Study of the Triplet State Properties of Tyrosines and Tryptophan in Azurins Using Optically Detected Magnetic Resonance

Optically detected magnetic resonance (ODMR) signals and phosphorescence spectra were seen of tyrosine in the P. aeruginosa and tryptophanless P.fluorescens azurins and of tryptophan in the former. This confirmed a conclusion from other experiments that the tryptophan of P. aeruginosa cannot effectively quench the singlet energy of both tyrosines. The ODMR signals were all very narrow, additional evidence that the chromophores are buried in the interior of the protein. Accurate values of the zero-field coupling constants D and E lead to a tentative correlation of D values with the red shift of the 0 → 0 peak of the phosphorescence spectrum. The environment of tryptophan in P. aeruginosa is the most hydrocarbon like of any tryptophan so far observed. The experiments raise a number of unanswered questions concerning rate processes. The intensities of the |2E| transition of tyrosine and the phosphorescence of both tyrosine and tryptophan are substantially reduced when the copper is oxidized. Nevertheless the phosphorescence lifetimes are unaffected. A hole cannot be burned in the ODMR resonances. The homogeneously broadened lines may conceivably be a result of low-temperature proton tunnelling.