Odor Quality and Chemical Structure - ACS Publications - American

9 Structure Recognition as a Peripheral Process in Odor Quality Coding Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 19, 2018 | https://pubs.acs.org Publication Date: March 20, 1981 | doi: 10.1021/bk-1981-0148.ch009

ALFRED A. SCHLEPPNIK Monsanto Company, Biomed Department, 1800 North Lindbergh Blvd., St. Louis, MO 63166

The odor q u a l i t y o f a compound i s , according t o BEETS ( l ) , d e f i n ed i n t r i n s i c a l l y by the chemical s t r u c t u r e : Odorant molecules encode the s t r u c t u r a l m o d a l i t i e s of the stimulant molecule i n a t r a n s d u c t i o n process, which, t a k i n g a l l changes o f o r i e n t a t i o n and conformation i n t o account, produces i n f o r m a t i o n a l m o d a l i t i e s . The l a t t e r are expressed as t o p o l o g i c a l l y defined s t r u c t u r a l f e a tures of high v a r i a b i l i t y and complexity. Odor/Structure C o r r e l a t i o n attempts t o e l u c i d a t e the mechanisms which mediate the information t r a n s f e r from s t r u c t u r a l features of a molecule t o a corresponding information p a t t e r n . The l a t t e r o r i g i n a t e s i n o l f a c t o r y neurons and i s encoded i n nerve impulses. It i s p r o j e c t e d f o r f u r t h e r a n a l y s i s , d i s c r i m i n a t i o n and recogn i t i o n t o the higher o l f a c t o r y centers of the CNS. T h i s informat i o n t r a n s f e r i n c l u d e s the t r a n s d u c t i o n process which converts chemical t o e l e c t r i c a l s i g n a l s . Many odor t h e o r i e s have been proposed i n the past, attempting t o e x p l a i n the multitude of o f t e n very complex phenomena observed i n human o l f a c t i o n . Most o f them were only p a r t i a l l y , i f at a l l , s u c c e s s f u l . Nevertheless, slowly a consensus developed and today i t i s g e n e r a l l y assumed that the primary process of chemorecept i o n takes p l a c e at the c e l l membrane of a sensory neuron and i n v o l v e s p h y s i c a l contact of the stimulant with p o t e n t i a l or act u a l receptor s i t e s which could be e i t h e r s p e c i a l i s t s - r e a c t i n g only with one s t r u c t u r a l c l a s s - or g e n e r a l i s t s which would r e a c t with a multitude o f s t r u c t u r a l c l a s s e s .

0097-6156/81/0148-0161$05.00/0 © 1981 American Chemical Society

Moskowitz and Warren; Odor Quality and Chemical Structure ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 19, 2018 | https://pubs.acs.org Publication Date: March 20, 1981 | doi: 10.1021/bk-1981-0148.ch009

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Then, i n t e r a c t i o n o f the stimulant molecule with the receptor s i t e regardless o f the nature o f the processes i n v o l v e d , has t o achieve the f o l l o w i n g r e s u l t s : a) Graded t r a n s d u c t i o n o f a chemical i n t o an e l e c t r i c a l s i g n a l ( i n t e n s i t y Grading) b) T r a n s c r i p t i o n o f a l l or s i g n i f i c a n t p a r t s o f the s t r u c t u r a l modality o f the stimulant molecule i n t o a set o f i n f o r m a t i o n a l m o d a l i t i e s which are combined i n a p r e c i s e and s p e c i f i c "Odor Information P a t t e r n " ( Q u a l i t y Coding) c) A m p l i f i c a t i o n o f the primary energy gained by adsorption of a few stimulant molecules t o a l e v e l high enough t o t r i g g e r the e l e c t r o g e n i c processes i n v o l v e d i n s i g n a l generation ( D e p o l a r i z a t i o n o f the sensory neuron, f i r i n g of a spike) d) High speed o f the t o t a l process t o create a p o t e n t i a l i n a few 100 msec. e) Do a l l t h i s without i n v o l v i n g the stimulus i n any chemic a l changes, but r e l e a s e i t unchanged r a p i d l y a f t e r t e r mination o f the t r a n s d u c t i o n process. In t h i s communication the focus i s on b ) : Odor/Structure Correlation. Most, perhaps a l l o f the odor t h e o r i e s advanced so f a r made the assumption that the t r a n s c r i p t i o n o f s t r u c t u r a l information encoded i n the stimulant molecule i n t o an odor information p a t t e r n i s an i n t e g r a l process: One o d o r i v e c t o r (AMOORE, 2) i n t e r a c t s with one receptor s i t e and t h i s i n t e r a c t i o n r e s u l t s i n t r a n s c r i p t i o n o f a l l s t r u c t u r a l components simultaneously i n t o t h e i r corresponding i n f o r m a t i o n a l m o d a l i t i e s . However, observation t e l l s us that o l f a c t o r y information i s i n h e r e n t l y complex: Ambergris f o r instance i s described (0HL0FF, 3) by s i x d i s t i n c t l y d i f f e r e n t notes. This would imply that i n an i n t e g r a l process o f the p e r i p h e r a l molecul a r i n t e r a c t i o n one s i n g l e neuron has t o detect at l e a s t s i x d i f f e r e n t p r o f i l e s with s i x d i f f e r e n t receptor s i t e s and p r o j e c t the i n f o r m a t i o n a l m o d a l i t i e s i n t a c t t o the higher centers. Since the s i n g l e b i t o f o l f a c t o r y information i s one spike o f the o l f a c t o r y neuron which i s independent o f the number and q u a l i t i e s of the detector s i t e s an insurmountable problem f o r q u a l i t y coding arises. One way t o avoid t h i s problem i s simply t o deny the e x i s tence o f s p e c i f i c receptor s i t e s and s p e c i a l i z e d detector c e l l s i n AMOORE s terms and r e p l a c e the s p e c i a l i s e d concept with a "General Concept" i n which q u a l i t y coding i s achieved through a s p a t i a l d i s t r i b u t i o n o f c o l l e c t i o n s o f a l a r g e number o f s t r u c t u r a l l y d i f f e r e n t g e n e r a l i s t receptor s i t e s which would i n t e r a c t with the stimulant molecule i n a l l i t s o r i e n t a t i o n s and conformations. In t h i s way a l l s t r u c t u r a l features o f the stimulant molecule - the s t r u c t u r a l m o d a l i t i e s - would be converted i n t o i n f o r m a t i o n a l mod a l i t i e s d i s t r i b u t e d over an information p a t t e r n with more or l e s s d i s t i n c t t o p o l o g i c a l c h a r a c t e r i s t i c s . Therefore the p r o f i l e i s not }


