Cooperative binding by a self-assembled receptor: metal in a structural and functional role

Metal complexes of the amino acid, 4,4[superscript]\u27-phosphinylidine bis phenylalanine (PBP*, 1), self-assemble to produce a macrocyclic receptor (Co[subscript]2PBP[subscript]2, 2) with a hydrophobic cavity capable of binding aromatic compounds from water. Here we demonstrate that the metal can play a structural and a functional role: the receptor binds substrates bearing both aromatic and polar functional groups ditopically, using the hydrophobic pocket as well as the metal itself. The doubly anionic Co[superscript]2+ macrocyclic complex binds neutral aromatic guests more tightly than it does most anionic guests, as would be expected. However, carboxylates tethered such that ligation of the metal is possible cause enhanced binding. NMR studies of these paramagnetic materials support ditopic binding, and reveal very large complexation induced shifts in both the host and the guest. Co[superscript]2+ macrocyclic complex demonstrates 9 fold enantioselectivity in substrates binding through ditopic binding. ftn*Please refer to dissertation for diagram

Enumeration of graph embeddings

AbstractFor a finite connected simple graph G, let Γ be a group of graph automorphisms of G. Two 2-cell embeddings ι: G → S and j: G → S of a graph G into a closed surface S (orientable or nonorientable) are congruent with respect to Γ if there are a surface homeomorphism h:S → S and a graph automorphism γϵΓ such that hoι=joγ. In this paper, we give an algebraic characterization of congruent 2-cell embeddings, from which we enumerate the congruence classes of 2-cell embeddings of a graph G into closed surfaces with respect to a group of automorphisms of G, not just the full automorphism group. Some applications to complete graphs are also discussed. As an orientable case, the oriented congruence of a graph G into orientable surfaces with respect to the full automorphism group of G was enumerated by Mull et al. (1988)