FINE STRUCTURE IN NEAR-ULTRAVIOLET CIRCULAR DICHROISM AND ABSORPTION SPECTRA OF TRYPTOPHAN DERIVATIVES AND CHYMOTRYPSINOGEN A AT 77 DEGREES K

To aid in identifying tryptophanyl circular dichroism bands in proteins, the circular dichroism spectra of model compounds have been characterized, using low temperature to enhance circular dichroism fine structure. The circular dichroism spectra of N-acetyl-L-tryptophanamide and seven other low molecular weight compounds may be grouped into four classes: (1) 1Lb bands intense, (2) 1La bands intense, (3) both 1La and 1Lb bands intense, and (4) fine structure whose origin was not readily identified. Both the 0-0 and 0 + 850 cm-1 1Lb transitions occur together and have the same circular dichroism sign. A number of 1La transitions could be identified, but their relative intensities varied greatly. In chymotrypsinogen A, the tryptophanyl circular dichroism bands arise mainly from 1La transitions. The results of these studies indicate that in proteins the circular dichroism fine structure occurring between 290 and 305 mμ, is most readily used to identify tryptophanyl circular dichroism bands. The effect of temperature upon the rotatory strength provides information about the interconversions among different conformations. In N-acetyl-L-tryptophanamide the rotatory strength at 77°K is 18 times greater than that at 298°K, which appears to indicate a high degree of conformational mobility at 298°K. In contrast, chymotrypsinogen shows only a 25% intensification upon cooling. Apparently the tryptophan residues in chymotrypsinogen have relatively rigid positions at 298°K, but a minor degree of mobility may exist. © 1969, American Chemical Society. All rights reserved.