currently commercially available. The larger sizes are a particularly active area of research since they can bind larger and more interesting guest molecules, thus expanding their potential applications. Cucurbit[10]uril is particularly difficult to isolate. It was first discovered by Day and coworkers in 2002 as an inclusion complex containing CB[5] by fractional crystallization of the cucurbituril reaction mixture. The CB[10]·CB[5] was unambiguously identified by single crystal X-ray structural analysis that revealed the complex resembled a molecular gyroscope. In this case, the free rotation of the CB[5] within the CB[10] cavity mimics the independent rotation of a flywheel within the frame of a gyroscope. Isolation of pure CB[10] could not be accomplished by direct separation methods since the compound has such a high affinity for CB[5]. The strong binding affinity for the CB[5] can be understood since it has a complementary size and shape to the cavity of the CB[10]. Pure CB[10] was isolated by Isaacs and coworkers in 2005 by introducing a more strongly binding melamine diamine guest that is capable of displacing the CB[5]. The melamine diamine guest was then separated from the CB[10] by reaction with acetic anhydride that converted the positively charged amine groups to neutrally charged amides. Cucurbiturils strongly bind cationic guests, but by removing the positive charge from the melamine diamine guest reduces the association constant to the point it can be removed by washing with methanol, DMSO, and water. The CB[10] has an unusually large cavity (870 Å3) that's free and capable of binding extraordinarily large guests including a cationic calix[4]arene. Applications Cucurbiturils have been used by chemists for various applications, including drug delivery, asymmetric synthesis, molecular switching, and dye tuning. Supramolecular host molecules Cucurbiturils are efficient host molecules in molecular recognition and have a particularly high affinity for positively charged or cationic compounds. High association constants with positively charged molecules are attributed to the carbonyl groups that line each end of the cavity and can interact with cations in a similar fashion to crown ethers. The affinity of cucurbiturils can be very high. For example, the affinity equilibrium constant of cururbit[7]uril with the positively charged 1-aminoadamantane hydrochloride is experimentally determined at 4.23*1012. Host guest interactions also significantly influence solubility behavior of cucurbiturils. Cucurbit[6]uril dissolves poorly in just about any solvent but solubility is greatly improved in a solution of potassium hydroxide or in an acidic solution. The cavitand forms a positively charged inclusion