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expressed at the molecular or microscopic, but at the macroscopic l e v e l , as defined areas o f the o l f a c t o r y epithelium. However i t has t o be noted that again, even i n t h i s d i f f u s e p l u r i f o r m i n t e r a c t i o n scheme the i n t e g r a l process i s used: One stimulant molecule i n t e r a c t i n g with one g e n e r a l i s t receptor s i t e i s s u f f i c i e n t f o r q u a l i t y coding. The a l t e r n a t i v e t o the i n t e g r a l process i s a d i f f e r e n t i a l process of the type o f a " M u l t i p l e P r o f i l e - M u l t i p l e Receptor S i t e " i n t e r a c t i o n f i r s t suggested by POLAK J r . (h). In a system o f t h i s kind the p r o f i l e and the receptor s i t e have t o be s t e r i c a l l y complementary l i k e a substrate t o an a c t i v e s i t e o f an enzyme; or a drug molecule t o i t s s p e c i f i c receptor s i t e ; o r a hormone t o i t s complementary r e g u l a t o r y s i t e o f a membrane bound adenyl c y c l a s e system. I t i s c h a r a c t e r i s t i c that almost a l l l i f e processes are regulated by i n t e r a c t i o n o f chemical messenger molecules with spec i f i c s i t e s o f a t e r t i a r y p r o t e i n s t r u c t u r e . Staying w i t h i n the w e l l e s t a b l i s h e d and accepted p r i n c i p l e s o f molecular biochemistry and assuming that there i s indeed no d r a s t i c d i f f e r e n c e between the p e r i p h e r a l processes o f substrate/enzyme-, d r u g / s p e c i f i c r e ceptor s i t e - and s t i m u l a n t / s p e c i f i c receptor s i t e i n t e r a c t i o n s one can p o s t u l a t e that the t e r t i a r y p r o t e i n s t r u c t u r e - the receptor s i t e - i s part o f the r e g u l a t o r y subunit o f an adenyl c y c l a s e system. The same r e g u l a t o r y subunit could contain a second r e g u l a t o r y site. Adenyl cyclases are h i g h l y complex enzyme systems c o n s i s t i n g o f s e v e r a l i n t e r a c t i n g subunits. The system described above contains a subunit with two r e g u l a t o r y s i t e s : One f o r the o d o r i v e c t o r which acts as an a c t i v a t o r f o r the c a t a l y t i c s i t e o f the adenyl c y c l a s e system imbedded i n a second subunit. The other r e g u l a t o r y s i t e i n the f i r s t subunit then can act as an a l l o s t e r i c r e g u l a t o r y s i t e for a c t i v a t o r s o r i n h i b i t o r s and i n t h i s manner r e g u l a t e the conformation o f the s p e c i f i c o d o r i v e c t o r receptor s i t e . The second subunit o f the adenyl c y c l a s e system i s the c a t a l y t i c subunit. I t forms a s t a b l e b i n a r y complex with the magnesium s a l t of adenosine triphosphate (ATP) i n i t s r e s t i n g s t a t e . The s t a b i l i t y o f the b i n a r y complex i s caused by the complexed ATP-molec u l e which acts as an undersized b l o c k i n g agent. A r r i v a l o f an o d o r i v e c t o r at t h e a c t i v a t i n g a l l o s t e r i c s i t e i n the regulatory subunit and the subsequent complex formation o f the o d o r i v e c t o r with the "Detector subunit" r e s u l t s i n conformational changes o f the detector subunit which are communicated through cooperative e f f e c t s t o the c a t a l y t i c subunit. The l a t t e r then can adapt again through conformational changes o f the t e r t i a r y s t r u c t u r e the c a t a l y t i c s i t e t o the substrate -ATP- which i t already contains. This "Induced F i t " (KOSHLAND, j>) a c t i v a t e s the c a t a l y t i c s i t e , t h e c a talytic (enzymatic) r e a c t i o n takes place very r a p i d l y and ATP i s converted t o 3 ,5 -adenosine monophosphate. This " c y c l i c adenosine f

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monophosphate (cAMP)" i s the second messenger: s t i l l a s i g n a l , the ubiquitous information c a r r i e r i n r e g u l a t o r y t i c processes.

chemical enzyma-

Adenyl cyclase systems i s o l a t e d from mammals contain as a t h i r d component an a d d i t i o n a l guanosine triphosphate s p e c i f i c subunit and in a l l probability even more components whose f u n c t i o n and s t r u c t u r e are not known as yet. I t i s i n t e r e s t i n g to note that adenyl c y c l a s e s , Na-K-activated adenosine phosphatases (ATPases) have been l o c a t e d i n the membrane of o l f a c t o r y neurons; and cAMP was found to have the highest concentration i n man i n the o l f a c t o r y mucosa. The second messenger, cAMP, couples the adenyl c y c l a s e which funct i o n s as a "Detector Enzyme" to another membrane bound enzyme, a Na-K-ATPase which operates as an i o n pump which moves ions i n aca c t i v e t r a n s p o r t against t h e i r concentration g r a d i e n t . Changes of the a c t i v i t y of the ATPase produce changes of the membrane potent i a l . Therefore r e g u l a t i o n of the i o n pump by the second messenger -cAMP- produces r e g u l a t i o n of the membrane p o t e n t i a l . Furthermore, assuming that both the detector enzyme (adenyl c y c l a s e ) and the transducer p r o t e i n (ATPase) are monomers of a heterogenic polymeri c enzyme system arranged i n a two dimensional p a t t e r n i n which act i v a t i o n of one coupled enzyme p a i r would, by p o s i t i v e cooperative effects, a c t i v a t e a l a r g e number of acceptor u n i t s (transducer + detector enzymes) not only a s i n g l e Na/K pump (the transducer enzyme), but a very l a r g e number of Na/K-pumps would be regulated. As a consequence of such a mechanism a powerful a m p l i f i c a t i o n f a c t o r would be introduced: The two dimensional multienzyme system operates l i k e a b i o a m p l i f i e r . A r r i v a l of a s i n g l e o d o r i v e c t o r molecule at i t s complementary specific receptor s i t e consequently leads to p a r t i a l d e p o l a r i z a t i o n of a l l transducer c e l l s i n v o l v e d i n the b i o a m p l i f i e r . The r e s u l t i n g change i n membrane p o t e n t i a l has been observed as the "Genera t o r P o t e n t i a l " . I f i t b u i l d s up high enough i t t r i g g e r s a t h i r d enzyme system which i n s t a n t l y d e p o l a r i z e s the o l f a c t o r y neuron. The r e s u l t i n g change i n membrane p o t e n t i a l i s a s i n g l e nerve imp u l s e , a spike. Since t h i s t h i r d enzyme system produces a strong s i g n a l on r e c e p t i o n of a weaker one i t works as a t r u e transponder which i n d i c a t e s by generation of a spike that a generator p o t e n t i a l had reached a c r i t i c a l l e v e l . The spike i s the s i n g l e b i t of chemoreceptory i n f o r m a t i o n a l modality t r a n s c r i b e d from s t r u c t u r a l modalties of the o d o r i v e c t o r . In short, l i g a n d formation of one o d o r i v e c t o r molecule with a r e ceptor s i t e having a complementary s t r u c t u r e to s t r u c t u r a l e l e ments of the o d o r i v e c t o r would r e s u l t i n formation of a s i n g l e b i t of chemoreceptory information. The acceptor system i s a modular system i n which the transducer and the transponder can remain un-


