fluidity, surface polarity, and membrane phase state. Moreover, the effect of hexanol, alone or in combination with aniline or PADPA, as a possible modifier of the AOT membrane, was also studied with the aim of evaluating whether the membrane fluidity and surface polarity is altered significantly by hexanol, which, in turn, may have an influence on the mentioned types of reactions. The data obtained indicate that the AOT vesicle membrane at room temperature and pH 4.3 (0.1 M NaHPO) is more fluid and has a more polar surface than in the case of fluid phospholipid vesicle membranes formed from 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Furthermore, the fluorescence measurements indicate that mixed AOT-hexanol membranes are less fluid than pure AOT membranes and that they have a lower surface polarity than pure AOT membranes. PADPA strongly binds to AOT and to mixed AOT/hexanol membranes and leads to drastic changes in the membrane properties (decrease in fluidity and surface polarity), resulting in Laurdan fluorescence spectra, which are characteristic for intramembrane phase separations (coexistence of ordered and disordered domains). This means that highly fluid AOT membranes transform upon the addition of PADPA into membranes that have ordered domains. Although the relevance of this finding for the enzymatic oligomerization of PADPA is not yet clear, it is also of interest if one likes to use heterogeneous vesicle membranes as additives for carrying out membrane surface-confined reactions that do not necessarily involve PADPA as a reactant.
The micro-polarity and micro-viscosity of liposome membranes were evaluated to develop a platform for the localization of hydrophobic substrates in aqueous solution. The distribution ratios of benzaldoxime (BO) onto the zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposome and onto the cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) liposome were almost the same, while those of benzonitrile oxide (BNO) onto DOTAP liposomes were higher than those of BO. Through the analysis of a polarity-sensitive 6-lauroyl-2-dimethylaminonaphthalene, the membrane surface of the DOTAP liposome was found to be dehydrated in the presence of substrates. Using a fluorescent probe 1,6-diphenyl-1,3,5-hexatriene, we found that the micro-viscosity of the DOTAP liposome membrane increased with BNO. These results indicate that the interaction of hydrophobic substrates induce variations in the microscopic membrane environment.
PhoPop5 and PhoRpp30 in the hyperthermophilic archaeon Pyrococcus horikoshii, homologues of human ribonuclease P (RNase P) proteins hPop5 and Rpp30, respectively, fold into a heterotetramer [PhoRpp 30(PhoPop5) 2 PhoRpp30], which plays a crucial role in the activation of RNase P RNA (PhopRNA). Here, we examined the functional implication of PhoPop5 and PhoRpp30 in the tetramer. Surface plasmon resonance (SPR) analysis revealed that the tetramer strongly interacts with an oligonucleotide including the nucleotide sequence of a stem-loop SL3 in PhopRNA. In contrast, PhoPop5 had markedly reduced affinity to SL3, whereas PhoRpp30 had little affinity to SL3. SPR studies of PhoPop5 mutants further revealed that the C-terminal helix (a4) in PhoPop5 functions as a molecular recognition element for SL3. Moreover, gel filtration indicated that PhoRpp30 exists as a monomer, whereas PhoPop5 is an oligomer in solution, suggesting that PhoRpp30 assists PhoPop5 in attaining a functionally active conformation by shielding hydrophobic surfaces of PhoPop5. These results, together with available data, allow us to generate a structural and mechanistic model for the PhopRNA activation by PhoPop5 and PhoRpp30, in which the two Cterminal helices (a4) of PhoPop5 in the tetramer whose formation is assisted by PhoRpp30 act as binding elements and bridge SL3 and SL16 in PhopRNA.Keywords: archaea/proteinRNA interaction/ Pyrococcus horikoshii/ribonuclease P/surface plasmon resonance.Abbreviations: 3-D, three-dimensional; PhopRNA, ribonuclease P RNA from P. horikoshii; pre-tRNA, precursor tRNA; RNase P, ribonuclease P; RNP, ribonucleoprotein; RRM, RNA recognition motif; SL3, stem-loop containing P3 helix; SL16, stem-loop containing P16 helix; SPR, surface plasmon resonance.Ribonuclease P (RNase P) is a ubiquitous trans-acting ribozyme that catalyses the processing of 5 0 leader sequences from tRNA precursors (pre-tRNA) and other noncoding RNAs in all living cells (1, 2). In contrast to eubacterial RNase P RNAs, the RNA components in archaea and eukaryotes alone have little catalytic activity in vitro and function in cooperation with protein subunits in substrate recognition and catalysis (3). Hence, archaeal and eukaryotic RNase Ps may serve as a model ribonucleoprotein (RNP) for studying how a functional RNA can be activated by protein cofactors and how the RNP enzymes catalyse biological processes.We earlier found that RNase P RNA (PhopRNA) and five proteins in the hyperthermophilic archaeon Pyrococcus horikoshii OT3 reconstituted RNase P activity that exhibits enzymatic properties like those of the authentic enzyme (4, 5). The P. horikoshii RNase P proteins were designated PhoPop5, PhoRpp21, PhoRpp29, PhoRpp30 and PhoRpp38, according to their sequence homology with the human RNase P proteins hPop5, Rpp21, Rpp29, Rpp30 and Rpp38, respectively (6). Biochemical and structural studies revealed that PhoPop5 and PhoRpp21 form a complex with PhoRpp30 and PhoRpp29, and the resulting complexes, PhoPop5PhoRpp30 and Pho Rpp21PhoRpp29, are involved i...
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