Handbook of Biochemistry

Section A Proteins, Volume III

Gerald D Fasman

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351089319
• Edition: 3
• Topic: Medicine
• Subtopic: Biochemistry in Medicine

This edition focuses solely on proteins, amino acids venom toxins and peptides, haemoglobin. It also gives us very detailed information regarding cell types, anti-bodies, infrared testing on protein cells and membrane studies.

Proteins

OPTICAL ROTATORY DISPERSION AND CIRCULAR DICHROISM OF PROTEINS^*

Jen Tsi Yang, G. Chi Chen, and Bruno Jirgensons

Biot in 1836 introduced the term specific rotation, [α], for the optical activity of a compound^{1, 2, 3}

${\left[\text{α}\right]}_{\text{λ}}\text{=}\text{α}/\ell \text{c}$

(1)

where

α	=	the observed rotation of the plane of polarized light in angular degrees at wavelength λ
ℓ	=	the optical path in decimeters;
c	=	the concentration of the compound in grams per milliliter.

The dimension is deg cm² decagram^–1. The traditional use of specific rotation at sodium D line, ${\left[\text{α}\right]}_{\text{D}}^{\text{t}}$, is now obsolete with the availability of modern spectropolarimeters. Here D refers to 589 nm and t the temperature in centigrade.

For biopolymers, the data are usually expressed in terms of mean residue rotation, [m]

$\left[\text{m}\right]\text{=}\left({\text{M}}_0/100\right)\left[\text{α}\right]$

(2)

where M₀ is the mean residue (molecular) weight. (Some workers prefer the symbols [ϕ] or [R] for [m] and MRW for M₀.) M₀ of a protein can be determined from its amino acid composition (for most proteins, M₀ ≅ 115). Moffitt⁴ introduced the term reduced mean residue rotation, [m′], by including a Lorentz field correction

$\left[{\text{m}}^{\prime }\right]=\left[\text{m}\right]\left[3/\left({\text{n}}^2+2\right)\right]$

(3)

which reduces [m] to that under vacuum, noting that the refractive index is unity in vacuum. The dimension of both [m] and [m′] is deg cm² dmol⁻¹.

Optical rotatory dispersion (ORD) in the visible region of most proteins obeys the Drude equation

${\left[\text{α}\right]}_{\text{λ}}\text{=}k/\left({\text{λ}}^2-{\text{λ}}_{\text{c}}^2\right)$

(4)

where λ_c and k are the dispersion and rotatory coefficients. A plot of λ² [α]_λ vs. [α]_λ yields a straight line with ${\text{λ}}_{\text{c}}^2$ as the slope and k as the intercept (at [α]_λ = 0).⁵ The Moffitt equation for α-helical polypeptides⁴

$\left[{\text{m}}^{\prime }\right]={\text{a}}_0{\text{λ}}_0{}^2/\left({\text{λ}}^2-{\text{λ}}_0{}^2\right)+{\text{b}}_0{\text{λ}}_0{}^4/{\left({\text{λ}}^2-{\text{λ}}_0{}^2\right)}^2$

(5)

has now replaced the Drude equation for the ORD of proteins (except collagen). By rearranging Equation 5 into

$\left[{\text{m}}^{\prime }\right]\left({\text{λ}}^2/{\text{λ}}_0{}^2-1\right)={\text{a}}_0+{\text{b}}_0/\left({\text{λ}}^2/{\text{λ}}_0{}^2-1\right)$

(6)

the b₀ and a₀of a protein can be determined from a plot of [m’](λ²/λ₀² – 1) vs. 1/(λ²/λ₀² – 1) with λ₀ usually preset at 212 nm. A b₀ of-630 deg cm² dmol⁻¹, based on the data of synthetic polypeptides, is regarded to represent a perfect helix. The Moffitt equation, now considered to be empirical, still provides a reasonable estimate of the helical content in a protein molecule. Both Equations 4 and 5 are only applicable over a certain wavelength range. Often a λ0 larger than 212 nm must be used when the data below 300 to 350 nm are included for the fitting of the Moffitt equation. For the convenience of comparison, we will list only the a₀ and b₀ values based on λ₀ = 212 nm (with a few exceptions).

The visible ORD can also be fitted with many two-term Drude equations

$\left[{\text{m}}^{\prime }\right]={\text{a}}_1{\text{λ}}_1{}^2/\left({\text{λ}}^2-{\text{λ}}_2{}^2\right)+{\text{a}}_2{\text{λ}}_2{}^2/\left({\text{λ}}^2-{\text{λ}}_2{}^2\right)$

(7)

One such equation has been proposed by Shechter and Blout⁶ with λ₁ = 193 and λ₂ = 225 nm. All these equations can be reduced to the Moffitt equ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• PROTEINS
• INDEX