The invention concerns compounds of the formula (I) wherein: Y1 and Y2 are independently a monosaccharide or disaccharide; X1 and X2 are independently —(R9—O)m—, —(R10)p—, —O—(R11—O)q—, —R16—O—R17—O— or a covalent bond; Q1 and Q2 are independently a nitrogen-containing heterocycle moiety; Z1 and Z2 are independently —(O—R7)—, —(O—C(═O)—R8)a-, —O—C(═O)—R12—C(=0)-R13—, —O—C(═O)—R14—C(═O)—R15 or a covalent bond; R7-R17 are each independently C1-C6 alkyl; R1-R6 are each independently a linear or branched alkyl group; b, c, d, e, f, and g are 0 or 1, provided b+c+d equals at least 2 and e+f+g equals at least 2; and a, m, p, and q are each an integer from 1-6.