Chem 524 Lecture Notes (Section 18)

XV. Circular Dichroism

A. Differential absorption of left and right circular polarized light by molecular transition

1. Measure polarization modulated transmission/need ratio to get transmission absorption

Imod = I0/2 (10-AL — 10-AR)

Itrans = Io/2 (10-AL + 10-AR) = K tan h (1.15 DA)

DA = AL - AR

2. Signals are small compared to absorption

electronic: DA/A ~ 103; vibration: DA/A ~ 10-4 ~ 10-5

need careful demodulation (lock-in typical)

3. Polarization created by linear polarizer

uv — crystal prism — IR — grid

and modulator — old electrooptic (KDP) / now — photoelastic — stress induce retard

optics, modulator materials: uv — qtz or CaF2; IR — CaF2 or ZnSe

4. Detector: PMT — UV, MCT (or like) — IR

B. UV Instruments

1. JASCO — market dominate — prism monochromator

good for uv penetration also polarize (linear)

2. AVIV — similar

3. OLIS — different — two chemical operation (beams from Rochan — out of phase at modulation)

4. Others — especially for stop-flow (dynamics) use grating optics — uv problem

C. IR instruments

Most home made until recently

a. Dispersive — TAK lab

advantage low resolution, single bands, bio syst

b. FT based — many labs

advantage multiplex (multiple trans), high res.

c. New — Bomem Biotools — full commercial design --FT — for mid IR — very good S/N

-- DSP-based — eliminate lockin -- full digitize time dependent signal

D. Applications

1. Molecular stereochemistry/absolute configuration

2. Chiral detector in separations

3. Biological polymer — conformational analysis (proteins, peptides, nucleic acid — secondary structure)

4. Protein folding — stop flow

5. Time dependent possible — detect elipticity with ns laser -- photolysis (chromophore)