| Abstract: | This article reports on both (1) the precision and capability of a computerized multidimensional spectrophotometric system recently developed in our laboratory and (2) the high-resolution study of the helix–coil transition of poly(L-glutamic acid)[poly(Glu)], especially with regard to the discovery of an overlooked transition which is attributable to order–disorder rearrangement of the poly(Glu) side chain in the α-helical conformation. This study was made possible by the high performance of the system used. The simultaneous and continuous measurement of the circular dichroism, the absorbance and light-scattering intensity, and the pH titration curve of poly(Glu) in aqueous salt solution was carried out under continuous scanning of pH ranging from 8 to 2. Besides the well-known random coil to α-helix transition that occurs at about pH 5.5, a highly cooperative transition, which is indicated as a small but definite step in several spectral dimensions, is observed for the first time at pH 4.3. The transition is ascribed to an order–disorder conversion of the side chain on the α-helix backbone. |
