The so-called “intrinsic” solubility of cyclodextrins (CDs) and their derivatives is always one of the first and important questions from students, newcomers to the CD field either pharmacists, analysts, or product managers. However, it is less crucial for synthetic chemists because they are using a wider scale of solvents in their reactions and finding the most suitable one is more target-dependent. The word “intrinsic” is a little misleading, particularly in the case of CDs. The examples below demonstrate some solubility related problems of CDs. In many cases, these examples are dominantly about the bCD derivatives, because the similar a– and gCD derivatives are usually more soluble in the majority of solvents.
Water is the most important and desired solvent in many processes and in pharmaceutical formulations. Although both α– and γCD are well-soluble in water but the most common – and the cheapest – βCD has low aqueous solubility. Although the aqueous solubilities of natural CDs have been published the question is obvious: can we believe the all old data just because there are no new publications?
The first problem is related to the water content of shelf-dry CDs which is mostly forgotten or neglected. The shelf-dry CDs contain a considerable amount (unsubstituted α/γ:≈10 %, β:≈14%) of water. Surfing on the web one can find data of βCD from 1.7-1.8% to a little higher than 2%. Many times, even in highly appreciated journal publications, the use of shelf-dry or dried CD version is unclear. While when somebody gets the bottle from the shelf, on the label the largest letters are ‘>90-95% content’ or ‘Pharma/Food/Technical Grade’. In the best case, in the small letter part, the label can show max. 14% (or so) of water. In the case of the βCD, the well-dried βCD is soluble in water at 1.8%, and a surprising experience can be that the preparation of a 15 mM (1.7%) solution of βCD from the bottle is easier than expected around the saturation concentration. From the shelf-dry βCD ~2.1% aqueous solution can be prepared. Of course, the CD content is around only ~1.8% (because for the diluted solutions the w/w% is very close to the more common w/v%). Although the use of dried CDs would be obvious, the water-free CDs have some unpleasant properties. Apart from the fast re-adsorption of water, they are very electrostatic which results in sticking the fine powders to everywhere.
The temperature-dependent solubilities of naked CDs are also important. Finding literature data is challenging; the data comparison is not always obvious. Molar ratios or molar concentrations are not weighing friendly units. Moreover, one can find many incorrect values which are inherited from book to book, from article to article.
The temperature-dependent aqueous solubility of most CD derivatives follows the general rule: the higher temperature means higher solubility. Among the most used CD derivatives, the methylated CDs represent a strange class. The very common randomly (sometimes called as statistically) methylated βCD, RAMEB, and also the same α- and γCD versions, are practically freely soluble in water. The also randomly but less methylated βCD derivative, CRYSMEB, follows the above solubility rule. But, when both hydroxyl CD rims are selectively permethylated, the 2,6-di-O-methylated (DIMEA/B/G) and 2,3,6-tri-O-methylated (TRIMEA/B/G) CDs, have considerably lower aqueous solubility in boiling water (dimethylated CDs near 30% at r.t., while the dimethylated α/βCD <5% in boiling water; permethylated CDs 8-12% at r.t, and <3% in boiling water). This phenomenon can be well utilized in their purification. And, at least, in these cases the water content of shelf-dry products can be really low, usually only 2-3%. However, it is necessary to emphasize that as the temperature is decreasing from boiling, the 2,6-di-O-methylated CDs start immediately to re-dissolve. While the DIMEB can be recrystallized very effectively from boiling water because the temperature window is large enough to filter the product with regular laboratory glassware, a considerable loss of the similar αCD derivative occurs. The γCD analogue practically cannot be isolated by recrystallization from hot water. By the way, it is also needed to mention that the labels on the bottles are usually also misleading! The cheapest DIMEBs are usually only RAMEB. The DIMEBs of moderate prices, in the best cases, contain 35-55% 2,6-heptakis methylated derivative, while the products with >90-95% 2,6-heptakis methylated βCD is expensive because of the multistep isolation precedure. As the heptakis 2,6-methylated content is increasing the r.t. dissolution becomes more and more difficult but decreasing the temperature helps to get clear solutions which then remain stable at r.t.
Of course, not only pure aqueous solubilities are important. The CD solubilities are changing from solvents to solvents and many solvents can form complexes with CDs. In those cases the solubility of CD complexes becomes important. In many cases, complexes of CDs/CD derivatives increase the CD content of solutions. Often occurs, particularly in cases of well-soluble drug molecule/CD complexes, that the CD solubility is higher than one can calculate from the solubility of the parent CDs. A good example is p-toluenesulfonic acid (or its salts)/βCD complex when the βCD content of the solution can go over 30% in water. Although the p-toluenesulfonic acid is considered also as hydrotropic material owing to its geometry and sulfonic acid residue, the inclusion complex formation also contributes to the enhanced CD solubility.
