CopC, a protein involved in copper resistance, is essentially constituted by two sheets forming a Greek key beta barrel motif. The aromatic ring of Trp83, sandwiched between the two beta sheets, has numerous contacts with residues in strands beta and stabilizes the protein fold. In the paper Trp83 was mutated to Leu to study the effect of this mutation on CopC by means of fluorescence spectra and UV spectra. The experiments indicate that the mutation bind Cu(2+) with a decreased formation constant of 3.95 x 10(11) M(-1) in 20 mM PB buffer at pH 7.0; mutagenesis make hydrophobic region to be exposed to an extent. Compared with the wild, thermal stability of the mutant was shown to decrease by stronger fluorescence of TNS at 80 degrees C. The important role of aromatic residue in structure is exhibited.