The action potential spreads throughout the myocardium in the following sequence:

SA node  Atrial internodal tracts and atria  AV node  Bundle of His  Purkinje system  Ventricles

TLS Online TPP Program

#Id: 8461

#Unit 1. Molecules and their Interaction Relevant to Biology

The interactions between R groups, particularly those spaced three (or four) residues apart; The assembly is said to have a coiled coil structure because each α helix itself follows a helical path.

TLS Online TPP Program

#Id: 8462

#Unit 1. Molecules and their Interaction Relevant to Biology

TLS Online TPP Program

#Id: 8463

#Unit 1. Molecules and their Interaction Relevant to Biology

Interactions between amino acid residues at the ends of the helical segment and the electric dipole inherent to the  helix.

TLS Online TPP Program

#Id: 8464

#Unit 1. Molecules and their Interaction Relevant to Biology

The overall effect is a significant net dipole for the a helix that gives a partial positive charge at the amino end and a partial negative charge at the carboxy end of the a helix. 

TLS Online TPP Program

#Id: 8465

#Unit 1. Molecules and their Interaction Relevant to Biology

The magnitude of this dipole moment corresponds to about 0.5-0.7 unit charge at each end of the helix. These charges would be expected to attract ligands of opposite charge and negatively charged ligands, especially when they contain phosphate groups and frequently bind at the N-termini of a helices. In contrast, positively charged ligands rarely bind at the C-terminus

TLS Online TPP Program

#Id: 8466

#Unit 1. Molecules and their Interaction Relevant to Biology

Polyleucine and polyalanine readily form a-helical structures.