F-actin is a helix with a random variable twist

Abstract
Image analysis of isolated F-actin filaments shows that the actin helix can be described by a constant rise per subunit but a considerably variable and randomized twist (number of units per turn). The ability of actin subunits to rotate through angles of the order of 10° from their helically ideal positions helps to explain actin's capacity to form many different polymorphic structures.