Optical Rotatory Dispersion and Circular Dichroism of Phosvitin at Low pH

Phosvitin undergoes a transition from an unordered conformation to a β structure if the pH of its aqueous solution is lowered sufficiently (1.8 to 1.9) to remove a major part of its negative charge. The characteristic optical rotatory dispersion features of this ordered conformation are ([m′] given in degree × cm2× decimole−1): trough at 230 mµ (−3,000), peak at 203 mµ (+15,000), and a crossover point at 220 mµ. The circular dichroism spectrum ([θ] given in degree × cm2× decimole−1) shows a minimum at 215 mµ (−11,800), a maximum at 194 mµ (+14,600), and a crossover point at 206 mµ. The rate of the transition, but not its extent, is a function of pH and of protein concentration. Salt affects the transition as well. The transition may be reversed nearly completely by an increase in pH if the duration of exposure to low pH is brief. Partial reversal appears to be possible in any case.