Insect excretes unusual six-coordinate pentavalent arsenic species

Abstract

Environmental context. Arsenate, in which oxidised arsenic is coordinated to four oxygen atoms, is common in the environment. We have found that a moth larva excretes an unusual form of oxidised arsenic which is bound to six oxygen atoms. Since the chemical groups which give rise to this species are abundant in environmental and biological systems, more research is needed into the possible presence of such six-coordinate complexes in natural systems. Abstract. Arsenate, in which pentavalent arsenic (AsV) is approximately tetrahedrally coordinated by oxygen, is common in biological or environmental systems. Octahedral coordination of AsV by oxygen is known chemically but hitherto has not been observed in natural systems. In an effort to understand the effect of elevated levels of arsenic on insects and other insectivorous animals in the food chain, larvae of the moth bertha armyworm (Mamestra configurata Walker) [Lepidoptera : Noctuidae] were examined under laboratory conditions. Synchrotron X-ray absorption spectroscopy was used to show that the exuvia (shed skin) and frass (fecal matter) contain an unusual AsV species six-coordinated by oxygen. The species is modelled as a low pH octahedral chelation complex with vicinal dihydroxyls such as glycerol or catechol. Structural characterisation using extended X-ray absorption fine structure (EXAFS) shows interatomic distances which are more similar to those of the glycerol complex and the near-edge also shows more similarity with the aliphatic chelator. The larvae may be using the six-coordinate AsV species as a specific excretory molecule. Since vicinal dihydroxyl species are common, more research is needed into the possible presence of such six-coordinate complexes in natural systems.