Helical Assembly of Flavin Mononucleotides on Carbon Nanotubes as Multimodal Near-IR Hg(II)-Selective Probes

The development of novel methods to detect mercury is of paramount importance owing to the impact of this metal on human health and the environment. We observed that flavin mononucleotide (FMN) and its helical assembly with a single-walled carbon nanotube (SWNT) selectively bind Hg²⁺ arising from HgCl₂ and MeHgCl. Absorption spectroscopic studies show that FMN preferentially forms a 2:1 rather than a 1:1 complex with Hg²⁺ at high FMN concentrations. On the basis of the analogy to the thymine-Hg-thymine complex, it is proposed that the 2:1 complex between FMN and Hg²⁺ comprises a Hg-bridged pair of FMN groups, regardless of the presence of SWNT. Upon addition of as little as a few hundred nanomoles of Hg²⁺, both FMN and FMN-SWNT exhibit absorption and photoluminescence (PL) changes. Moreover, FMN-SWNT displays simultaneous multiple sigmoidal changes in PL of SWNT tubes having different chiral vectors. Assessment of binding affinities using the Hill equation suggests that 2:1 and 1:1 complexes form between Hg²⁺ and FMN groups on the FMN-SWNT. Theoretical calculations indicate that optical changes of the FMN-SWNT originate from Hg-mediated conformational changes occurring on the helical array of FMN on the SWNT. High-resolution transmission electron microscopy revealed that the presence of Hg²⁺ in complexes with the FMN-SWNT enables visualization of helical periodic undulation of FMN groups along SWNT without the need for staining. Circular dichroism (CD) study revealed that FMN-SWNT whose CD signal mainly originates from FMN decreases dichroic bands upon the addition of Hg²⁺ owing to the formation of a centrosymmetric FMN-Hg-FMN triad on SWNT. The binding mode specificity and multimodal changes observed in response to Hg²⁺ ions suggest that systems based on FMN-SWNT can serve as in vivo NIR beacons for the detection of various mercury derivatives.

The first phosphoramide-mercury(II) complex with a Cl2Hg-OP[N(C)(C)]3 segment

Mercury(II) exhibits a strong preference for linear coordination which has been attributed to relativistic effects splitting the 6p orbitals and promoting sp hybridization. If the two ligands attached to the mercury(II) ion are weak donors, the metal ion can act as a good Lewis acid and expand its coordination number. Moreover, mercury has a special affinity for softer bases, such as S and N atoms, and has much less affinity for hard bases, such as those including an O atom. The asymmetric unit of dichlorido[tris(piperidin-1-yl)phosphane oxide-κO]mercury(II)-dichloridomercury(II) (2/1), [HgCl2{(C5H10N)3PO}]2·[HgCl2], is composed of one HgCl2{(C5H10N)3PO} complex and one half of a discrete HgCl2 entity located on an inversion centre. The coordination environment around the Hg(II) centre in the complex component is a distorted T-shape. Bond-valence-sum calculations confirm the three-coordination mode of the Hg(II) atom of the complex molecule. The noncovalent nature of the Hg...Cl and Hg...O interactions in the structure are discussed.