In certain embodiments, X4 is selected from {HEAD ~ D proline}, D arginine and {TAIL ~ D} phenylalanine 
In certain embodiments , X4 is selected from { HEAD ~ D proline } , D arginine and { TAIL ~ D } phenylalanine
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH