Overview of Protein Structure:- The Peptide Bond Is Rigid and Planar

Covalent bonds also place important constraints on the conformation of a polypeptide. In the late 1930s, Linus Pauling and Robert Corey embarked on a series of studies that laid the foundation for our present understanding of protein structure. They began with a careful analysis of the peptide bond. The α carbons of adjacent amino acid residues are separated by three covalent bonds, arranged as Cα-C-N-Cα. X-ray diffraction studies of crystals of amino acids and of simple dipeptides and tripeptides demonstrated that the peptide C-N bond is somewhat shorter than the C-N bond in a simple amine and that the atoms associated with the peptide bond are co-planar. This indicated a resonance or partial sharing of two pairs of electrons between the carbonyl oxygen and the amide nitrogen (Fig. 4–2a). The oxygen has a par tial negative charge and the nitrogen a partial positive charge, setting up a small electric dipole. The six atoms of the peptide group lie in a single plane, with the oxy gen atom of the carbonyl group and the hydrogen atom of the amide nitrogen trans to each other. From these findings Pauling and Corey concluded that the peptide C-N bonds are unable to rotate freely because of their partial double-bond character. Rotation is permitted about the N-Cα and the Cα-C bonds. The backbone of a polypeptide chain can thus be pictured as a series of rigid planes with consecutive planes sharing a common point of rotation at Cα (Fig. 4–2b). The rigid pep tide bonds limit the range of conformations that can be assumed by a polypeptide chain.

By convention, the bond angles resulting from rotations at Cα are labeled  ϕ (phi) for the N-Cα bond and ψ (psi) for the Cα-C bond. Again by

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مواضيع ذات صلة

Overview of Protein Structure:- A Protein’s Conformation Is Stabilized Largely by Weak Interactions

Protein Sequences and Evolution:- Protein Sequences Can Elucidate the History of Life on Earth

The Covalent Structure of Proteins:- Amino Acid Sequences Provide Important Biochemical Information

The Covalent Structure of Proteins:- Small Peptides and Proteins Can Be Chemically Synthesized

The Covalent Structure of Proteins:- Amino Acid Sequences Can Also Be Deduced by Other Methods

The Covalent Structure of Proteins:-Large Proteins Must Be Sequenced in Smaller Segments

The Covalent Structure of Proteins:- Short Polypeptides Are Sequenced Using Automated Procedures

The Covalent Structure of Proteins:-The Amino Acid Sequences of Millions of Proteins Have Been Determined

The Covalent Structure of Proteins:-The Function of a Protein Depends on Its Amino Acid Sequence

Working with Proteins:- Unseparated Proteins Can Be Quantified

Working with Proteins:- Proteins Can Be Separated and Characterized by Electrophoresis

Working with Proteins:- Proteins Can Be Separated and Purified