The pH is a third important factor in the solubilities. It has been known for decades that the secondary hydroxyls of CDs are very weak acids. The pH where the secondary hydroxyls are starting to be ionized is around 12. This phenomenon could be utilized well in some derivatizations where dominant secondary hydroxyl substitution – due to steric reason, this practically means O(2) – is requested. The special case is the 6-O-monotosylation of βCD. The common synthetic methods require relatively diluted solutions to keep the probability of the monosubstitution as high as possible. The application of an appropriate cation, Cu2+, forms a dimeric salt with βCD at pH >12, which blocks the secondary hydroxyls and not only the CD content is increased up ~5 % but the effective preparation of 6-O-monotosylated βCD is also achieved. When the pH drops below 7-8 the product starts to crystalize. Unfortunately, the reactivity of the 6-O-monotosylated βCD does not allow its purification using this pH-dependent solubility. Another characteristic example can be the 6-monoamino-βCD. Although a general rule is that the ionized form of an ionizable derivative is more soluble in water than the neutral form, the 6-amino-βCDs are somehow special cases. The 6-peramino CDs are easy to recrystallize from water by playing with the pH, the 6-monoamino case is more challenging. While the 6-peramino CDs are practically insoluble, at a little higher than neutral pH – where some amine moieties are still ionized, the 6-monoamino-βCD is soluble, even at pH 8-8.5. When the pH is further increased the 6-monoamino-bCD starts to crystallize but around pH 10.5-11 a redissolution can be observed, as the secondary hydroxyls are going to be ionized. Careless “neutralization” can lead to not only the loss of product but more difficult filtration of the crystals. Fortunately, by using a volatile base, ammonia the eventual overuse of base (unlike to NaOH) can be avoided, because the excess of base is easy to remove.
And finally, especially the aqueous solubilities, can be affected with solvent modifiers, like hydrotropic materials or tensides which do not shift the solutions’ pH to strongly basic enough to ionize the CD hydroxyls. Some short alkyl chain alcohols and dipolar aprotic solvents can also be used as co-solvents, too, for increasing the aqueous solubilities. Usually, there is a solvent:water ratio where the natural CDs solubilities have a maximum. In organic solvents/water mixtures, the complex formation can also affect the CD solubilities. But, it is also true that despite the well-established CD/aqueous organic solvent studies owing to the molar ratios used in the publications the extraction of data in a usable format is difficult. DMSO is a good example for the effect of complexation, as DMSO shows an anomalous solubility curve. In dry DMSO the βCD is very well soluble, and the βCD solubility is increasing parallel with the DMSO content. But, above 90 % of DMSO, the βCD solubility unreasonably starts to decrease. And, finally, when no water in the system the βCD solubility is again higher than before. At low water content the DMSO/βCD complex formation occurs, which is less soluble in DMSO than the free βCD, virtually the βCD solubility decreases. Another example can be the permethylated CDs. For the permethylations dimethyl sulfate, in the presence of DMSO and NaOH, is efficiently used. When the reaction is complete only the insoluble inorganic salts are solids but destroying the unreacted dimethyl sulfate (aqueous ammonia solution is commonly used for that), the formed permethylated CDs precipitate to allow the isolation of the crude product with simple filtration.
Moving to the organic solvents the solubilities can become more complicated. The natural CDs are usually very well soluble in dipolar aprotic organic solvents (like DMF, DMSO, dimethyl acetamide or N-methyl pyrrolidone). Apart from the mass differences in weighing, a consequence of the water content is that usually, even in a non-aqueous environment, the host/guest complexes are formed. One example can be the αCD/pyridine case but it is less known that the very hydrophobized CDs, like peracetylated CDs, can also adsorb a few percents, meaning usually 1-2 mole, of water. The completely dried peracetylated βCD is moderately soluble in toluene and upon complexation of toluene, slow precipitation starts and the complexation almost completely removes all the macrocycles from the solution. This process can be very slow, depending on the residual water content of the CD derivative. A drop of water into the well-stirred solution makes the precipitation very fast, practically to a minute reaction. Additionally, these peracetylated derivatives are usually recrystallized in methanol – to remove impurities and residual solvent -, and the water content of the incompletely dried peracetylated CDs makes this purification also difficult.
While the role of water is not so obvious in the case of hydrophobic CD derivatives, the solubility of dried αCD in pyridine well demonstrates its effect on the complex formation. A completely and freshly dried αCD is soluble at 10-15% in pyridine, but after a couple of minutes, the traces of water induce the formation of αCD/pyridine complex. Due to either the residual water of the incompletely dried CD, pyridine, or eventually the slow weighing process at the air, the αCD/pyridine solution turns to a hardly-to-stir gel or suspension because the αCD/pyridine solubility is low (<2%).
The presence of impurities can also affect the solubilities. In many cases this can happen with the selectively persubstituted CDs. The crude product is soluble only until the impurities (reagents, solvents, or randomly substituted residues) are not removed. An example is the heptakis(2,6-di-O-ethyl)-βCDs (or TRIMEB, prepared under different conditions than that mentioned above). In the synthesis 20-30% randomly/incompletely ethylated version is formed and the crude contains these side-products and DMSO. This crude can be dissolved in n-heptane and the symmetrically derivatized βCD precipitates slowly and the impurities remain in solution. But, the n-heptane solubility of isolated symmetrically derivatized product is very limited so the n-heptane cannot be used for further purification.
As can be seen, the solubilities of CDs and their derivatives are not always obvious and clear stories. A reparametrized solubility/temperature equations and recalculated solubilities in water/organic solvent mixtures are published in a recent article shown below.
L. Jicsinszky, Some comments on the cyclodextrin solubilities. MOJ Bioorg. Organic Chem. DOI: 10.15406/mojboc.2019.03.00091 (https://medcraveonline.com/MOJBOC/MOJBOC-03-00091.pdf)