Poly mutarotation

In addition to the substituted camphors, " mutarotations have been observed for solutions of menthyl benzoylformate, poly(L-... 

In the absence of steric effects the activation energy of the form I — form II transition should approximate the energy barrier to rotation about the peptide linkage (21 kcal/mole) provided that mutarotation involves the proposed cis — froas-isomerization. Downie and Randall (1959) measured the rates of forward mutarotation of poly-L-proline I in acetic acid at various temperatures and obtained an activation energy, AE = 23 kcal/mole. The rate of the reaction was independent of concentration over a sevenfold dilution of the polymer. That is, at any stage of mutarotation (as measured by [ ][,) the velocity constant, fc, was found to be independent of concentration. On the other hand k decreased from 15 X 10 sec to 2.5 X 10 sec during the course of mutarotation. 

The kinetics of the optical rotatory changes of poly-L-proline in various solvents and at various temperatures have also been studied by Steinberg et al. (1960a). In acetic acid the course of the forward mutarotation reaction was found to be independent of concentration (over the range 0.25 to 2.0 gm/KK) ml) but, as observed by Downie and Randall, the rate constant depends on the degree of mutarotation. An activation enthalpy, AH = 21 kcal/mole, was determined for both the forward mutarotation of form I in acetic acid and the reverse mutarotation of form II in acetic acid-w-pro-panol. 

We have seen that the form I —> form II interconversion may take place along different pathways depending on the solvent system, with differing over-all mutarotation kinetics. In acetic acid the apparent order of the forward mutarotation reaction is 1.33 (0). It will be seen from Eq. (8) that in this case K t) should decrease with time, in agreement with the experimental observations. When 0 = 1, the mutarotation proceeds with first-order kinetics. This behavior is seen in the reverse mutarotation of poly-L-proline II in acetic acid-propanol. When /3 < 1, K (t) increases with time. This situation obtains in the initial stage of the forward mutarotation in acetic acid-water solvent which proceeds with zero-order kinetics. 

Fig. 6. Reduced viscosity (C = 1%) of poly-n-proline at SCO as a function of [a]". —, data taken during forward mutarotation in glacial acetic acid A—Ai data taken during forward mutarotation in acetic acid-water (7 3 v/v) O—O, data taken during reverse mutarotation in acetic acid- -propanol (1 9 v/v) A, value in aqueous 12 M LiBr. (Prom Steinberg et al., 1960b. Reproduced with kind permission of the American Chemical Society.)...

Downie and Randall (1961) have recently measured the mutarotation of poly-O-acetyl-L-hydroxyproline in formic acid and the reverse mutarotation in Ar,iV -dimethylformamide at various temperatures and polymer concentrations. The activation energy of forward mutarotation was 22.4 kcal/mole and of reverse mutarotation, 23.8 kcal/mole, consistent with a tram- m-isomerization mechanism involving rotation about the peptide bonds. 

Mutarotation phenomena are also exhibited by another derivative of poly - L - hydroxyproline, i.e., poly-O-p-tolylsulphonylhydroxyl - l - proline (Kurtz et al., 1957, 1958a). On dissolution in acetic acid, this polymer gives... 

The poly-L-proline II configuration propagates outward from these nuclei along single gelatin chains. This process is responsible for the more rapid, concentration-independent portion of the mutarotation phenomenon. 

Recently it has been found that formation of the biuret complex also suppresses mutarotation almost completely, while acidification (and consequent destruction of the complex) reactivates the mutarotation process (von Hippel and Wong, 1961). It was inferred from these results that chelation of the cupric ion across the peptide bond inhibits the development of the poly-L-proline Il-type helix by restraining adjacent residues in sterically unfavorable configurations. 

The anomeric forms derived from equilibration of aldoses give rise to multiple peaks when trimethylsilylated and gas chromatographed [311]. A method of overcoming this problem, assuming that mutarotation itself is not under study, is to modify the aldose. It can be oxidised and lactonised to the aldonolactone, for example, and characterised as its TMS derivative [322]. Alternatively for the identification of aldoses and alditols, more use may be made in the future of the separations achievable on open tubular columns of the poly-0-acetylaldonic nitriles (18) produced from aldoses and the poly-acetyl esters from alditols [323]. Figure 1.18 shows the separation of 32 assorted polyols and aldoses. 

A study of the rate of mutarotation of a-D-glucose in dimethyl-fonnamide in the presence of poly(methacrylic acid), PMA, using polyelectrol5de samples of a different type and degree of stereoregulaiity, has recently been reported (J5). The results of this study indicate that the rate constant for mutarotation of glucose is higher with isotactic PMA than with syndiotactic or conventional PMA. Conventional... 

The concept of bifunctional catalysis was first introduced to account for the unusually large catalytic efficiency of 2-hydroxypyridine in the mutarotation of tetramethylglucose (42). In this reaction, phenol acts as an acid catalyst and pyridine as a basic catalyst and it was, therefore, concluded that a compound with the phenolic hydroxyl and the basic nitrogen at the proper spacing should be able to produce a concerted attack on the sensitive bond of the reactive molecule, with a corresponding reduction of the required activation energy. A similar effect was invoked to explain the unusual pH dependence of the hydrolysis of p-nitrophenyl acetate in the presence of poly-4(5)-vinylimidazole (PVI) (43). 

Dukor, R.K. and Keiderling, T.A. Mutarotation studies of poly-L-proline using FTIR, electronic and vibrational circular dichroism. Biospectroscopy, 2, 83, 1996. 

The fact that the final values in the mutarotation were not zero indicates that the polymers that have undergone helix-helix transition are not equimolar mixtures of right- and left-handed helices. The right- and left-handed helices of poly(D2PyMA) are not completely enantiomers although the polymer chain is isotactic where the main-chain stereocenters are basically pseudoasymmetric. This is because the stereocenters in the vicinity of chain ends are true asymmetric centers whose absolute configurations are considered to be either RRR— or SSS—, that is, homochiral. This can result in an unbalance in the contents of the helices. The rather small mutarotation of the polymers with DP = 45 and 81 may mean that the helices of these polymers are relatively stable due to the formation of aggregates. ... 

The semi-empirical conformational energy calculations of poly-cis-5-ethylproline (PC5EP) predict that the helical structure may exist in two conformational forms such as I and II. Experimental results confirmed that in solution two major conformations may be assumed by the poly-cis-5-ethyl-D-proline. However, the calculations for poly-trans-5-ethyl-D-proline indicated that only one form may be allowed.Spectroscopic data (Circular Dichroism, NMR) showed the polypeptide exists in a poly-L-proline form-I-type helix and changes slowly to some intermediate conformation. The slow muta-rotation is partially due to the steric interactions of the ethyl group with the carbonyl group of the amide during the mutarotation. 

The work described here is concerned with the effect of the ethyl group at the 3-position of the pyrrolidine ring on the ordered helical array and the mutarotation when compared with unsubstituted poly-L-proline. An examination of the space-filling models of poly-trans-3-ethylproline indicates the existence of two different forms. The rotational isomerism of the side chain ethyl group shows that the carbonyl group is literally locked on one side, which suggests that mutarotation processes may be hindered. 

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