Steric effects threshold

The importance of lipophilicity to bitterness has been well established, both directly and indirectly. The importance of partitioning effects in bitterness perception has been stressed by Rubin and coworkers, and Gardner demonstrated that the threshold concentration of bitter amino acids and peptides correlates very well with molecular connectivity (which is generally regarded as a steric parameter, but is correlated with the octanol-water partition coefficient ). Studies on the surface pressure in monolayers of lipids from bovine, circumvallate papillae also indicated that there is a very good correlation between the concentration of a bitter compound that is necessary in order to give an increase in the surface pressure with the taste threshold in humans. These results and the observations of others suggested that the ability of bitter compounds to penetrate cell membranes is an important factor in bitterness perception. 

The transport factors described above are influenced by the morphology of the adsorbent particles as well as by their surface chemistry. Pore sizes influence chromatographic properties in two main ways. The first effect is due to steric hindrance. Molecules can only penetrate into pores of sizes above a certain threshold. For bigger molecules the theoretically available surface area is only partly accessible. The second factor is related to pore size distributions. 

The use of computer techniques in the correlation of biological activity with substrate physical-chemical properties has received much attention in the area of medicinal chemistry. The use of these techniques, denoted Quantitative Structure Activity Relationships (QSAR), were developed mostly by Hansch and his coworkers eind have been reviewed by Tute te), Purcell et. al. (9) and Dunn (10). These techniques were utilized by Greenberg (1 l) ln the correlation of odor threshold and suprathreshold data with Log P, the log (n-octanol/water partition coefficient). In the same study it was reported that steric and polar effects as measured by the Taft Steric and Polar Constants poorly correlated with odor intensity data. 

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