wherein X??? is a spacer group selected from the group consisting of alkylenes, substituted alkylenes, arylenes, cycloalkylenes and substituted alkylenes; G??? is a functional group selected from amines, thiols, carboxyls, aldehydes or alcohols; A??? is a specific-protecting group thereof; R1 is an amino acid side chain such as H, CH3, etc., optionally bound with a specific protecting group; and B