400 PHOTOMECHANICAL CONSIDERATIONS 



other ions used hitherto), while a partial (20 per cent) reversibility 

 of the bleaching could be demonstrated by addition of ascorbic acid or 

 sodium dithionite. 



In solutions other than alcoholic ones a change analogous to the one 

 represented by Fig. 1-^Fig, 2 was not observed with bacteriochloro- 

 phyll. Dilution of a concentrated methanolic bacterio chlorophyll solu- 

 tion with water yields a colloidal nonfluorescent solution. The near- 

 infrared spectrum of such a solution may show a complex structure 

 (cf. e.g., Komen, 4) under favourable conditions (crude pigment 

 solution, quick dilution). Addition of oxidants affects the two bands in 

 a different way (Fig, 3). 



400 500 600 700 800 900 mu 



Fig. 3. Absorption spectrum and reversible bleaching of colloidal bacterio- 

 chlorophyll, formed by addition of water to a concentrated methanolic pigment 

 solution. The absorption band with maximum around 785 mju is bleached re- 

 versibly to some extent; the band around 845 mju is very unstable. It also dis- 

 appeared upon a slight heating or illumination. 



Reversible oxidation by addition of oxidising and reducing ions can 

 also be demonstrated with in vivo bacteriochlorophyll. In Fig. 4, re- 

 versible oxidation of Rho do spirillum r?/6r?/;;/ chromatophores is shown. 

 The weak absorption band at about 800 m/i is not bleached reversibly, 

 but shows a marked stability against oxidation. Its position, however, 

 is reversibly shifted about 5 m/i towards the blue (cf. Goedhccr, 5). 

 This band disappears completely at a pH below 4 (cf. Thomas, Goed- 

 heer, Komen, 6). 



A prerequisite for the occurrence of reversible oxidation is the 

 presence of the central magnesium atom. No reversible oxidation could 

 be demonstrated with bacteriopheophytin. 



