On the orange color of Z-Trp-ONPo

Out of all nitrophenyl esters of N-alpha-protected alpha-amino acids Z-Trp-ONPo is the only one which is deep orange colored in the crystalline state. Any change in N-alpha-protection, nature of amino acid, spatial separation between Trp and the ester-group or position of the nitro-substitutent in the aromatic ring of the ester function results in a loss of this characteristic property. We solved the molecular and crystal structure of Z-L-Trp-ONPo by X-ray diffraction analysis and investigated its color changes and visible (vis) and infrared (IR) absorption spectra in the solid state as a function of the amino acid derivative/KBr (w/w) ratio in the pellets. This investigation was extended to toluene solutions of different Z-Trp-ONPo concentrations by use of vis absorption and proton magnetic resonance spectroscopic techniques.