- Peptide coupling of unprotected amino acids through in situ p-nitrophenyl ester formation
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Several series of dipeptides and tripeptides were prepared via an activation-coupling method involving the in situ formation of a p-nitrophenyl ester of an (N-protected) amino acid, followed by coupling with an unprotected amino acid in partially aqueous solutions. The resulting peptide is easily isolated by precipitation. In general, the yields obtained are good to excellent and racemization is minimal. This method is particularly advantageous with respect to its simplicity and lack of obligatory side chain protection/deprotection steps.
- Gagnon, Paul,Huang, Xicai,Therrien, Eric,Keillor, Jeffrey W.
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p. 7717 - 7719
(2007/10/03)
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- Thermodynamic and (1)H NMR Study of Proton Complex Formation of Histidine-containing Cyclodipeptides in Aqueous Solution
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A thermodynamic and (1)H NMR study of proton complex formation in aqueous solution of some L-histidine-containing cyclic L-dipeptides has been carried out.The enthalpic and entropic changes associated with protonation of the cyclodipeptides, obtained by potentiometric and calorimetric measurements, together with the (1)H NMR data and NOESY experiments, enable the role played by non-covalent interactions in proton complex formation to be assessed.In addition, a comparison with c(Gly-His) permits the influence of side-chain residues on the conformation of protonated species to be observed.
- Arena, Giuseppe,Impellizzeri, Giuseppe,Maccarrone, Giuseppe,Pappalardo, Giuseppe,Sciotto, Domenico,Rizzarelli, Enrico
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p. 371 - 376
(2007/10/02)
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- Correlative Variations in Enzyme-Derived and Substrate-Derived Structures of Catalytic Transition States. Implications for the Catalytic Strategy of Acyl-Transfer Enzymes
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Acetylchymotrypsin, acetyl elastase and (carbobenzyloxy)glycyl elastase all undergo hydrolysis with the same overall solvent isotope effect, which arises from a single protonic site n/k1 = 2.45(1-n + n/2.45)>. chymotrypsin, however, shows a larger effect arising from at least two sites n/k1 = 3.34(1-n + n/1.85)2>.Formylchymotrypsin and acetylchymotrypsin undergo deacylation with α-deuterium and β-deuteium secondary isotope effects, respectively, that suggest fractional tetrahedral character at the transition state of about 0.44 (vs. 0.58 - 0.66 for similar nonenzymic reactions) when compared to equilibrium isotope effects for complete addition.The effect for acetyl elastase suggests much less tetrahedral character (0.27).Addition of an N-acyl function leads to a more inverse isotope effect, per deuterium, and thus to an apparent increase in tetrahedral character: to 0.84 for chymotrypsin; to 0.43 for (carbobenzyloxy)glycyl elastase.It is concluded that enzyme-substrate interactions at the transition state can alter both enzyme structure, as shown by the solvent isotope effects, and substrate structure as shown by the substrate isotope effects.Such alterations, in the combination of enzyme with natural substrate, probably adjust both structures for optimal catalytic interaction.
- Stein, Ross L.,Elrod, James P.,Schowen, Richard L.
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p. 2446 - 2452
(2007/10/02)
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- CATALYTIC EFFICIENCY OF SYNTHETIC MICELLAR CATALYSTS BEARING A MERCAPTO GROUP AS THE REACTION CENTER.
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In order to obtain a clue to understanding the micro-environmental effect on the reactivity of a mercapto group placed in a reaction center of enzymes, micellar surfactants bearing a mercapto group were synthesized and their catalytic activity in the degradation of p-nitrophenyl carboxylates was studied. N-Hexadecyl-N** alpha -glutaryl-L-cysteinamide (AM multiplied by (times) Cys-1) has an ability to form anionic micelles in aqueous media. The catalytic activity of AM multiplied by (times) Cys-1 was compared with that of another synthetic surfactant, N-hexadecanoyl-L-cysteine (AM multiplied by (times) Cys-2). These surfactants below their critical micelle concentations markedly accelerated the degradation of several p-nitrophenyl carboxylates. On the contrary, the concentration-rate profiles for the degradation of p-nitrophenyl dodecanoate (PNPL) as catalyzed by the surfactants indicate that the reactivity of the mercapto group is reduced upon formation of the anionic micelles.
- Murakami,Nakano,Matsumoto
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p. 2996 - 3004
(2007/10/05)
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