REPORT
tion spectra in terms of three pa rameters associated with each tran sition or absorption band: the wavelength corresponding to maxi mum absorption, λκ, the half-width of the band, ΔΛ-, and the dipole strength for the transition, DK. The last parameter is proportional to the area under the absorption band and may be defined for the /vtli transition by the expression (34) : 3Ac Γ " kK(\) n s r j V i Jo λ where h is Planck's constant, c is the speed of light, ΝΊ is the number of absorbing molecules per cubic centimeter of solution, and kK is the partial absorption coefficient for the Kth transition. I f the absorp tion band is approximated by a Gaussian curve, its dipole strength, DK. may be shown to be directly proportional to the product fc (\K) ' ΔΛ·, where k(XK) is the peak ab sorption coefficient and ΔΛ- is now defined as the half width of the absorption band at 1/e of its maxi mum value. Circular Optical
Dichroism Rotatory
and Dispersion
The phenomena of circular di chroism and optical rotation are closely related to the phenomena of ordinary absorption and dispersion described above. The difference in absorption coefficients of an op tically active medium for left and right circularly polarized light is
termed circular dichroism, while the difference in the refractive in dices of the medium for the two circular polarizations, circular bi refringence, is a measure of optical rotation. Variations, as a function of wavelength, of circular dichro ism (CD) and optical rotation (op tical rotatory dispersion, OED) give even more structural informa tion on dissymmetric molecules than ordinary absorption or dis persion. The influence of an op tically active absorbing sample on a linearly polarized wave of electro magnetic radiation is illustrated in Figure 2: the incident linearly polarized wave may be resolved into two circular components, in phase, with equal amplitudes but opposite sense; the two components traverse the medium with unequal speeds due to the circular bire fringence, ?);, — n,t, of the medium; further, the two circular compo nents suffer unequal absorption due to the circular dichroism, k,. — kJl} of the medium. The transmitted components exhibit unequal ampli tudes and are no longer in phase. These components combine to pro duce a resultant wave which is elliptically polarized: the major axis of elliptical vibrations is ro tated relative to the direction of the incident linear polarization by an angle a called the optical rotation, and the arctangent of the ratio of minor to major axes of elliptical vibration, Θ, is referred to as the angle of ellipticity. Measured
For rapid, easy Concentration of
Proteins ...use
S&S
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Apparatus is simple, inexpensive S&S Collodion Bags require a spe cial glass holder, which is shown on the illustration above. The apparatus is inexpensive, simple in construc t i o n , and easy to operate. Ask your laboratory supply house for S&S Collodion Bags—an impor tant a d v a n c e m e n t in p r e p a r a t o r y techniques for albumen determina t i o n t h a t saves t i m e , e f f o r t , a n d contributes to increased accuracy of measurement. For free data sheet and prices, mail coupon below. Information w i l l be sent w i t h no obligation.
M A I L C O U P O N FOR FREE DATA Carl Schleicher & Schuell Co.
S&S Keene, New Hampshire, Dept
0-6(
Please send free data sheet and prices on S&S Collodion Bt Name, INCIDENT LI8HT LINEARLY POLARIZED
TRANSMITTED L I 6 H T ELLIPTICALLY POLARIZED
Company Address-
Figure 2. ized l i g h t
I n f l u e n c e of a n o p t i c a l l y a c t i v e a b s o r b i n g s a m p l e o n l i n e a r l y
polar
City
Zone
State
Circle No. 85 on Readers' Service Card VOL. 38, NO. 7, JUNE I960
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31 A
