In {HEAD ~ other embodiments}, X2 is selected from D leucine, D valine, {TAIL ~ glycine} and D proline 
In { HEAD ~ other embodiments } , X2 is selected from D leucine , D valine , { TAIL ~ glycine } and D proline
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH