Effect of aggregation on the optical rotatory dispersion of poly(α, L ‐glutamic acid)

Light‐scattering measurements show that poly(α,L‐glutamic acid) (PGA) is strongly aggregated at the low pH's (4.0–4.3) usually used to measure the mean residue rotation at the trough, [R′]233, and that [R′]233 becomes more negative with aggregation. Concurrent weight‐average molecular weight measurements by light‐scattering and [R′]233 measurements by optical rotatory dispersion as a function of pH show that a maximum negative value for [R′]233 (unaffected by aggregation) is obtained at higher pH's (4.53–4.82). Precautions should be taken to insure that the Na‐PGA sample used as standard does not aggregate at the low pH's required for its complete conversion to the helical form. An Na‐PGA sample meeting the above requirements gives [R′]233 of −14,600° ± 1.5% over the pH range 4.53–4.82 in 0.05M NaCl.