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changed and only a change of the detector enzyme i n the r e g u l a t o r y subunit of the detector enzyme i s r e q u i r e d (and s u f f i c i e n t ) t o provide f o r the accommodation of a p r a c t i c a l l y u n l i m i t e d number and v a r i e t y of s t r u c t u r a l features of the o d o r i v e c t o r through complementary s t r u c t u r a l features of the receptor s i t e . At f i r s t glance t h i s seems t o r e s u r r e c t the o l d " S p e c i a l i s e d Concept" i n which a s p e c i f i c receptor s i t e f o r a " t y p i c a l odorivect o r s t r u c t u r e " and i t s congeners would engage i n l i g a n d formation i n an i n t e g r a l process. This would be i n sharp contrast t o experimental r e s u l t s obtained i n s i n g l e c e l l e l e c t r o p h y s i o l o g i c a l studi e s . These demonstrate that at l e a s t i n v e r t e b r a t e s the o l f a c t o r y neurons are not s p e c i a l i s t s , but GENERAL!STS AS FAR AS THE OVERALL STRUCTURE OF THE MOLECULE IS CONCERNED: They i n t e r a c t with a multitude of s t r u c t u r a l l y d i f f e r e n t o d o r i v e c t o r s . This observat i o n i n s i n u a t e s that not the t o t a l sum of a l l s t r u c t u r a l features of the o d o r i v e c t o r s , NOT THE OVERALL STRUCTURE, i s encoded, but a SPECIFIC PARTIAL STRUCTURAL FEATURE which may very w e l l be part of many otherwise t o t a l l y d i f f e r e n t o v e r a l l s t r u c t u r e s of o d o r i vectors. M. G. J . BEETS (6) has introduced the term " P r o f i l e " f o r t h i s type of p a r t i a l - or submolecular - s t r u c t u r e . This p r i n c i p l e and the term were adopted, but i n the system discussed now - the ENZYME MODEL OF OLFACTION - the meaning of " p r o f i l e " was defined more sharply. In i t the term " p r o f i l e " describes a l i m i t e d number of w e l l defined substructures of the o d o r i v e c t o r . In the EMO a prof i l e of the o d o r i v e c t o r c o n s i s t s of a three dimensional s p a t i a l arrangement of a sequence of atoms i n a w e l l defined o v e r a l l geometry. I t can be present e x p l i c i t l y , preformed i f the odorivect o r or a s i g n i f i c a n t part of i t has a r i g i d s t r u c t u r e with p r a c t i c a l l y almost no conformational freedom. However the p r o f i l e can be contained i m p l i c i t l y i n o d o r i v e c t o r molecules with v a r y i n g degrees of conformational freedom. Such molecules have e i t h e r " e l o quent" s t r u c t u r e s with high degrees of conformational freedom, capable of expressing t h e i r s t r u c t u r a l m o d a l i t i e s i n many d i f f e r ent ways; or f l e x i b l e molecules with a l i m i t e d range of conformat i o n a l freedom i n which one or a few conformations would be v a s t l y p r e f e r r e d and others excluded. I t follows that eloquent, and t o a l e s s e r degree, f l e x i b l e molecules can, a l l other s t e r i c a l requirements provided, assume the same p r o f i l e as one preformed i n a r i g i d o d o r i v e c t o r s t r u c t u r e . However, with i n c r e a s i n g conformational freedom the p r o b a b i l i t y of assuming a s p e c i f i c prof i l e diminishes r a p i d l y . It furthermore follows that a p r o f i l e may c o n s t i t u t e only a s i g n i f i c a n t part of the o v e r a l l s t r u c t u r e of the o d o r i v e c t o r molecule: e i t h e r a shape - the Van der Waals molecular o u t l i n e proposed by AMOORE - which can degenerate t o a m o l e c u l a r l y defined plane; or i t can be a f u n c t i o n a l group i n the t r a d i t i o n a l sense


