The effects of cooperativity, finite chain length, and field-induced changes of the conformational equilibrium on the electric birefringence of polypeptides in the range of the helix–coil transition

✍ Scribed by Gerhard Schwarz; Uwe Schrader

• Publisher: Wiley (John Wiley & Sons)
• Year: 1975
• Tongue: English
• Weight: 719 KB
• Volume: 14
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.1975.360140607

✦ Synopsis

Abstract

An electric birefringence apparatus with an He–Ne laser is described, which permits a high sensitivity of measurement for field strengths up to 20 kV/cm and pulse durations between 10^−5^ to 1 sec. It is applied to poly(γ‐benzyl‐L‐glutamate) and poly(β‐benzyl‐L‐aspartate) in a nonaqueous solvent mixture, especially under the conditions of the helix‐coil equilibrium. A theoretical treatment of the problem shows that the usual neglect of end effects is only justified for comparatively long chains. On the basis of a computational procedure taking into account the finite chain lengths, the experimental data for 15 kV/cm reveal an obvious enhancement of the coil‐to‐helix transition caused by the electric field. This effect is quantitatively confirmed by a pertinent theory. The cooperative parameter σ may be best determined from the initial linear increase of the bire‐fringence versus the square of the field strength where the conformational equilibrium is not affected.