Circular dichroism of modified tryptophan residues β-3-oxindolyl- l-alanine

Abstract

1. 1. The near-ultraviolet circular dichroism (CD) spectra of β-3-oxindolyl- l-alanine (β-[3-(2-indolinone)]- l-alanine) and α-N- acetyl-β-3- oxindolyl- l-alanine ethyl ester have been studied. The longest wavelength CD band is centered at 280–285 nm. Another band occurs at 245–254 nm. In the case of β-3-oxindolyl- l-alanine, the intensities of these CD bands are affected by pH; both bands are positive at pH 1.5 and negative at pH 7. α-N- Acetyl-β-3- oxindolyl- l- alanine ethyl ester has negative CD bands in a variety of solvents ( Δε M about −3 at 252 nm and about −0.4 M −1 · cm −1 at 283 nm). 2. 2. The CD spectra anticipated from tryptophan oxidized in a protein are discussed. Depending upon which tautomeric form of oxindole is produced, the modified tryptophanyl side chain may exhibit one of several different types of CD spectra. In all cases, the modified side chain may have CD bands that overlap the naturally occurring tryptophanyl CD bands.