CARBOHYDRATES, CHITIN AND CUTIN 



309 



0.012 



0.010 



0.008 



0.006 



0.004 



0.002 







-0.002 



-0.004 



137 141 145 149 153 1.57 1.61 IdSn^ 



xylene, iodobenzene in alcohol). From this he concludes that the 

 source of the negative birefringence is not natural chitin, but chitin 

 chancred chemically by, say, potassium mercuric iodide. This con- 

 clusion is, however, incorrect, for Diehl and Van Iterson (1955) 

 found with mixtures of glycerol and quinoline, and Schmidt (1936) 

 with a-monobromo-naphthalene (mixed 

 with xylene) negative minima of the 

 rodlet birefringence curve, even though 

 these curves are not identical for various 

 imbibition mixtures (Fig. 1 5 1). What was 

 demonstrated in cellulose (Frey-Wyss- 

 LiNG, 1936b) probably applies here, viz., 

 that the difference in the adsorptive 

 power of the micellar frame with respect 

 to the components of the imbibition li- 

 quid is responsible for the displacement of 

 the curves. From the data now available, 

 therefore, it may confidently be asserted 

 that the submicroscopic chitin rodlets 

 have a negative intrinsic birefringence. 



Cellulose likewise becomes optically 

 negative by nitration and complete acetyl- 



ation (triacetyl cellulose), i.e., by the esterification of the polar OH- 

 groups. It may therefore reasonably be presumed that it is the acetyl 

 side chains of the chitin which cause the negative birefringence. It is 

 nevertheless a curious fact that for chitin only one acetyl group per 

 glucose residue is required for this, whereas three are necessary in 

 cellulose; presumably, therefore, the amino group of the glucosamine 

 also tends to produce negative birefringence. 



The negative intrinsic birefringence of chitin does not hamper the 

 approach to the micellar texture of chitinous composite bodies by 

 polarizing optics if the imbibition agents used are Hquids whose 

 refractive index is below 1.48, i.e., water or glycerol. It should, 

 however, be realized that the determination of the micellar orientation 

 does not then take place on the basis of the positive intrinsic aniso- 

 tropy of the submicroscopic frame of the membrane, as in cellulose, 

 but rests on the positive rodlet birefringence of the chitin skeleton. 

 Another interesting fact has been discovered based on the optical 



Fig. 15 1. Rodlet birefringence curve 

 of chitin sinews (from Diehl 

 and Van Iterson, 1935); A with 

 quinoline-glycerol, B with mer- 

 curic iodide of potassium. Ab- 

 scissa: Refractive index n^ of the 

 imbibition liquid. Ordinate: Bi- 

 refringence n^-no. 



