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