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of "Osmophores" (RUZICKA's odor theory, 7 ) which may c o n t r i b u t e s t e r e o e l e c t r o n i c f e a t u r e s , such as P i - e l e c t r o n clouds e t c . In d i s c u s s i o n s o f Odor/Structure correlations the odorivector can be t r e a t e d as a c o l l e c t i o n o f a number o f e x p l i c i t o r i m p l i c i t p r o f i l e s which are e i t h e r d i r e c t l y connected o r imbedded i n a l a r g e r "frame s t r u c t u r e " . The r e s u l t i n g molecular weight o f the s t r u c t u r e s generated i n t h i s way has l i t t l e i n f l u e n c e on t h e i n t e r a c t i o n s with o l f a c t o r y receptor s i t e s as long as the fugac i t y o f the o d o r i v e c t o r i s high enough t o allow a s u f f i c i e n t number o f o d o r i v e c t o r molecules t o reach receptor s i t e s . Small mol e c u l e s with a molecular weight o f l e s s than 100 Daltons d i s p l a y i n a d d i t i o n t o t h e i r normal i n t e r a c t i o n with complementary recept o r s i t e s p r o j e c t i n g s i g n a l s i n t o the o l f a c t o r y nerve, strong i n t e r a c t i o n s with a branch o f the trigeminus nerve, causing the w e l l known e f f e c t s o f i r r i t a t i o n and i n t e r f e r e n c e with odor perc e p t i o n (CAIN and MURPHY, 8 ) . T y p i c a l o d o r i v e c t o r s - most "odorant molecules" - have a molecul a r weight i n the range o f 100 t o about 350 Daltons. They cont a i n t h e r e f o r e enough " s k e l e t a l " atoms t o b u i l d frame s t r u c t u r e s for e x p l i c i t f u n c t i o n a l groups o r d i s t i n c t shapes. I t i s t h i s type o f o d o r i v e c t o r with one f u n c t i o n a l p o l a r group attached t o or imbedded i n an o f t e n very complex frame which i s the one most commonly encountered. Since t h e frame part can p o t e n t i a l l y cont a i n a p l u r a l i t y o f p r o f i l e s the t o t a l o d o r i v e c t o r i t s e l f can c a r r y a vast amount o f s t r u c t u r a l information. Conversely, t h e small molecules with a molecular weight below 100 Daltons have only very small "frames", i f any at a l l , and consequently c a r r y only a l i m i t e d amount o f s t r u c t u r a l information beyond t h e i r i n herent trigeminus i r r i t a n t c o n t r i b u t i o n t o the o v e r a l l sensory perception. In any case, whatever amount o f s t r u c t u r a l modality may be cont a i n e d i n the o d o r i v e c t o r s t r u c t u r e has t o be t r a n s c r i b e d t o t a l l y or p a r t i a l l y i n the t r a n s d u c t i o n process. More p r e c i s e l y , t h i s t r a n s c r i p t i o n process has t o be e f f e c t e d i n the p e r i p h e r a l i n t e r a c t i o n o f the o d o r i v e c t o r with the receptor s i t e l e a d i n g t o l i gand formation. The r e s u l t i n g complex i s bound by weak and r e v e r s i b l e bonds, such as hydrogen bonds or Van der Waals f o r c e s . In most cases t h e receptor s i t e i s the proton donor, most l i k e l y through f r e e t h i o l groups. In some s p e c i a l cases the reverse process, i n which the o d o r i v e c t o r acts as a proton donor, may be operational. In order t o achieve weak bond formation the l i g a n d has t o f i t i n to the receptor s i t e i n such a way as t o b r i n g weak bond forming s i t e s o f the o d o r i v e c t o r and the receptor s i t e w i t h i n s t r i k i n g d i s t a n c e . T h i s i s the same process as the one assumed i n drug/ receptor i n t e r a c t i o n s . I t was recognized i n molecular pharmacolo-


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gy that the l i g a n d could be construed t o c o n s i s t o f an " A f f i n i t y Part" and an " I n t r i n s i c A c t i v i t y P a r t " (ARIENS, 9 ) . This concept i s l o o s e l y comparable t o the d e s c r i p t i o n o f a "normal o d o r i v e c t o r molecule" as a "frame plus one p o l a r f u n c t i o n a l group". The a f f i n i t y part determines t h e ease with which the complementar i t y o f p r o f i l e and receptor s i t e i s achieved. This i s dependent of the equlibrium s t r u c t u r e o f both the p r o f i l e and the receptor s i t e and the energy r e q u i r e d t o change the conformation o f e i t h e r component or both. The " I n t r i n s i c A c t i v i t y P a r t " determines t h e ease o f weak bond formation o f the f u n c t i o n a l group with the a c t i v e group o f the receptor s i t e . I t has been suggested that t h i s weak bond format i o n occurs as the f i r s t step and provides a p i v o t f o r the a f f i n i t y part which encodes the "Shape p r o f i l e s " and t h e r e f o r e has to assume the proper o r i e n t a t i o n before the complex formation i s f i n i s h e d by induced f i t o f the receptor s i t e . I t i s noteworthy that i n such cases weak bond formation t o the f u n c t i o n a l group does not encode the s t r u c t u r a l modality o f the f u n c t i o n a l group, but o f that o f the shape. F u n c t i o n a l groups have t h e i r own, i n most cases s t e r i c a l l y l e s s demanding, s p e c i f i c receptor s i t e s . From t h i s follows that increased s i z e and s t e r i c a l complexity o f the frame ( a f f i n i t y moiety) p o t e n t i a l l y provides a l a r g e r number of s h a p e - p r o f i l e s . Assuming that a l l these s t e r i c a l m o d a l i t i e s are expressed i n i n f o r m a t i o n a l m o d a l i t i e s the c o n t r i b u t i o n o f f u n c t i o n a l groups becomes p r o p o r t i o n a l l y l e s s d i s t i n c t i v e , and, given a s u f f i c i e n t l y e f f e c t i v e s t e r i c hindrance o f the f u n c t i o n a l group may render i t s i n f o r m a t i o n a l modality i n the o v e r a l l odor information pattern negligeable. In t h i s way t h e o l d e r "Funct i o n a l Group Odor Theory" o f RUZICKA (1920) and the "Stereochem i c a l Theory o f Odor" o f AM00RE (1962) are r e c o n c i l e d : Both are t o t a l l y compatible with t h e "Enzyme Model o f O l f a c t i o n " and ded u c i b l e from the general molecular requirements o f l i g a n d formation. Odorivector molecules can contain an almost u n l i m i t e d number o f profiles. Of these are only a few e x p l i c i t , but with i n c r e a s i n g conformational freedom a r a p i d l y i n c r e a s i n g number o f i m p l i c i t ones are p o t e n t i a l l y p o s s i b l e . This r a i s e s t h e question about t h e number o f complementary receptor s i t e s necessary t o deal unambiguously and e f f i c i e n t l y with the t r a n s c r i p t i o n o f s t r u c t u r a l i n t o i n f o r m a t i o n a l m o d a l i t i e s . The concept o f m u l t i p l e p r o f i l e - mult i p l e receptor s i t e s provides no i n d i c a t i o n how the a c t u a l number o f receptor s i t e types could be deduced. However the minimum number r e q u i r e d t o encode the t o t a l o l f a c t o r y spectrum perceived by man can be estimated by means o f b a s i c p r i n c i p l e s o f informat i o n theory. For that a few simple assumptions have t o be made:


