In another embodiment, R6 is a side atom or group of a natural amino acid, such as H (from glycine), ???CH2(CH2)3NH2 (from leucine), ???CH2CH2CH2NHC(NH)NH2 (from arginine), 4-hydroxybenzyl (from tyrosine), CH2COOH (from aspartic acid) or CH2CH2COOH (from glutamic acid).