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N'-Acryloyl-L-histidine was synthesized and copolymerized with N-vinylpyrrolidone using a radical initiator. The resultant copolymers having a variety of compositions were used as catalysts for the hydrolysis of p-nitrophenyl esters of carboxylic acids in 20% aqueous dioxane, and the second-order rate constant was determined. Because of the hydrophobic character of these copolymer catalysts, p-nitrophenyl laurate that carries a long acyl group was condensed along the copolymer chain by hydrophobic interactions, hence the copolymer catalysts were almost five times as reactive as Nz-acryloyl-L-histidine for the hydrolysis at pH 7,92. Increasing the pH to 10,07, the rate constant ratio increased to almost 15. This was explained in terms of the enhanced catalytic activity due to the increased cooperations between neutral imidazole functions. Since the rate increase attained was much smaller than that observed in the enzyme, the contribution to the enzyme efficiency of fixing the functional groups in a specific arrangement was relieved. ZUSAMMENFASSUNG:N'-Acryloyl-~-histidin wurde synthetisiert und radikalisch mit N-Vinylpyrrolidon copolymerisiert. So wurden Copolymere mit unterschiedlicher Zusammensetzung erhalten, die als Katalysatoren fur die Hydrolyse von p-Nitrophenylestern verschiedener Carbonsauren verwendet wurden. In 20proz. waDrigem Dioxan wurden die Geschwindigkeitskonstanten zweiter Ordnung bestimmt. Wegen des hydrophoben . Charakters dieser Copolymere wird p-Nitrophenyllaurat, das einen langkettigen Acylrest tragt, durch hydrophobe Wechselwirkungen langs der Polymerkette angereichert. Dadurch sind die Copolymere bei einem pH-Wert von 7,92 etwa fiunfmal so reaktiv als Katalysator fur die Hydrolyse wie N7-Acryloyl-L-histidin. Bei einer Steigerung des pH-Wertes auf 10,07 *) Revised manuscript of November 27, 1975. Y. Imanishi, Y. Amimoto, T. Sugihara, and T. Higashimura steigt das Verhaltnis der Geschwindigkeitskonstanten auf fast 15. Diese gesteigerte katalytische Wirksamkeit wird durch die Zunahme kooperativer Effekte zwischen neutralen Imidazoleinheiten erklart. Da der erhaltene Geschwindigkeitszuwachs vie1 kleiner ist, als bei Enzymen beobachtet werden kann, wird dadurch der Beitrag einer bestimmten raumlichen Anordnung der funktionellen Gruppen fur die Wirksamkeit von Enzymen hervorgehoben.

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ABSTRACT:Polymerizations of a series of phenylalanine NCA's carrying various ring substituents, such as p- CHsO,, and m-N02, initiated by poly(2-vinylpyridine) were investigated in different solvents, such as dimethylformamide, nitrobenzene, methyl benzoate, and acetophenone; the polymerization rates were compared with those induced by a-picoline. The binding constants of the a-amino acid NCA's for different initiators were also determined. It was concluded that in methyl benzoate or acetophenone poly(2-vinylpyridine) is particularly effective in the polymerization of electron-accepting NCA because the electron-donating polymer binds NCA through hydrogen bonding as well as through a charge-transfer interaction. In nitrobenzene only the hydrogen bonding between the polymer and the NCA's was involved in the accelerated polymerization by poly(2-vinylpyridine). In dimethylformamide no interactions were operating between the polymer and the NCA's, and consequently the polymerization by poly(2-vinylpyridine) which is a weaker base than a-picoline was slower. These findings suggest the possibility of substrate-selective polymerization using a polymer initiator with a suitable choice of solvent.KEY WORDS Ring-Substituted Phenylalanine NCA / Polymer Initiator / Solvent / Hydrogen Bond / Charge-Transfer Interaction / Selectivity / The efficiency and the specificity of enzyme catalysis have been considered to come partly from the complexation of a substrate by the enzyme. 1 The complexation takes place through secondary valence forces such as hydrogen bonding, charge-transfer interaction, hydrophobic interaction, and electrostatic interaction. 2 In order to transplant features of the enzyme into a synthetic catalyst, the enzyme catalysis has been investigated by using simplified model systems. 3 These models have usually been macromolecular compounds, which possess sites for substrate binding as well as sites for catalytic action. The present authors have investigated the polymerization of a-amino acid N-carboxyanhydride (NCA) initiated by polymeric amines. The binding of NCA by the polymeric amine through the hydrogen bonding resulted in a sizable increase of the polymerization rate. 4 -7 The charge-transfer interaction between the polymeric amine and NCA was also found to affect the polymerization rate. 8 Furthermore, it has been suggested that the hydrogen bonding and the charge-transfer interaction affect the polymerization rate concurrently. 9 A combination of different types of interactions, such as hydrogen bonding and charge-transfer interaction, could lead to a more efficient and more selective catalyst, as in the case of enzymes. To this end poly(2-vinylpyridine) was chosen as an initiator for the NCA polymerization. The pyridyl group, a tertiary amine which initiates the polymerization of NCA, 10 is a hydrogen-bond acceptor and an electron donor. Therefore poly(2-vinylpyridine) could be an efficient and selective initiator for the polymerization of NCA, which carries an active hydrogen and an electron-accepti...

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