"Synthetic amino acids encompassed by the present invention include, for example,naphthylalanine, L-hydroxypropylglycine, L-3,4-dihydroxyphenylalanyl, .alpha.-amino acids such as L-.alpha.-hydroxylysyl and D-.alpha.-methylalanyl, L-.alpha.-methyl-alanyl, .beta. amino-acids such as .beta.-analine, and isoquinolyl.D-amino acids and other non-naturally occurring synthetic amino acids can also be incor" . . . .