The influence of hydrogen bonding on the diffusion behaviour of diastereoisomeric tripeptide derivatives

The diffusion behaviour of the diastereoisomers of Z-Ala-Phe-Val-OMe and Z- Ala-Leu-Val-OMe was studied in solutions of carbon tetrachloride and toluen e. The capillary method according to Anderson was used for the diffusion ex periment. The loss of the concentration of the tripeptide derivatives in th e course of the time was monitored by infrared spectroscopy using their NH stretching vibrations. In general, the diffusion rate of the substances in toluene was 50-100 time s larger than in carbon tetrachloride. Also the diastereoisomers differ in their diffusion properties. In carbon tetrachloride this effect is very sma ll but still significant. It can be explained by the strong intramolecular hydrogen bonding of the peptides which leads to C-5 and C-7 rings. In tolue ne the different configuration of the compounds whose changes are connected with the change in their polar properties are responsible for the observed diffusion rates. The diffusion rate will be discussed in terms of equilibr ium constants describing the intramolecular association behaviour and molec ular descriptors of the tripeptide derivatives obtained from HPLC measureme nts in polar solvents. It will be shown that the diffusion rate correlates with the McGowan volume V-x and in part with the effective hydrogen bond ac idity Sigma alpha(2)(H). (C) 1999 Elsevier Science B.V. All rights reserved .