DYNAMICS OF HELIX-COIL TRANSITION IN POLY-L-ORNITHINE

Ultrasonic attenuation measurements have been carried out in solutions of poly-L-ornithine in water (0.2 M NaCl)-methanol (85:15) at 25˚ over the frequency range 10-165 MHz and the pH range 10-11.85. A single relaxation process is observed, and the associated relaxation time and maximum chemical absorption go through a maximum at a pH where circular dichroism measurements indicate a helix-coil transition is occurring. The maximum value of the relaxation time is 1.7 × 10-8 sec. The observed relaxation process appears to be directly associated with the kinetics of the helix-coil transition, and the data are in quantitative agreement with current theories of the dynamics of this transition. The rate constant for formation of a helical segment at the end of an already helical section is estimated to be 7.4 × 1010 sec-1, and the volume change associated with this process is approximately ±0.6 cc/mol of monomer. © 1969, American Chemical Society. All rights reserved.