BARBARA W. LOW AND JOHN T. EDSALL 



direct stereochemical mechanism for turning the helical axis 

 through 180° in a left-handed helix has been suggested by 



f 



10 



Fig. 8a. Photograph of scale models of left-handed and right- 

 handed a-helices. One proline residue has been introduced into each 

 chain configuration. In the left-handed helix the regular chain con- 

 figuration is completely disrupted, as shown. In the right-handed helix 

 some minor discontinuities are introduced with loss of hydrogen bond- 

 ing. (See opposite page for exploded drawings of the regions L and R.) 



412 



