PH-INDUCED HELIX-COIL TRANSITION OF POLY-L-LYSINE AND POLY-L-GLUTAMIC ACID AND 238-MMU DICHROIC BAND

The pH-induced conformational changes of poly-L-lysine and of poly-L-glutamic acid have been investigated by CD spectral measurements in the wavelength region 200-250 mµ. The dichroic spectra of the non-ionized polymers are typical of α-helically organized polypeptide structures characterized by a triply inflected spectrum (negative minima at 221-222 and 208-209 mµ and a positive maximum at 190-191 mµ). In the ionized form both polymers exhibit doubly inflected CD curves (small positive maximum at 218 mµ and a large minimum at 197-198 mµ). The dichroic changes at the 221-mµ extremum as a function of pH result in symmetrical S-shaped titration curves and the dichroic spectral titrations for both polymers exhibit a single point of crossing, an isodichroic point, at 204 mµ with an ellipticity of –20,400 ± 1400 deg cm2/dmol. The linear combination of dichroic contributions from the limiting curves in proportions corresponding to the α-helical and uncoiled contents [estimated by the recently developed isodichroic method17] generates all the observed intermediate curves, including the usually reported triply inflected dichroic spectrum near neutral pH's. It is suggested that the pH-induced conformational change of the two electrolytes is a single-step process involving two conformationally distinguishable forms, i.e., the α-helical and the uncoiled; the latter is characterized by a doubly inflected CD spectrum (small maximum at 218 mµ and large skewed minimum at 198 mµ). The 238-mµ minimum in the CD curves of polymers near neutral pH's is not due to any discrete electronic transition, i.e., has no molecular origin, but is rather an artifact because of the summation of a small amount of α-helical structural contribution (2-5 %) to a predominantly disordered form spectrum. © 1969, American Chemical Society. All rights reserved.