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3)

k) 5)

CHEMICAL STRUCTURE

Each receptor s i t e produces only one i n f o r m a t i o n a l modal i t y , a monoosmatic component. Each o l f a c t o r y neuron contains e x c l u s i v e l y or v a s t l y predominantly receptor s i t e s s p e c i f i c f o r only one p r o f i l e . The c o n t r i b u t i o n s - nerve impulses - of the i n d i v i d u a l a c t i v e neurons are summated i n the next higher center, the glomeruli o f the o l f a c t o r y bulb. I f the combined act i v i t y of 20.000 - 25.000 o l f a c t o r y neurons, which a l l feed i n t o one glomerulus, excede a t h r e s h o l d value, the glomerulus i s a c t i v a t e d - turned on t o produce a s i g n a l which i n d i c a t e s a s p e c i f i c monoosmatic component. A l l s p e c i f i c monoosmatic components are combined i n s t i l l higher centers t o produce an "Odor Information P a t t e r n " . Each d i s c e r n i b l e odor has a s p e c i f i c unique i n d i v i d u a l odor information p a t t e r n .

It has been observed that the d i s c r i m i n a t o r y c a p a b i l i t i e s of human o l f a c t i o n are tremendous: I t was estimated that an untrained person could d i f f e r e n t i a t e up t o ten m i l l i o n odors, perhaps even s i g n i f i c a n t l y more than t h a t . Information theory then shows that i n order t o encode the q u a l i t i e s of ten m i l l i o n odors i n a simple b i n a r y mode (Monoosmatic components on or o f f , t h e i r i n t e n s i t y , a l b e i t important, i s i n t h i s connection disregarded) only 2k t o 27 s p e c i f i c p r o f i l e s , d i s r e g a r d i n g p o s s i b l e and probable redundancies, and t h e r e f o r e the same number of complementary receptor s i t e s would be required. Assuming furthermore that s a i d redundancy, i n which the i n f o r m a t i o n a l m o d a l i t i e s of two d i f f e r e n t s p e c i f i c receptor s i t e s of two d i f f e r e n t o l f a c t o r y neurons are c o n f l u ent i n one c o l l e c t o r c e l l and t h e r e f o r e c o n t r i b u t e t o the express i o n of only one monoosmatic component i s indeed o p e r a t i o n a l i t becomes necessary t o increase the t o t a l number o f types o f s p e c i f i c receptor s i t e s t o 2U-30. This means that only 2^-30 specific detector p r o t e i n s are r e q u i r e d f o r s t r u c t u r e r e c o g n i t i o n i n the t r a n s d u c t i o n process. This compares t o about U000 enzyme systems i n d i f f e r e n t stages of a c t i v i t y estimated to be present i n a c e l l any time. The next question a r i s i n g i s that about the minimum number of monoosmatic components r e q u i r e d to encode an odor q u a l i t y . I t has been recognized by BEETS that an inherent " P r i n c i p l e of informat i o n a l complexity" makes the perception of even a s i n g l e odorant molecular species i n f o r m a t i o n a l l y complex, even i f the odor i n f o r mation p a t t e r n i s dominated by the t e r m i n a l d e r i v a t i v e (monoosmat i c component) of a s i n g l e chemoreceptory modality. But there has t o be something l i k e a minimum complexity s t i l l . In terms of the EM0 there must be a minimum number of monoosmatic components ess e n t i a l to produce a minimal odor information p a t t e r n . Again, s i n c e t h i s problem i s not i n the domain of p e r i p h e r a l processes, the Enzyme Model of O l f a c t i o n cannot provide an answer. However, experimental r e s u l t s obtained by P0LAK (10) i n d i c a t e that one


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s i n g l e i n f o r m a t i o n a l modality does not encode s i g n a l s the presence o f an odorant.

a q u a l i t y but

169

only

As a consequence the minimum number of monoosmatic components r e quired t o encode an odor q u a l i t y i s two. This seems to r u l e out the concept of primary odors. However, t a k i n g i n t o account the r e l a t i v e i n t e n s i t i e s of the monoosmatic components one could expect that an odor p r o f i l e with two monoosmatic components of which one dominates d e c i s i v e l y would s i g n a l an odor q u a l i t y approaching the s i m p l i c i t y of a primary odor. A r r i v a l of the o d o r i v e c t o r i n i t s p r e f e r e d o r i e n t a t i o n s and conformations at the o l f a c t o r y epithelium leads to simultaneous complex formation o f many o d o r i v e c t o r molecules through d i f f e r e n t p r o f i l e s contained as s t r u c t u r a l m o d a l i t i e s i n the o v e r a l l s t r u c t u r e with t h e i r corresponding complementary receptor s i t e s . This leads t o s i g n a l generation and s i g n a l m o d i f i c a t i o n and produces an odor information p a t t e r n i n which each monoosmatic component i n d i c a t e s the presence of a d i s t i n c t chemical s t r u c t u r a l f e a t u r e . Consequently the odor information p a t t e r n denotes not only a w e l l d e f i n e d odor q u a l i t y , but, by s i g n a l i n g the presence of s p e c i f i c f u n c t i o n a l groups, c h a r a c t e r i s t i c shapes and e l e c t r o n d i s t r i b u t i o n , expresses an abridged q u a l i t a t i v e a n a l y s i s o f the odorivector. Therefore i n any attempt of odor-structure c o r r e l a t i o n not the t o t a l (or o v e r a l l ) s t r u c t u r e o f the molecule should be considered but the i n d i v i d u a l c o n t r i b u t i o n s of the molecular p r o f i l e s . Perhaps t h i s could be done by a combination o f computer a s s i s t e d conformational a n a l y s i s of the o d o r i v e c t o r s which would provide i n formation about the nature o f the e x p l i c i t and implicit profiles as w e l l as the p r o b a b i l i t y of the formation of the l a t t e r , with multidimensional s c a l i n g of the h i g h l y processed information the odorivectors d e l i v e r . Furthermore the o d o r i v e c t o r s could be t r e a t e d the same way, with the same methods, as drug molecules are i n QSAR ( Q u a n t i t a t i v e S t r u c t u r e A c t i v i t y C o r r e l a t i o n ) . A computerized approach t o b i o chemical q u a n t i t a t i v e s t r u c t u r e - a c t i v i t y - c o r r e l a t i o n s was i n t r o duced by the HANSCH APPROACH ( l l ) . D e f i n i t i o n of a l l the e s s e n t i a l p r o f i l e s , those capable of being expressed i n monoosmatic components, would a f f o r d the foundation on which an algorithm f o r the c a l c u l a t i o n of odor q u a l i t y based on the chemical s t r u c t u r e of the o d o r i v e c t o r conceivably could be designed. Up t o t h i s point only speculations have been presented. They were based on the assumption that the p e r i p h e r a l process i n o l f a c t i o n i s mediated by s p e c i f i c receptor s i t e s of a group of membrane bound adenyl cyclases and that the M u l t i p l e P r o f i l e - M u l t i p l e Receptor S i t e concept i s v i a b l e . I f these assumptions are c o r r e c t the f o l l o w i n g e x t r a p o l a t i o n s could be made:


