LACK OF WATER TRANSPORT BY AMINO-ACID SIDE-CHAINS OR PEPTIDES ENTERING A NONPOLAR ENVIRONMENT

Water has been reported to enter cyclohexane in association with 3-met hylindole, a model compound representing the side chain of tryptophan. Entrainment of water would cloud the interpretation of measured parti tion coefficients as a simple index of hydrophobicity. NMR and isotope -exchange experiments indicate that the reported entrainment of water resulted from unrecognized exchange of H-3 from water into the -NH- gr oup of the indole ring. A more detailed analysis shows that no signifi cant amounts of excess water (less than 0.1 molecule/molecule of solut e) enter cyclohexane with molecules representing the side chains of tr yptophan, phenylalanine, threonine, lysine, or the peptide bond itself .