When X is ???O???(CH2)n???CH(ZR5)2 where n is a number of 1-6, Z is O or S, and R5 is H or an alkyl group, Y is preferably ???O2CCH2CH2CO2R6 where R6 is H or NHS. When X is ???O???CH2???CO2CH(CH3)CH2CONHS, the second functional group Y is preferably CH2???CH???CO2???.