XX CONTENTS 



VI. Carotenoids and Related Compounds (continued) 



(3) Echinenone 591 



(4) Other Animal Carotenoids 592 



a. Sulcatoxanthin 592 



b. Pentaxanthin 592 



c. Pectenoxanthin 592 



d. Cynthiaxanthin 593 



5. Structure and Occurrence of Carotenoids Having Less Than Forty 



Carbon Atoms 593 



( 1 ) Bixin and Related Compounds 593 



a. Labile and Stable Bixins 594 



b. Norbixins 596 



c. Bixin Dimethyl Esters 597 



(2) Crocetin 597 



a. Crocetins 598 



b. Dimethylcrocetins 599 



c. Occurrence 599 



d. Related Compounds 600 



(a) Crocin 600 



(3) Azafrin 600 



a. Structure 601 



b. Occurrence 601 



c. Related Compounds 601 



(a) Azafrin Methyl Ester 601 



(b) Azafrinone 602 



6 . Properties of the Carotenoids 602 



(1) Melting Points and Optical Rotation 602 



(2) Spectral Absorption 602 



(3) Solubility 613 



a. Separation of the Carotenes from the Xanthophylls 613 



b. Solubilities of Individual Carotenoids 620 



c. Colloidal Suspensions of Carotene 620 



(4) Stereochemical Behavior of the Carotenoids 621 



a. Introduction 621 



b. Nomenclature 622 



c. Configuration of Natural Carotenoids 623 



d. Possibilities for Steric Changes in the Polyenes 625 



e. Methods of Producing Stereoisomeric Changes 629 



(a) CIS-trans Isomerization Produced by Melting Crystalline 



Carotenoids 632 



(b) Thermal cis-trans Isomerization in Solution 632 



(c) cis-trans Isomerization Produced by Iodine Catalysis at 



Room Temperature 635 



(d) cis-trans Isomerization Produced by Acid Catalysis 636 



(e) cis-trans Isomerization Produced by Photochemical Ac- 



tion 637 



f . Properties of Carotenoids Containing cis Bonds 638 



(a) General Properties 638 



(b) Spectral Characteristics as Affected by trans-cis Iso- 



merism 638 



(c) The cis- Peak Effect 644 



