Biological Physics: Energy, Information, Life

9.5. Thermal, chemical, and mechanical switching[[Student version, January 17, 2003]] 319

-15

-10

-5

0

5

10

15

10 20 30 40 50

T, ◦C

optical rotation, degrees

alpha

helix

random

coil

N=1500

N=46

N=26

Figure 9.6: (Experimental data with fit.) Alpha-helix formation as a function of temperature for solutions of
poly-[γ-benzyl-l-glutamate] (an artificial polypeptide), dissolved in a mixture of dichloroacetic acid and ethylene
dichloride. At low temperature all samples displayed an optical rotation similar to that of isolated monomers,
whereas at high temperatures the rotation changed, indicating alpha-helix formation. Topdots: polymer chains
of weight-average length equal to 1500 monomers. Middle dots:46-monomer chains.Lower circles: 26-monomer
chains. The vertical axis gives the optical rotation; this value is linearly related to the fraction of all monomers
in the helical conformation.Topsolid curve: the large-Nformula (Equations 9.25 and 9.24), obtained by fitting
the values of the five parameters ∆E,Tm,γ,C 1 ,andC 2 to the experimental data. The lower two lines were then
predictions of the model (see Sections Section 9.5.3 on page 323 and 9.5.3′on page 344), with no further fitting
done. [Experimental data from Zimm et al., 1959.]

source polarizer

monochromater

sample concentration c

d

θ

analyzer

detector

E

Figure 9.7:(Schematic.) A polarimeter. The wavy line depicts the strength of the electric field in a ray of light
emerging from the source; the arrows show the direction of the field. These arrows are shown rotating by an angle
θas they pass through the sample; the rotation shown corresponds to the positive valueθ=+π/2. By convention,
the plus sign means that an observer looking into the oncoming beam sees the electric field rotating in the clockwise
direction as the beam advances through the medium. Try looking at this figure in a mirror to see that the optical
rotation changes sign.

Wecan monitor conformational changes in polypeptides without having to look at them individ-
ually, by looking at changes in the bulk properties of their solutions. When studying the helix–coil
transition, the most telling of these changes involves the solution’s effect on polarized light.