Structure and Dynamics of Metallomacrocycles: Recognition of Zinc Xylyl-Bicyclam by an HIV Coreceptor

As platforms for the design of metal-based therapeutic and diagnostic agents, macrocycles are
rigid enough to provide strong metal binding sites and orient functional groups stereoselectively, yet flexible
enough to accommodate structural changes required for induced-fit recognition of biological targets. We
consider the recognition of the Zn(II) complex of the bis-tetraazamacrocycle xylyl-bicyclam, a potent anti-HIV agent, by the coreceptor CXCR4, a G-protein-coupled receptor used by HIV for membrane fusion and
cell entry. NMR studies show that the macrocycles of Zn(II)2−xylyl-bicyclam perchlorate exist in aqueous
solution as two major configurations, trans-I (nitrogen chirality R,S,R,S), and trans-III (S,S,R,R). Acetate
addition induced a major structural change. X-ray crystallography shows that the acetate complex contains
the unusual cis-V cyclam configuration (R,R,R,R and folded) with bidentate coordination of acetate to Zn(II)
plus second-coordination-sphere double H-bond formation between diagonal NH protons on the opposite
cyclam face and acetate carboxylate oxygens. Detailed 1D and 2D NMR studies show that the major
configuration of Zn(II)2−xylyl-bicyclam acetate in aqueous solution is cis-V/trans-I. Molecular modeling shows
that an analogous cis-V site can be formed when Zn(II)2−xylyl-bicyclam binds to CXCR4, involving the
carboxylate groups of Asp262 (Zn(II) coordination) and Glu288 (double H-bonding). The second cyclam
can adopt the trans-I (or trans-III) configuration with Zn(II) binding to Asp171. These interactions are
consistent with the known structure−activity relationships for bicyclam anti-HIV activity and receptor mutation.
Consideration of the anti-HIV activity of xylyl-bicyclam complexes of other metal ions suggests that affinity
for carboxylates, configurational flexibility, and kinetic factors may all play roles in receptor recognition.
For example, Pd(II) cyclam complexes interact only weakly with axial ligands and are inflexible and inactive,
whereas Co(III) cyclams bind carboxylates strongly, are configurationally flexible, and yet have low activity.
Our findings should aid the design of new generations of active macrocycles including highly specific
chemokine receptor antagonists.