Glutamic acid helical content

One interesting aspect of this helix-based hypochromicity is that equimolar amounts of right- and left-handed helices will mutually cancel in optical rotatory effect, but that this will not be the case for absorptivity. Indeed, Rosenheck and Doty deduce the existence of appreciable helical content in poly-oL-glutamic acid at pH 3.0 from the decrease in absorptivity (corrected for side chains) vis- -vis a pH 8 solution (random conformation). 

As yet, however, infrared spectra cannot differentiate various helices that have been proposed (Donohue, 1953 Low and Edsall, 1956), nor is it clear that the random coil, which is in general the alternative conformation in solution, can be reliably distinguished from helices. Although the amide I frequency of helical poly-L-glutamic acid in DjO shifts from 1638 to 1644 cm upon forming the random coil (Klemperer and Doty, 1960) and similar shifts are observed in copolymers of glutamic acid and lysine upon loss of helical content (Blout and Idelson, 1958), this carbonyl frequency can be almost identical in the two forms (Elliott et al., 1957b, 1958). Additional techniques are therefore required to provide firm evidence for the a-helical conformation in solution. 

What accommodations, then, must be made in the pattern of analysis developed for standard synthetic polypeptides if it is to yield quantitative estimates of partial helical content in proteins The requirements are much the same as those set out for poly-L-lysine (see Section III, G, 3), yet since globular proteins, like copolymers of L-lysine and L-glutamic acid, cannot be made completely helical in aqueous solution, a helical reference conformation must be taken either from other standard molecules or from the nonaqueous behavior of the protein in question. This latter procedure involves solvent changes with little bearing on native conditions and may be impossible to carry out in the face of restraints imposed by proline and cross-links, so that standard helical dispersion can more feasibly serve as this reference conformation. 

A value of be = —630 is well established for synthetic polypeptides and fibrous proteins. The considerations leading to the reasonable approximation that be of the disordered chain is zero, together with the implied equality of Xo and for this state, have been discussed in Section III, C, 1, and incorporated into the pattern of analysis for partial helical content as set out in Section III, G, 2. A value of ao = 4-650 has been obtained for poly-L-glutamic acid, poly-L-lysine, and Pinna nobilis tropomyosin under the appropriate conditions. As has been stressed, these constants have conformational significance only when a value of 212 nm is used for Xo. 

Pig. 2. Helical content of poly(R-glutamic acid) - luthenium(lll) complex in aqueous solution (7). DP 100 [COOHI/lRul, o no Ru, 10, A 5, o 2.5 room temperature ionic strength 0.2... 

L-glutamic acid) (PGA) by Cho et al Aqueous PGA-solutions exhibit a similar aging-effect at constant pH = 5.7 and 25 °C as PAAm and PAAm/AAcNa. Time dependent estimations of the a-helix content from circular dichroism measurements show a decrease during the storage of the aqueous solutions from 52.3 0 (1 day) to 7A% (26 day). The decrease in helicity was... 

From Fig. 95 it can be seen that [a]x, say at 500 m/i, is more negative for the randomly coiled configurations than for the helical ones. On the basis of similar observations on a variety of polypeptides, it appears possible to generalize and state that the conversion of the helix to the random coil is accompanied by a decrease in [a]x. This is illustrated in Table XVII for a series of copolymers of L-glutamic acid and L-lysine in water solution, the per cent helix depending on the composition. If one wishes to obtain a very crude estimate of helical content from [ajx, one can assign some value such as 90° to 100° as the difference between the helix and random coil (on the basis of observations such as those indicated in Fig, 95 for PBG) and then express the per cent helical content in terms of the ratio of the observed value of [a]x to the assumed total change of 90° or 100°. 

The above considerations are summarized in Fig. 97 for data on poly-L-glutamic acid. The dispersion data were plotted for the helical (pH 4.7) and randomly coiled (pH 6.6) forms in dioxane-0.2 molar NaCl. The intermediate curves were obtained by interpolation. The following points may be noted from Fig. 97 (1) The curves are linear (simple dispersion) up to about 30 to 40% helical content (2) in this region of simple dispersion the value of increases (3) the value of fajo, shown by the dashed line, increases progressively from — 77° to — 46° over the same interval. 

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