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CHEMICAL STRUCTURE


1) Adenyl c y c l a s e s are r e g u l a t o r y enzymes and themselves subject t o a l l o s t e r i c r e g u l a t i o n of t h e i r s p e c i f i c r e ceptor s i t e . Therefore i t should be p o s s i b l e t o regul a t e the a c t i v i t y , and hence the s e n s i t i v i t y of the det e c t o r subunit. 2) Regulation o f the detector system of a s p e c i f i c r e g u l a t o r y subunit would r e s u l t only i n the change of the cont r i b u t i o n of one monoosmatic component - the one whose detector s e n s i t i v i t y i s changed. Therefore the odor i n formation p a t t e r n would remain unchanged except f o r the c o n t r i b u t i o n of that s i n g l e monoosmatic component whose corresponding receptor s i t e has been r e g u l a t e d ( a c t i v a t ed or i n h i b i t e d ) . 3)

Regulation of a s i n g l e monoosmatic component could l e a d t o n o t i c e a b l e changes i n odor q u a l i t y . I n h i b i t i o n would reduce or even e l i m i n a t e the c o n t r i b u t i o n of a dominant or s i g n i f i c a n t l y modifying monoosmat i c component and thereby cause a n o t i c e a b l e antagonist i c e f f e c t . Further r e d u c t i o n of a minor monoosmatic component or i t s e l i m i n a t i o n would go i n a l l p r o b a b i l i t y undetected. A c t i v a t i o n could r a i s e the c o n t r i b u t i o n o f a minor monoosmatic component to e i t h e r modifying or dominant status and thus create a n o t i c e a b l e s y n e r g i s t i c e f f e c t . Both a n t a g o n i s t i c and s y n e r g i s t i c e f f e c t s are very common i n multicomponent o d o r i v e c t o r systems and are w e l l known t o experienced perfumers.

k) These observed s y n e r g i s t i c and a n t a g o n i s t i c e f f e c t s i n d i c a t e that the r e g u l a t o r y a c t i v i t y has t o be encoded i n an o d o r i v e c t o r present i n the mixture, i n a l l probab i l i t y i n the same way as the a c t i v a t o r s of the detect o r adenyl c y c l a s e - as an " A c t i v e P r o f i l e " In terms of e s t a b l i s h e d p r i n c i p l e s o f enzyme chemistry there i s no d i f f e r e n c e between the i n t e r a c t i o n o f a molecular p r o f i l e with i t s complementary receptor s i t e and that of an a c t i v e prof i l e with i t s corresponding complementary r e g u l a t o r y s i t e . In both cases normal l i g a n d formation through weak bonds takes place. The a c t i v e p r o f i l e can be a s p e c i f i c r e g u l a t o r y one which does not i n t e r a c t with a normal detector s i t e . Consequently i t would not produce a monoosmatic component and the r e g u l a t o r y a c t i v i t y would be independent of the i n t r i n s i c odor of the r e g u l a t o r y mol e c u l e i t s e l f . The other p o s s i b i l i t y o f course i s that the same


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molecular p r o f i l e could d i r e c t l i g a n d formation with a complementary receptor s i t e and c o n t r i b u t e i n t h i s way t o the format i o n of the corresponding monoosmatic component. Beyond that i t could d i r e c t l i g a n d formation with a r e g u l a t o r y s i t e and i n t h i s way i n t e r f e r e with the t r a n s c r i p t i o n of the structural information m o d a l i t i e s of a copresent o d o r i v e c t o r . F i n a l l y , an active profile i n an o d o r i v e c t o r could i n t e r f e r e with i t s own transcription. In a l l of these examples of r e g u l a t i o n of t r a n s c r i p t i o n of prof i l e s by a l l o s t e r i c r e g u l a t i o n o f r e c e p t o r s i t e s by a c t i v e profiles THE DUAL NATURE OF 0D0RIVECT0RS manifests i t s e l f . T h i s i s a new p r i n c i p l e p o s t u l a t e d t o be p e r t i n e n t i n a l l mixtures of odorivectors. In i t s most extended scope t h i s p r i n c i p l e s t a t e s that a l l o d o r i v e c t o r s have two f u n c t i o n s : To d i s p l a y t h e i r own i n t r i n s i c odor and at the same time act as r e g u l a t o r i n the odor perception of a copresent o d o r i v e c t o r . The l a t t e r i s achieved by a l l o s t e r i c r e g u l a t i o n i n a p e r i p h e r a l process. The Dual Nature of Odorivectors explains the observed n o n l i n e a r a d d i t i v i t y of odors. In odor mixtures the c o n t r i b u t i o n of each component i s not n e c e s s a r i l y the odor q u a l i t y i t would d i s p l a y i f presented as a s i n g l e odorant - the i n t r i n s i c odor - but an odor q u a l i t y which i s changed by the a l l o s t e r i c r e g u l a t i o n caused by a co-present o d o r i v e c t o r . The extent of t h i s change i s a funct i o n of the concentration of the r e g u l a t o r y o d o r i v e c t o r s present. The concept of the Dual Nature of Odorivectors furthermore exp l a i n s a l l observed i r r e g u l a r i t i e s , s y n e r g i s t i c and a n t a g o n i s t i c e f f e c t s at l e a s t i n part by assuming the causative processes take place at the periphery and not e x c l u s i v e l y at the CNS-level as has been g e n e r a l l y assumed so f a r . That t h i s p e r i p h e r a l i n t e r a c t i o n of o d o r i v e c t o r s i s a r e a l i t y and not j u s t a p o s t u l a t e r e s u l t i n g from lengthy speculations has been confirmed by s t a t i s t i c a l l y s i g n i f i c a n t experimental proof obtained i n malodor/"antimalodor"-interaction s t u d i e s (12), and on a more general base, i n o d o r / o d o r - i n t e r a c t i o n s . These r e s u l t s give i m p l i c i t proof that s p e c i f i c receptor s i t e s f o r molecular and a c t i v e p r o f i l e s e x i s t . The "Antimalodors" (AMALs) mentioned above were discovered i n a chance observation i n 1968 (13). In a r o u t i n e screening program of new aroma chemicals i t was found that s e v e r a l new compounds had the unique property t o suppress the perception of malodors caused by molecules which have pronounced proton donor or proton acceptor p r o p e r t i e s . The most commonly encountered malodors belong i n t h i s group: lower f a t t y a c i d s , phenols, mercaptans, amines e t c . Even more important was the observation that these "Antimalodors" produced a very s p e c i f i c counteraction effect



