Poly-L-valine

Komori and Nonaka132,133 electrochemically oxidized methyl, isopropyl, n-butyl, isobutyl, r-butyl and cyclohexyl phenyl sulfides (108) and cyclohexyl p-tolyl sulfide (109) to their sulfoxides using a variety of polyamino acid-coated electrodes to obtain the range of e.e. values shown in parentheses. The highest enantiomeric purities were obtained using an electrode doubly coated with polypyrrole and poly(L-valine), an electrode which also proved the most durable of those prepared. 

Very low asymmetric induction (e.e. 0.3-2.5%) was noted when unsymmetrical sulphides were electrochemically oxidized on an anode modified by treatment with (— )camphoric anhydride or (S)-phenylalanine methyl ester299. Much better results were obtained with the poly(L-valine) coated platinum electrodes300. For example, t-butyl phenyl sulphide was converted to the corresponding sulphoxide with e.e. as high as 93%, when electrode coated with polypyrrole and poly(L-valine) was used. 

Several alkyl aryl sulfides were electrochemically oxidized into the corresponding chiral sulfoxides using poly(amino acid)-coated electrodes448. Although the levels of enan-tioselection were quite variable, the best result involved t-butyl phenyl sulfoxide which was formed in 93% e.e. on a platinum electrode doubly coated with polypyrrole and poly(L-valine). Cyclodextrin-mediated m-chloroperbenzoic acid oxidation of sulfides proceeds with modest enantioselectivity44b. 

Subsequently, a number of reactions at poly-L-valine coated carbon electrodes 237-243) gj.g reported to yield optically active products. Reductions, e.g. of citraconic acid or l,l-dibromo-2,2-diphenylcyclopropane as well as the oxidation of aryl-alkyl sulfides proceeded with chiral induction at such electrodes... 

TaUe 4. Asymmrtric electrochemical reactions at poly-L-valine coated carbon electrodes... 

In an enantiomer-differentiating oxidation at a poly-(L-valine)-coated Pb02 anode, rac-2,2-dimethyl-l -phenyl-1 -propanol was partially oxidized leaving 43% optically pure (5)-alcohol [371]. At a TEMPO-modified graphite felt anode rac-1-phenyl-ethanol has been enantioselectively oxidized in the presence of (-[-sparteine leaving 46% of the (/ [-alcohol with 99.6% ee [372]. However, under the same conditions, an exclusive dehydrogenation of (-[-sparteine to the iminium salt without oxidation of the alcohol was found [373]. 

Julid and Colonna showed that, as expected, poly-(L)-leucine and poly-(i)-isoleucine could be employed in place of poly-(L)-alanine, while poly-(L)-valine gave poorer results in terms of yields and stereoselectivity [13]. Some other eno-nes were tried without success and nucleophiles other than peroxide anion were... 

From such a background, some kinds of polypeptide blend samples have been studied by solid state NMR.27,72 74 Especially, detailed information for four kinds of blend samples such as poly(L-alanine) (PLA)/poly(L-valine) (PLV), PLA/poly(L-isoleucine) (PLIL), poly(D-alanine) (PDA)/PLV and polyglycine (PG)/PLV blends, have been reported. Here, let us describe some reasons why PLA/PLV, PDA/PLV, PLA/PLIL and PG/PLV blends are interesting systems. PLA and PDA in the solid-state can take the a-helix and (3-sheet forms due to intra- and intermolecular HBs, respectively. PG in the solid-state can take the 3j-helix (PG-II) and (3-sheet (PG-I) forms due to intra- and intermolecular HBs, respectively. However, PLIL and PLV in the solid state can predominantly take the (3-sheet form as the stable conformation. For this reason, it is interesting to know whether an isolated a-helix or 3i-helix form polypeptide surrounded by a major polypeptide in the (3-sheet form can take the helical conformation, or not, due to the balance between intramolecular and intermolecular hydrogen bonds. In addition, we would like to know whether a polypeptide in the (3-sheet form surrounded by a major polypeptide in the a-helix or 3 -helix form can take the (3-sheet form. 

Similarly, good results were obtained with poly-(L)-alanine, poly-(L)-leucine and poly-(L)-isoleucine, whereas with poly-(L)-valine or random copolymers reduced chemical yields as well as asymmetric induction were obtained. Chemical and optical yields are closely related in all cases. The enantiomeric excesses obtained increased as the average chain length of the catalyst increased varying from 10 to 30 amino acids (ee values between 50 to 99%). For high ee values it seems to be essential that the polymer chain is at least 10 units154. The degree of asymmetric induction decreases as the temperature is raised. The amount of catalyst only influences the chemical yield, not the optical yield of... 

