i66 



Plant Pigments 



[March, 



has prepared products from melletic and pyromelletic acids, which 

 he cannot under any circumstances regard as quinonic in structure. 

 It is worthy of note, in this connection, that such simple Com- 

 pounds as quinoneimine, O = CqR^ — NH, and quinonediimine, 

 HN = C6H4 = NH, are colorless. Lately Willstätter^^ has suc- 

 ceeded in isolating colorless ortho-quinones. To differentiate these 

 from the isomeric orange variety, the Superoxid f ormula, 



rv? 



'\/-o 



has been assigned to them. This also serves the purpose of em- 

 phasizing the absence of the chromophoric group.^^ 



Chemical interrelationships.^^ General observations. Un- 

 til recently the chemistry of plant pigments had not become a subject 

 for systematic research. Thus far only a "few Standard types of 

 such pigments have been fairly well identified, the great mass of 



11 Willstätter : Ber. d. d. ehem. Gesell, 1904, xxxvii, p. 4744. 



12 Many interesting problems of a more specific nature were taken up, 

 among others, by Hantzsch (Ber. d. d. ehem. Gesell, 1906, xxxix, p. 1073) in his 

 study of the colored nitro-phenol ethers, and his subsequent development of 

 " chromo-isomerism " (isomerism exhibited in change of color) ; Willstätter 

 {Ber. d. d. ehem. Gesell, 1908, xli, p. 1465) and Hewitt {Trans. Chem. Soc., 1907, 

 xci, p. 1251; Zeit. Physik. Chem., 1900, xxxiv, p. l), who has attempted to har- 

 monize his theory of fluorescence with that of color. " Symmetrical Compounds," 

 he says, " capable of equal tautomeric displacements in either of two directions, 

 should be those to exhibit the phenomena of fluorescence, for the molecule 

 would Swing between the two extreme positions like a pendulum, the energy 

 absorbed of one wave-length being degraded and given out with slower fre- 

 quency." (Thorpe, Die, Applied Chem., 1912, ii, p. 59.) Thus, in fluorescein, 

 we have : 



o 



o 



o 



0= 



/\/\/\0H _ HO|/\/\/^OH __ HO/\/\f^ 



\A/\/ 



c 



I 



C6H4— COOK 



c o 



I I 



C6H4— CO 



=0 



c 



C6H4— COOK 



This brings fluorescence in relationship to color, the quinonoid structures being 

 the fluorescent substances. 



13 For a detailed description of individual plant pigments see West : Biochem. 

 Bull., 1915, iv, p. 151 (preceding paper). 