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which d i d not i n t e r f e r e with the p e r c e p t i o n o f a l l other odor q u a l i t i e s . On t o p o f t h a t t h e AMAL-activity was h i g h l y s p e c i f i c and r a p i d l y and t o t a l l y r e v e r s i b l e . In other words, the AMALs induced s p e c i f i c r e v e r s i b l e anosmia. L a t e r on, i n an extensive screening program i t was demonstrated t h a t the AMAL-activity o f o d o r i v e c t o r s was t o t a l l y independent o f t h e i r i n t r i n s i c odor q u a l i t i e s . Psychophysical p r e c i s i o n measurements f i n a l l y showed that at the very high d i l u t i o n s o f t h e i r a p p l i c a t i o n l e v e l s the AMALs were a l l subthreshold. A l l these observations combined make i t obvious t h a t we had i n deed a r e g u l a t o r y i n t e r a c t i o n and not one o f t h e t r a d i t i o n a l malodor counteractions such as simple overpowering or masking. T h i s view i s supported by t h e f a c t t h a t t h e antimalodors have no s t r u c t u r a l s i m i l a r i t y t o most o f the common malodors but present "normal" aroma chemical types, with molecular weights w e l l above 100 Daltons and d i f f e r e n t degrees o f p o l a r i t y . In c o n t r a s t t h e malodors - proton donors or proton acceptors with no exception - are a l l small molecules with molecular weights w e l l below 100 Daltons, they are h i g h l y p o l a r compounds and have l i t t l e o r no s t e r i c requirements. They share no s t r u c t u r a l features (Methyl mercaptan - t r i m e t h y l amine - i s o b u t y r i c a c i d - phenol) and a l l are strong i r r i t a n t s . These observations l e a d t o two very important a.

conclusions:

The oberved i n h i b i t i o n o f malodor p e r c e p t i o n cannot be caused by competitive i n h i b i t i o n . In such a mechanism t h e AMAL-molecule would block t h e common recept o r s i t e f o r a l l p r o t o t r o p i c malodors and the s t e r i c requirements f o r t h e malodors and t h e AMALs would have t o be very s i m i l a r i n order t o make l i g a n d f o r mation o f both types with t h e same r e c e p t o r s i t e poss i b l e . As has been pointed out already e x a c t l y t h e opposite i s t h e case: AMALs with t h e i r sterically w e l l d e f i n e d a c t i v e p r o f i l e s r e q u i r e s t e r i c a l l y equall y w e l l d e f i n e d r e c e p t o r s i t e s f o r l i g a n d formation, whereas t h e r e c e p t o r s i t e f o r the common e n t i t y o f a l l malodors has no s t e r i c a l requirements at a l l . This p o i n t s t o a l l o s t e r i c r e g u l a t i o n o f the c r i t i c a l receptor s i t e common t o a l l malodors. The a l l o s t e r i c s i t e undergoes l i g a n d formation with t h e AMAL a c t i v e p r o f i l e . T h i s process causes changes i n t h e conformat i o n o f t h e r e g u l a t o r y subunit o f the detector enzyme which a l t e r s the o v e r a l l geometry o f the s t e r i c a l ly indifferent c r i t i c a l r e c e p t o r s i t e t o such an extent t h a t i t s a c t i v i t y f o r l i g a n d formation with malodors i s decreased or t o t a l l y i n h i b i t e d . Consequent-



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l y , i f the AMAL reaches i t s r e g u l a t o r y receptor site p r i o r t o the a r r i v a l o f a malodor molecule at t h e c r i t i c a l receptor s i t e t h e ensuing conformational change o f the l a t t e r makes l i g a n d formation d i f f i c u l t or impossible, t h e t r a n s d u c t i o n process i s slowed down or t o t a l l y i n h i b i t e d . Consequently t h e c o n t r i bution o f the monoosmatic component common t o a l l malodors i s reduced o r t o t a l l y e l i m i n a t e d . As a r e s u l t the malodor cannot be p e r c e i v e d i n i t s o r i g i n a l i n t e n s i t y or not at a l l . b.