N 017 "Conformational Analysis of Macromolecules. IV. Helical Structures of PolyfL-alanine), Poly(L-valine), Poly(p-methyl-L-aspartate), Polyly-methyl-l.-glutamate),... 

A particular case which should be pointed out is that of the bond angle around tetrahedral carbon atoms. Very often, all of the angles around a given carbon atom are assigned the value of 109-5°. However, in some cases, such as for aliphatic carbon chains, the value of r[CCC] has been observed to be as large as 115°. In calculations on poly-L-valine (Ooi,... 

Fig. 26. Energy contours for poly-L-valine helices, in kcal mole-1. The energy at each value of

Fig. 27. Side chain and backbone conformation for the right-handed a-helix of poly-L-valine of minimum energy. = 132°, xji—123°, xi = 290 (Ooi et al., 1967).

In addition to this we have several examples of which the polymer conformation of the polymeric complex leads the asymmetrical selectivity Hydrogenation reactions of 1-methylcinnamic acid and 1-acetamidocinnamic acid by several poly(L-amino acid)-Pd complexes are observed (142-144). Poly(L-valine) (/3-form) and poly(/3-benzyl-L-aspartate) (a-helix, sinistral) give dextrorotative products, and poly(L-leucine) and poly( 3-benzyl-L-aspartate) (a-helix, dextral) do levo-rotatory products. Also, optical active poly-/3-hydroxyl esters-Raney Ni catalyst (145) and Ion-exchange resin modified by optical active amino acid-metal complex (146,147) are observed in asymmetrically selective hydrogenations. 

Chothia153 has observed that the (3 sheets in globular proteins have a right-handed twist. Computations on model 3 sheets, for example, the parallel and antiparallel structures of poly(L-valine) sheets,154 illustrated in Figure 18, have accounted for these observations.8 10 154 In general, side chain-backbone interactions within each strand result in a preference for a right-handed twist for L-amino acids, although there are exceptions. In addition, interstrand side... 

Figure 22 s-versus-T curves for poly(L-valine) in water. The squares are the experimental results (Figure 19), and the line is the calculated result.16S... 

M. Go, F. T. Hesselink, N. Go, and H. A. Scheraga, Macromolecules, 7, 459 (1974). Molecular Theory of the Helix—Coil Transition in Poly(amino acids). IV. Evaluation and Analysis of s for Poly(L-valine) in the Absence and Presence of Water. 

Thibodeaux, S.J., Billiot, E., and Warner, I.M. Enantiomeric separations nsing poly(l-valine) and poly(l-leucine) surfactants investigation of steric factors near chiral center. J. Chromatog. A. 2002, 966, 179-186. 

Incidental to this work is more evidence that the a-helix exists at the air-water interface. While some have appeared reluctant to accept this view, no good theoretical reason exists why it should not be stable. Where the nature of the side chain might provoke other conformations [as in poly(l-valine)] or the molecular weight is low or monolayers are spread from poor solvents miscible with water, other conformations are detectable, and the monolayer properties are significantly different (5). 

Alkyl aryl sulfides electrochemically oxidized on electrodes whose surfaces were modified by coating them with optically active compounds like camphoric acid [89] and poly(amino acids) [90, 91] afford mixtures of sulfoxides with variable enantiomeric excess. An optical yield of 93% is claimed when f-butyl phenyl sulfide is oxidized at a Pt electrode doubly coated with polypyrrole and poly(L-valine) [91]. 

Komori and Nonaka [499] reported the first example of an electrochemical enantiomer-differentiating reaction When racemic 2,2-dimethyl-1-phenyl-1-propanol was oxidized at a poly(L-valine)-coated anode, 43% optically pure (S)-(—)-2,2-dimethy 1-1-phenyl-1-propanol was recovered as an unreacted part. Yamagishi and Aramata [500] also found electrooxidative optical resolution of a racemic Co(l,10-phenathroline)3 complex by a chiral clay-coated anode, and Yoshinaga and coworkers [501] electrore-ductively resolved racemic Co(acetylacetonato)3 by using optically active supporting electrolytes. 

Poly (I.-valine) (PL Vchain-length dependence of H chemical shifts. Figure 24 shows the H CRAMPS NMR spectra of poly(L-valines) (A) [Val]n-1 (DPn is about 5, /3-sheet form) and (B) [Val]n-5 (DPn = 100, /3-sheet) in the solid state. The shoulder peak around 1.2-1.5 ppm in spectrum A is assigned to the —N—CH2— peak of the n-butylamide (—NHBu) group. It is well known that PLV is easily stabilized in /3-sheet form whereas it is very difficult to take an... 

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