The logic for hotstart PCR is as follows, most DNA samples used as template will have some single-stranded DNA. 

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#Unit 1. Molecules and their Interaction Relevant to Biology

Counter-clockwise rotation starting at 0º gives negative dihedral angles from -1º through

90º ending at -180º (the same dihedral angle  as +180º).

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#Unit 1. Molecules and their Interaction Relevant to Biology

Because they cannot rotate, each peptide bond holds 6 atoms in a plane.

However, most possible angles of Phi (φ) and Psi (ψ) are impossible due to clashes

between atoms. These balls are much smaller than the atoms they represent.

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#Unit 1. Molecules and their Interaction Relevant to Biology

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#Unit 1. Molecules and their Interaction Relevant to Biology

In fig, Phi is fixed at 0º. Clashes occur at ALL Psi angles. Phi = 0º is physically

impossible.

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#Unit 1. Molecules and their Interaction Relevant to Biology

The Ramachandran Principle is that alpha helices, beta strands, and

turns are the most likely conformations for a polypeptide chain to 

adopt, because most other conformations are impossible due to

steric collisions between atoms.