The CRC Handbook of Chemistry and Physics, 97th ed., Physical Constants of Organic Compounds, p. 3-48, describes biotin solubility: soluble in water, ethanol; slightly soluble in diethyl ether and chloroform.
Descriptive terms for solubility, p. 1-38 (solubility – parts of solvent required for 1 part of solute (recalculated to mass%)):
- very soluble – less than 1 (> 50%)
- freely soluble – 1 to 10 (50% to 9.1%)
- soluble – 10 to 30 (9.1% to 3.2%)
- sparingly soluble – 30 to 100 (3.2% to 0.99%)
- slightly soluble – 100 to 1000 (0.99% to 0.1%)
- very slightly soluble – 1000 to 10000 (0.1% to 0.01%)
- practically insoluble, or insoluble – >= 10000 (< 0.01%)
Aqueous Solubility and Henry’s Law Constants of Organic Compounds, p. 5-141, states that biotin solubility is 0.35 g kg−1 H2O (0.035 mass%) at 25 °C.
This solubility value would be described as very slightly soluble.
I don't know how to explain this inconsistency.
Octanol/water partition coefficient
.. However, low water solubility does not necessarily mean high fat solubility. A standard measure of lipophilicity is octanol/water partition coefficient (or its decadic logarithm, log P). Positive means more soluble in octanol (fats), negative means more soluble in water.
Experimental log P values for fat-soluble vitamins:lit
- vitamin A +5.68
- vitamin D3 +10.24
- vitamin E +12.18
- vitamin K0 +2.20
Typical water soluble vitamin:
Some B vitamins:
- vitamin B1 −3.93
- vitamin B2 −1.46
- vitamin B4 −0.09
- vitamin B7 +0.39
- vitamin B8 −1.68
- vitamin B13 −0.38
Biotin (vitamin B7) is interestingly slightly lipophilic, or little more fat-soluble than water-soluble.
Even though biotin solubility in water is quite low, it is still classified as water soluble. Originally, biotin (named vitamin H, originally) was discovered in extracts from egg yolk or liver, which are rather fatty tissues, in methyl ester form, practically insoluble in water.
On the other hand, biotin was originally not found in egg white, because it contains avidin protein, which binds biotin so strongly, that it's one of the strongest non-covalent interactions known.
Biotin is often bound in dietary sources to proteins as amide on lysine. By digestion, it's metabolised to biocytin, then to free biotin.
So the fat or water solubility view may be somewhat simplistic.