Raman tensors for the tryptophan side chain in proteins determined by polarized Raman microspectroscopy of oriented N-acetyl-l-tryptophan crystals

Abstract

Polarized Raman spectra have been obtained from oriented single crystals of N-acetyl-l-tryptophan by use of a Raman microscope and 488.0 nm argon excitation. The crystal, of orthorhombic space group P2 12 12 1, provides the relative Raman intensities I aa , I bb and I cc corresponding to the aa, bb and cc components of the crystal Raman tensor. The polarized Raman spectra of the crystal have been combined with depolarization ratios from solution spectra of randomly oriented N-acetyl-l-tryptophan and l-tryptophan to yield Raman tensors for each of the following vibrational normal modes of the indole moiety: N 1H stretch (≈3416 cm −1), WI (≈1617 cm −1), W2 (≈1576 cm −1), W3 (≈1557 cm −1), W4 (≈1487 cm −1), W5 (≈1458 cm −1), W6 (≈1424 cm −1), W7 (≈1357 cm −1), W7′ (≈1332 cm −1), W16 (≈1010 cm −1) and W18 (≈757 cm −1). These Raman tensors determined for the tryptophan residue in N-acetyl-l-tryptophan are proposed as being transferable to tryptophan side chains in proteins. A knowledge of Raman tensors for the tryptophan side chain should facilitate the determination of indole ring orientation in biological complexes amenable to investigation by the method of polarized Raman microspectroscopy.