Far-UV Circular Dichroism (CD) Spectropolarimetry: Secondary Structure Deconvolution & Melting Curve ($T_m$)
Circular Dichroism (CD) is the differential absorption of left- and right-circularly polarized light by optically active, chiral molecules:
$$Delta A = A_L – A_R = (epsilon_L – epsilon_R) cdot c cdot l = Delta epsilon cdot c cdot l$$
Because differential absorption transforms circularly polarized light into elliptically polarized light, CD is conventionally reported in terms of ellipticity ($theta$) in millidegrees ($text{mdeg}$), where $theta = 32.98 cdot Delta A$.
In proteins, peptide bonds ($-text{CONH}-$) are identical achiral chromophores, but when organized into regular secondary structures, chiral exciton coupling between peptide electronic transitions ($pi to pi^*$ at $sim 190text{ nm}$ and $n to pi^*$ at $sim 222text{ nm}$) generates unique, diagnostic Far-UV CD signatures:
- $alpha$-Helix: Characteristic double negative minima at $208text{ nm}$ ($pi to pi^*$ polarized parallel to helix axis) and $222text{ nm}$ ($n to pi^*$ transition), and a strong positive peak at $193text{ nm}$ ($pi to pi^*$ perpendicular).
- $beta$-Sheet: A single broad negative minimum at $218text{ nm}$ ($n to pi^*$) and a positive maximum at $195text{ nm}$.
- Random Coil / Unfolded: Strong negative band at $198text{ nm}$ and near-zero ellipticity above $220text{ nm}$.
To compare proteins independently of molecular weight, raw ellipticity ($theta_{text{obs}}$, mdeg) is normalized to Mean Residue Ellipticity ($[theta]_{text{MRE}}$, $text{deg}cdottext{cm}^2cdottext{dmol}^{-1}$):
$$[theta]_{text{MRE}} = frac{theta_{text{obs}} times 100}{c times l times N_{text{res}}}$$
where $c$ is protein molar concentration ($text{M}$), $l$ is optical pathlength ($text{cm}$), and $N_{text{res}}$ is total amino acid residue count. Percentage $alpha$-helical content can be estimated directly from $[theta]_{222}$:
$$% alphatext{-helix} = left(frac{-[theta]_{222} – 3,000}{36,000}right) times 100$$
- Spectropolarimeter: Jasco J-815 or Chirascan Far-UV spectropolarimeter equipped with Peltier temperature controller and continuous dry $text{N}_2$ gas purge ($5.0text{ L/min}$).
- Cuvette: Precision synthetic suprasil quartz demountable cuvette with $0.100text{ cm}$ ($1.0text{ mm}$) pathlength.
- Proteins: Equine Skeletal Muscle Myoglobin ($>95%$, predominantly $alpha$-helical) and Bovine Ribonuclease A.
- Far-UV Transparent Buffer: $10text{ mM Potassium Phosphate}$, $text{pH } 7.0$ (Phosphate buffers have minimal Far-UV absorbance down to $185text{ nm}$; chlorides, Tris, and DTT are strictly avoided as they absorb heavily below $210text{ nm}$).
Initiate high-purity nitrogen purge ($>99.999%$, $5text{ L/min}$) for 20 minutes prior to igniting the $150text{ W}$ Xenon arc lamp to eliminate atmospheric ozone and molecular oxygen which absorb Far-UV photons below $200text{ nm}$. Set parameters: Wavelength range $190–260text{ nm}$, bandwidth $1.0text{ nm}$, digital integration time (D.I.T.) $2text{ s}$, data pitch $0.2text{ nm}$, scan speed $50text{ nm/min}$, temperature $20.0^circtext{C}$.
Fill the clean $1.0text{ mm}$ quartz cell with $300 mutext{L}$ of $10text{ mM Potassium Phosphate}$ buffer ($text{pH } 7.0$). Verify that High Tension (HT) voltage remains $< 600text{ V}$ across $195–260text{ nm}$. Collect 5 consecutive scans and average to create the baseline subtraction file.
Dilute Myoglobin to $0.15text{ mg/mL}$ ($8.57 mutext{M}$) in phosphate buffer. Load into cuvette without microbubbles. Collect 5 accumulations at $20^circtext{C}$. Subtract buffer baseline. Export baseline-corrected raw ellipticity $theta$ in millidegrees.
Fix wavelength monitor at $222.0text{ nm}$. Ramp Peltier temperature from $20.0^circtext{C}$ to $95.0^circtext{C}$ at a constant rate of $1.0^circtext{C/min}$ with a temperature sampling pitch of $0.5^circtext{C}$. Plot $[theta]_{222}$ vs Temperature ($^circtext{C}$) and fit to a two-state unfolding model to determine the midpoint melting temperature ($T_m$).
| Wavelength λ (nm) | Observed θ (mdeg) | HT Voltage (V) | [θ]_MRE (deg·cm²/dmol) | Structural Spectral Feature |
|---|---|---|---|---|
| 193.0 nm | +38.4 mdeg | 510 V | +29,050 | Strong positive π → π* helix peak |
| 200.0 nm | −2.1 mdeg | 420 V | −1,590 | Zero-crossing crossover |
| 208.0 nm | −34.8 mdeg | 365 V | −26,320 | 1st Characteristic α-Helix Minimum |
| 215.0 nm | −31.2 mdeg | 330 V | −23,600 | Inter-minimum saddle |
| 222.0 nm | −36.1 mdeg | 310 V | −27,310 | 2nd Characteristic α-Helix Minimum (n → π*) |
| 250.0 nm | −0.4 mdeg | 270 V | −300 | Baseline return |
Deconvolution & Secondary Structure Estimation:
Sample parameters: $c = 8.57 times 10^{-6}text{ M}$, $l = 0.1text{ cm}$, $N_{text{res}} = 154text{ residues}$.
$$[theta]_{222} = frac{-36.1 times 100}{8.57 times 10^{-6} times 0.1 times 154} = mathbf{-27,310text{ deg}cdottext{cm}^2cdottext{dmol}^{-1}}$$
$$% alphatext{-helix} = left(frac{27,310 – 3,000}{36,000}right) times 100 = mathbf{67.5%}$$
Thermal Denaturation Fit: Fitting the sigmoidal $[theta]_{222}$ transition vs temperature yields a two-state cooperative melting transition with $T_m = mathbf{78.4^circtext{C}}$ and unfolding enthalpy $Delta H_v = 412text{ kJ/mol}$, demonstrating high conformational stability.
