RAMAN TENSORS FOR THE TRYPTOPHAN SIDE-CHAIN IN PROTEINS DETERMINED BYPOLARIZED RAMAN MICROSPECTROSCOPY OF ORIENTED N-ACETYL-L-TRYPTOPHAN CRYSTALS

Polarized Raman spectra have been obtained from oriented single crysta ls of N-acetyl-L-tryptophan by use of a Raman microscope and 488.0 nm argon excitation. The crystal, of orthorhombic space group P2(1)2(1)2( 1),21, provides the relative Raman intensities I-aa, I-bb and I-cc cor responding to the aa, bb and cc components of the crystal Raman tenser . The polarized Raman spectra of the crystal have been combined with d epolarization ratios from solution spectra of randomly oriented N-acet yl-L-tryptophan and L-tryptophan to yield Raman tensors for each of th e following vibrational normal modes of the indole moiety: N-1-H stret ch (approximate to 3416 cm(-1)), W1 (approximate to 1617 cm(-1)), W2 ( approximate to 1576 cm(-1)), W3 (approximate to 1557 cm(-1)), W4 (appr oximate to 1487 cm(-1)), W5 (approximate to 1458 cm(-1)), W6(approxima te to 1424 cm(-1)), W7(approximate to 1357 cm(-1)), W7' (approximate t o 1332 cm(-1)), W16(approximate to 1010 cm-l) and W18 (approximate to 757 cm(-1)). These Raman tensors determined for the tryptophan residue in N-acetyl-L-tryptophan are proposed as being transferable to trypto phan side chains in proteins. A knowledge of Raman tensors for the try ptophan side chain should facilitate the determination of indole ring orientation in biological complexes amenable to investigation by the m ethod of polarized Raman microspectroscopy.