The fluorescence of a protein is due to tryptophan, tyrosine and phenylalanine (assuming the proteins doesn’t contain a separate fluorescent prosthetic group). Iodide ions are known to quench the fluorescence of tryptophan. If a protein is known to contain only one tryptophan residue and iodide fails to quench its fluorescence, what can you conclude about the location of the tryptophan?