On the magnitude of the cotton effects of poly‐L‐glutamic acid

Abstract

The recent improvement in spectropolarimeters makes it possible to measure the Cotton effects of polypeptides and proteins between 185 and 240 rnp.Ir2 The helical conformation, quite unlike the coiled form, shows a strong trough and peak at 233 and 108 mp. The reduced mean residue rotations, [m']233 and [112']198, have thus been suggested as two measiires of the helicity in protein moleciiles. At present, poly-L-glutamic acid (PGA) in aqueous solution is providing the reference values for the two conformations. The reported value for [m']233 (helix), however, varies from -13,000 to -18,000, thiis making siich an estimation very inc certain.^ Such a wide variation in reported results is puzzling because t,he PGA samples have all been prepared by the same meth0d.l We offer a comparative stitdy of 4 samples from 4 laboratories; all stork solutions were

A Conformational Study of Glucagon

“…In the presence of 6 M guanidine hydrochloride ( Fig. 1) the amplitude of the Cotton effect diminishes, and approaches the value expected for a random coil [7,8].…”

“…Increasing the ionic strength by adding sodium chloride has no effect on the optical rotation. Taking values of -118,500" and -1,800" 19* [7,8] for the average molar residue rotations for an a-helix and a random coil, and neglecting end-effects, the acid, neutral and alkaline samples all possess approximately 10°/, of a-helix, which for a chain of 29 residues corresponds to about one turn. In the presence of 6 M guanidine hydrochloride ( Fig.…”

“…The formation of @-structures when glucagon aggregates is a phenomenon hitherto known to be paralleled only by poly-L-lysine in alkaline solution [7,46,47] and silk fibroin, on appropriate adjustment of the solvent conditions [48] ; the formation of a-helices in alkaline aggregation [34, 51 and presumably crystallisation [2] appears to be unique.…”

Physical Chemical Studies on Proteins and Polypeptides

“…The sum of the above data argues strongly that, for a-hellces, the main (70-90 per cent) source of optical rotatory power in the spectral range between 200 m~ and 600 m/, lies in the optically active absorption bands located between 185-225 m/z. This does not rule out the possibility that absorption bands lying deeper in the ultraviolet may make a small but significant contribution to the dispersions of helical polypeptldes (90,91).…”

“…The empirical nature of this equation emphasized by the fact that the parameters A19~ and A~5 cannot be directly related to the rotatory strengths of the individual electronic transitions which are ultimately responsible for the rotatory properties. A major problem in .the use of the two-term Drude equation is the choice of a reference compound in the determination of A19~ and A*** for 100 per cent a-hellcal structures (90)(91)(92)(93). For instance, as pointed out above, if poly-L-meth~onlne in chloroform is chosen as being 100 per cent helical, then PLGA in dioxane has about 90 per cent helix.…”