# Binding of Arsenic by Common Functional Groups: An Experimental and Quantum-Mechanical Study
## 1 Introduction  
Arsenic (As) is an element easily detectable in the environment as a consequence of many natural and anthropogenic activities. It is a minor but ubiquitous element of the Earth's crust, and some chemical species are volatile and highly soluble in aquatic systems. In general, arsenite and arsenate minerals are highly soluble in aqueous solution, often proving to be sources of arsenic contamination [1, 2, 3, 4, 5]. The effects of arsenic compounds on humankind are strictly related to contamination. In fact, from the environment arsenic may reach the human organism through many pathways: by inhalation, responsible for respiratory system problems; by dermal exposure,which can lead to lesions and aggressive skin cancers; or by ingestion of contaminated foods and water. In particular, the latter, i.e., the consumption of As-contaminated drinking water, is considered--among the As-induced diseases--as the primary cause of human illness, and in 1992, this prompted the World Health Organization (WHO) to reduce the recommended value for arsenic in drinking water from 50 \(\mathrm{\SIUnitSymbolMicro g}\)/L (fixed value in 1963) to 10 \(\mathrm{\SIUnitSymbolMicro g}\)/L, as a consequence of the suspicion of carcinogenicity. Arsenic is nowadays recognized as one of the most dangerous pollutants, and the International Agency of Research on Cancer (IARC) classified it as a class I carcinogen that exhibits acute and chronic toxicity depending on the type of exposure [6]. Ingestion of large doses of arsenic leads to cardiovascular, gastrointestinal, and central nervous system disorders and, in some cases, to death. Chronic arsenic exposure represents a serious public health concern because As affects almost all organ systems, resulting in diseases of the vascular, respiratory, neurological, reproductive, and endocrine systems and causing tumors of the skin, lung, and urinary bladder [6; 7], in addition to an increased risk of diabetes mellitus. Despite the carcinogenic potential of arsenic has been widely studied, the mechanisms by which it causes cancer and other diseases remain poorly understood. Similarly to other chemical elements, toxicity strongly depends on the arsenic speciation, i.e., on the form under which it is present in different environmental compartments and human organs and tissues. As shares properties with both metals and nonmetals, and it is thus classified as a metalloid. It holds four oxidation states (\(-\)3, 0, +3, +5) and can be found in a variety of inorganic and organic forms exhibiting different toxicities and bioavailability [8]. Distinguishing between species is important due to the differences in properties and mechanisms of toxicity. Different species have different toxicity and bioavailability, but the knowledge of the speciation is also important to define and develop suitable removal procedures [1; 9; 10; 11; 12; 13]. Inorganic As(III) represents the most toxic arsenic form because of its high affinity toward sulfhydryl groups of proteins. Recently, in order to evaluate the strength of As(III)-SH interactions, experimental and computational investigations have been performed on mixed thiol-carboxylic ligands (thiol, thiolamic, and dimercaptosuccinic acids) [14; 15]. Notwithstanding these studies confirmed the pronounced As(III)-thiol group interaction, they also evidenced the contribution of carboxylate groups to the whole stabilization of complex species. In light of this, the study on the interactions established by As(III) with some important functional groups typically present in proteins (carboxylic and amino) has been undertaken to evaluate their contribution to the stability of the formed complexes. Three classes of ligands were taken into account: simple carboxylic acids and linear amines, but also amino acids to evaluate the presence of carboxylic and amino groups simultaneously. Carboxylic acids are compounds naturally involved in different stages of life cycles, but they can also be synthesized industrially by oxidation reaction of aldehyde or primary alcohol as well.