a substitution of {HEAD ~ the amino acid} which occupies the position with {TAIL ~ a different amino acid}, 
a substitution of { HEAD ~ the amino acid } which occupies the position with { TAIL ~ a different amino acid } ,
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH