Host-guest complexes of phenoxy alkyl acid herbicides and cyclodextrins. MCPA and β -cyclodextrin E. MANUELA GARRIDO, MARCOS SANTOS, PAULO SILVA, FERNANDO CAGIDE, JORGE GARRIDO and FERNANDA BORGES The chlorophenoxy herbicide MCPA (4-chloro-2-methylphenoxyacetic acid), widely used for the control of broad-leaf weeds primarily in cereal and grass seed crops, still remains one of the most often used herbicides in Portugal. As the formation of inclusion complexes with cyclodextrins can improve its solubility properties, the interaction between the herbicide MCPA and β -cyclodextrin was investigated. The stability constants describing the extent of formation of the complexes have been determined by phase-solubility studies. Different analytical techniques [ultraviolet-visible spectroscopy (UV-Vis), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy ( 1 H NMR)] were employed for a thorough investigation of the structural characteristics of the obtained complexes, which exhibited distinct features and properties from both “guest” and “host” molecules. FTIR and 1 H NMR data obtained for the MCPA/β -CD complexes gave information about the interaction between MCPA and the nonpolar cyclodextrin cavity. The dramatic change observed in band frequency and proton displacements of OCH 2 group and H6 aromatic proton confirmed the inclusion of MCPA in β -CD. The formation of an inclusion complex between MCPA and β -CD increased the aqueous solubility of this herbicide which could be a particularly advantageous property for some specific applications, namely to improve commercial formulation and for environmental protection. Keywords: Phenoxyacetic acid herbicides, MCPA, β -cyclodextrin, inclusion complex. Introduction As plants constitute the world’s primary food source there has been a tendency, in recent years, to maximize agricul- tural yields due to demographical growth. The increase in productivity has been achieved through the development of new high-yield crops and the use of fertilizers and pes- ticides. At the global level, the annual losses due to the presence of destructive weeds are calculated to be around 10–15% of production among staple crops. Therefore, it is not outlandish that 47.5% of the 2 million tons of pesticides consumed globally each year are herbicides. [1] The risk of environment contamination by pesticides and their degradation products has encouraged the interest in the development of new formulations for pesticide appli- cation. Slow-release formulations of pesticides maintain the threshold concentration of the active ingredient in the soil or plant by its release at the required rate, reducing its level in the environment because lower amounts are required for biological effect. These novel delivery approaches have been recently ascribed to encompass either economical or environmental advantages. [2–4] Microencapsulation is a simple and cost-effective prac- tice that allows enclosing bioactive materials with the pur- pose of protecting and/or releasing the enclosed substances in a controlled way under specific conditions. The encapsu- lation process involves coating or entrapment of a material or a mixture into another material. Current encapsulation techniques included a variety of techniques, namely spray drying, spray chilling and cooling, coacervation, fluidized bed coating, liposome entrapment, rotational suspension separation, extrusion and inclusion complexation. The last method is generally achieved by us- ing cyclodextrins (CDs) as the encapsulating material. The most important structural feature of these compounds is their toroid shape, with lipophilic inner cavities and hy- drophilic outer surfaces, which make them capable of in- teracting with a large variety of guest molecules to form noncovalent inclusion complexes. [5, 6] Among them, β -CD is receiving increasing attention due its low cost and high capacity to interact with a wide variety of molecules, in- cluding pesticides and drugs. In fact, several studies have