Hydrogen bonding. 47. Characterization of the ethylene glycol-heptane partition system: hydrogen bond acidity and basicity of peptides.

Twelve measured ethylene glycol-heptane partition coefficients, Peh, have been combined with 20 measured literature values and 44 indirectly determined values to give a set of 76 values. Excluding one value for benzamide, the log Peh values are correlated through our general solvation equation, log Peh = 0.336 - 0.075R2 - 1. 201pi2H - 3.786 Sigmaalpha2H - 2.201 Sigmabeta2H + 2.085Vx with r2 = 0.966, sd = 0.28, and F = 386. The solute descriptor R2 is the excess molar refraction, pi2H is the dipolarity/polarizability, Sigmaalpha2H and Sigmabeta2H are the overall hydrogen bond acidity and basicity, and Vx is the McGowan volume. The log Peh equation has then been used to obtain descriptors for eleven peptides, all of which are end-protected. It is shown that for these end-protected peptides, hydrogen bond basicity makes a greater contribution to log Peh than does hydrogen bond acidity.