C. J. p. SPRUIT AND A. SPRUIT-VAN DER BURG 117 



conclusion that has received considerable support from the work of 

 Spruit and Schuiling ( 1945 ) . So we know at least one chemical prop- 

 erty of this compound which is in agreement with the chemical struc- 

 ture of the naphthoquinones. 



It may be argued that although certain derivatives of naphtho- 

 hydroquinone possess absorption spectra quite similar to van der Kerk's 

 inactivation spectrum, it might well be that other unrelated classes of 

 compounds exist with similar spectral properties. During later years 

 we have had opportunity to study the ultraviolet absorption spectra 

 of various compounds in more detail, and the results of this work 

 make it appear justifiable to reconsider the matter of an identification 

 of the bacterial luciferin. We should like to present some of the 

 relevant material here. 



How typically characteristic is the absorption spectrum mentioned 

 of this particular class of compounds? To test this, one can investigate 

 the influence upon the absorption spectrum of alterations in the sub- 

 stituents attached to the naphthalene nucleus. The second method is 

 to change the character of the aromatic nucleus itself. If we take 

 naphthyl-2-methylketones as a starting point, it is clear from theoreti- 

 cal considerations that substitution in the methyl group should not 

 alter the absorption spectrum profoundly. This is confirmed by the 

 experiments (Spruit, 1949c). On the other hand, keeping in mind 

 that the long-wavelength absorption band in the inactivation spec- 

 trum is somewhat more to the red than the corresponding band in 

 the naphthohydroquinone compound, we are interested in the effects 

 of other substituents in the nucleus. Figure 12 gives examples of what 

 happens if one shifts around with the OH groups. The result demon- 

 strates that the exact position of the long-wavelength band should not 

 pose much of a problem. Further examples can be found elsewhere 

 (Spruit, 1949b). 



Quite another problem is posed by the nature of the aromatic nu- 

 cleus. As far as the quinones are concerned, the absorption spectra of 

 anthraquinones are very similar to or almost identical with those of 

 the analogous naphthoquinones. The quinone nucleus forms an effec- 

 tive barrier between the conjugated systems on both sides of it. This 

 observation is amply confinned by the spectra of a number of other 

 tricyclic paraquinones containing heterocyclic ring systems (Spruit, 



