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j. Soc. Cosmet. Chem., 47, 363-375 (November/December 1996) Cross-adaptation of sweaty-smelling 3-methyl-2-hexenoic acid by its ethyl esters is determined by structural similarity JOHN D. PIERCE, J•t., DAVID. H. BLANK, EVGUENY V. ARONOV, ZHENRONG GUO, GEORGE PRETI, and CHARLES J. WYSOCKI, Monell Chemical Senses Center, 3500 Market Street, Philadelphia, PA 19104-3308 (J.D.P., D.H.B., E.V.A., Z.G., G.P., C..].W.) and Department of Dermatology, School of Medicine (G. P.), and Department of Animal Biology, School of Veterinary Medicine (C..]. W.), University of Pennsylvania, Philadelphia, PA. Accepted jSr publication February 1, 1997. Synopsis Cross-adaptation, the decrease in sensitivity to one odorant after exposure to a different odorant, is affected by perceptual similarity. Recent evidence has shown that structural similarity, in the absence of perceptual similarity, can also influence cross-adaptation. We recently demonstrated cross-adaptation of a 10:1 mixture of (E)- and (Z)-3-methyl-2-hexenoic acid (3M2H), a principal component of human underarm odor, by a 3:1 mixture of the fruity-smelling (E)- and (Z)-ethyl esters of 3M2H (EE3M2H). To further explore the structural basis for this cross-adaptation, we synthesized and purified the individual E- and Z-isomers of EE3M2H and tested them separately for cross-adaptation to a 10E:1Z mixture and a 10Z:1E mixture of 3M2H. The E-isomer of EE3M2H significantly cross-adapted both 3M2H mixtures the Z-isomer of EE3M2H cross-adapted the 10Z:lE mixture only. In threshold tests, the Z-isomer was detected at lower concentrations than were either the E-isomer or a 3E:IZ EE3M2H mixture. These results provide strong evidence that structural similarities underlie the cross-adaptation between 3M2H and its ethyl esters. INTRODUCTION Cross-adaptation, the decrease in sensitivity to one odorant after exposure to a different odorant, has commonly been interpreted as a measure of the degree to which odors share common sensory channels (1-4). Although it has long been known that perceptual similarity influences cross-adaptation, recent evidence has provided insight into how structural similarity, in the absence of perceptual similarity, can also influence cross- adaptation. The present study further investigated the role of structural similarity as it affects cross-adaptation in perceptually distinct compounds: sweaty-smelling 3-methyl- 2-hexenoic acid and its fruity-smelling ethyl ester. Most work in olfactory cross-adaptation from a psychophysical approach has focused on the perceptual relationship between the adapting stimulus and the test stimulus as it determines cross-adaptation. It is now well-established that perceptual analogs, i.e., 363