The receptor s i t e common t o a l l p r o t o t r o p i c malodors - the " c r i t i c a l " receptor s i t e - then has t o have t h e a b i l i t y t o recognise the presence or absence o f a proton donating group (-C00H, Ar-OH, R-SH, e l e c t r o p h i l e s ) or a proton accepting group (RR'R ^, nucleop h i l e ) . Several s t r u c t u r e s which would have t h i s prop e r t y and can be assembled from f u n c t i o n a l groups common i n p r o t e i n s , such as c a r b o x y l - , mercapto- or p r i mary amino groups. A "Reinforced i o n i c bond" formed from a c a r b o x y l - and a primary amino group would give through formation o f two hydrogen bonds between two hydrogens on n i t r o g e n and the two oxygens i n the carboxylate anion a resonance s t a b i l i s e d s i x membered r i n g system. A proton donor would donate a proton t o t h e r i n g system which would then open t o give an ammonium c a r b o x y l i c a c i d ; whereas a proton acceptor would a c cept a proton and break the resonance s t a b i l i s e d s i x membered r i n g t o g i v e an amino carboxylate anion. Since both f u n c t i o n a l groups can be part o f d i s t a n t amino a c i d s brought i n t o proximity i n t h e t e r t i a r y p r o t e i n s t r u c t u r e formation o f a resonance s t a b i l i s e d reinforced i o n i c bond could s t a b i l i z e one conformat i o n and i t s r i n g f i s s i o n could b r i n g about profound conformational changes. 1

The f i n a l unequivocal experimental proof that the observed e f f e c t s were indeed p e r i p h e r a l ones was obtained i n psychophysical experiments. T e r t i a r y b u t y l mercaptan was used as t h e t a r g e t i n a monorhinal p r e s e n t a t i o n . I t s p e r c e i v e d odor i n t e n s i t y remained unchanged when i n a d i c h o r h i n a l experiment the c o n t r a l a t e r a l nar i s was exposed t o a very low i n t e n s i t y o f U-cyclohexyl-U-methyl2-pentanone (CMP, 13). However, i n agreement with e s t a b l i s h e d crossover a d d i t i v i t y , the t o t a l p e r c e i v e d o v e r a l l odor i n t e n s i t y showed a small, but s t a t i s t i c a l l y s i g n i f i c a n t i n c r e a s e . Then t h e two separate odorant streams o f the d i c h o r h i n a l experiment were combined and t h e mixture o f malodor ( t . - b u t y l mercaptan) and AMAL (CMP) presented t o the subjects again. Perceived o v e r a l l i n t e n s i t y was reduced by lh% and perceived malodor i n t e n s i t y by &5% at a s i g n i f i c a n c e l e v e l o f 5%.


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S i m i l a r experiments with other malodor/AMAL-combinations gave t h e same r e s u l t s . For example when i s o v a l e r i c a c i d and CMP were used p e r c e i v e d o v e r a l l i n t e n s i t y i n the monorhinal p r e s e n t a t i o n o f t h e mixture was reduced by 6k% i n comparison with t h e d i c h o r h i n a l pres e n t a t i o n ; and perceived malodor i n t e n s i t y by 96%. When l i n a l y l acetate was used as t h e t a r g e t i n p l a c e o f the p r o t o t r o p i c malodors i n t h e same experimental p r o t o c o l no d i f f e r e n c e between monorhinal p r e s e n t a t i o n o f t h e t a r g e t and mono- and dichor h i n a l p r e s e n t a t i o n o f t a r g e t and AMAL (CMP) was observed. These r e s u l t s cannot be explained with any o f t h e o l d e r t h e o r i e s o f o l f a c t i o n whereas the Enzyme Model o f O l f a c t i o n not only can do t h a t e f f o r t l e s s l y , but a c t u a l l y allows t o p r e d i c t these e f f e c t s on the b a s i s o f g e n e r a l l y accepted p r i n c i p l e s o f molecular b i o chemistry. The concept o f "STRUCTURE RECOGNITION AS PERIPHERAL PROCESS IN ODOR QUALITY CODING" represents only t h e s p e c i a l a p p l i c a t i o n o f a more general mechanism o f s t r u c t u r e r e c o g n i t i o n i n p e r i p h e r a l processes t o t h e problems o f q u a l i t y coding i n o l f a c tion.

Acknowledgement The author g r a t e f u l l y acknowledges the support by colleagues and management o f MONSANTO FLAVOR/ESSENCE from 1968 - 1978. The r e search i n i t i a t e d at M F/E i s now being continuated by Bush Boake A l l e n , Inc, Montvale, NJ.

Abstract Interaction of odorivectors with receptors leading to signal generation and subsequent formation of an odor information pattern composed of a limited number of monoosmatic components can be v i sualized to proceed by either an integral or differential process. The integral process is molecular: The total odorivector molecule is involved in a single interaction which triggers a transduction process capable of producing a multicomponent information pattern. The differential process is based on a multiple profile/multiple receptor site mechanism: Many odorivector molecules interact independently through different submolecular profiles with complementary specific receptors in profile specific transduction processes, each of which leads to formation of a specific monoosmatic component of the final odor information pattern. In this mechanism therefore specific regulation of formation of monoosmatic components should be possible and should lead to distinct changes in perceived odor quality caused by the resulting selective synergistic and antagonistic effects. Implications of the concept of the differential process and experimental results of selective specific antagonistic effects are discussed in this communication.


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Literature Cited 1. BEETS, M. G. J. "Structure-Activity Relationships in Human Chemoreception"; Applied Sciences Publishers Ltd.: Barking, Essex, England, 1978. 2. AMOORE, J. E. "Molecular Basis of Odor": American Lecture Series, Publication No. 773; Ch. C. Thomas, Springfield, Illinois; 1970. 3. OHLOFF, G. "Relationship between Odor Sensation and Stereochemistry of Decalin Ring Compounds" in G. OHLOFF and A. F. THOMAS, Eds. "Gustation and Olfaction"; Academic Press New York-London; 1971; p. 178 4. POLAK, E. H., J.Theor.Biol. 1973, 40, 469 5. KOSHLAND Jr., D. E . , Ann.Rev.Biochemistry, 1968, 37, 359 6. BEETS, M. G. J., "Molecular Approach to Olfaction" in: E. J. ARIENS, Ed., "Molecular Pharmacology"; Academic Press New York-London; 1964, Vol. II, p. 1 7. RUZICKA, L . , Chemikerztg. 1920, 44:19, 129 8. CAIN, W. S.; MURPHY, C. L . , Nature, 1980, 284, 255 9. ARIENS, E. J., Arch.int.pharmacodyn. 1954, 99:1, 32 10. POLAK, E. H., J.Am.Oil Chem.Soc., 1968; 45, 680A 11. GOULD, Robert F., Ed. "Biological Correlations - the Hansch Approach", Advances in Chemistry Series 114; American Chemical Society, Washington, D.C.; 1972 12. SCHLEPPNIK, Alfred A., to be published 13. SCHLEPPNIK, Alfred A., VANATA, S. G., U.S. 4,009,253, Feb. 22, 1977 RECEIVED

October 29, 1980.


Odor Quality and Chemical Structure - ACS Publications - American

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