title: On the orange color of Z-Trp-ONPo.
abstract: Out of all nitrophenyl esters of N(alpha)-protected alpha-amino acids Z-Trp-ONP(o) 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-ONP(o) 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-ONP(o) concentrations by use of vis absorption and proton magnetic resonance spectroscopic techniques. The onset of the orange color correlates closely with the appearance of a concentration-dependent absorption band near 500 nm and concentration-dependent shifts of the urethane and indole NH proton resonances. Our observations can be explained by the formation of an intermolecular charge transfer complex involving the Trp indole and the -ONP(o) nitrophenyl as the donor and the acceptor moieties, respectively.
authors.affiliation: Institute of Biomolecular Chemistry, CNR, Department of Chemistry, University of Padova, 35131 Padova, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Moretto
authors.forename: F
authors.initials: F
authors.lastname: Formaggio
authors.forename: M
authors.initials: M
authors.lastname: Crisma
authors.forename: B
authors.initials: B
authors.lastname: Kaptein
authors.forename: Q B
authors.initials: QB
authors.lastname: Broxterman
authors.forename: C
authors.initials: C
authors.lastname: Toniolo
mesh_terms: D003116:Color; D003460:Crystallization; D018360:Crystallography, X-Ray; D008968:Molecular Conformation; D009578:Nitrobenzenes / Q000737:chemistry*; D012996:Solutions; D013057:Spectrum Analysis; D055672:Static Electricity; D014364:Tryptophan / Q000031:analogs & derivatives* / Q000737:chemistry*
chemical_list: D009578:Nitrobenzenes; D012996:Solutions; C502273:benzyloxycarbonyl-tryptophan 2-nitrophenoxy ester; D014364:Tryptophan