1. A compound represented by formula I: wherein R represents independently for each occurrence H or alkyl; Y represents independently for each occurrence —C(O)— or —S(O)—; n represents independently for each occurrence 1, 2, 3, or 4; and X represents independently for each occurrence an optionally substituted alkyl, heteroalkyl, alkenyl, or -[(alkyl-NR 1 C(O)) m -alkyl]-, wherein m is 1, 2, 3, or 4; and R 1 is H or alkyl.