Evaluating hydrogen-bond donor strength

A parameter that measures hydrogen-bond donor (HBD) strength of solutes is useful in modeling many biological interactions. The solvents octanol and chloroform, have about equal HBD strength and thus will accommodate the hydrogen-bond acceptor (HBA) groups in solutes about equally well. Because the solvent octanol has a strong acceptor oxygen, solutes with HBD groups will favor it over chloroform on that basis. With its eight alkane carbons, octanol also favors solutes with a significant amount of alkane character, a property referred to in this paper as 'excess alkane affinity' (XAA). On the other hand, it is easier to form a cavity in the solvent chloroform, so larger solutes tend to favor that solvent. After allowing for XAA and molecular volume, the difference between log P(oct) and log P(clf) is a measure of the effective sum of HBD. This value is given the symbol epsilonalpha and appears to be on the same scale as Abraham's summation operatoralpha(2)(H).