A LONG LIFETIME COMPONENT IN THE TRYPTOPHAN FLUORESCENCE OF SOME PROTEINS

The tryptophan fluorescence of two membrane proteins (outer membrane p rotein A and lactose permease), a 21-residue hydrophobic peptide, thre e soluble proteins (rat serum albumin, ribonuclease T-1, and azurin), and N-acetyltryptophanamide (NATA) was investigated by lime-resolved m easurements extended over 65 ns. A long lifetime component with a char acteristic time of 25 ns and an amplitude below 1% was found for outer membrane protein A, lactose permease, the peptide in lipid membranes, and azurin in water, but not for rat serum albumin, ribonuclease T-1, and NATA in water. When outer membrane protein A was dissolved and un folded in guanidinum hydrochloride, the long lifetime component disapp eared. Hence, a hydrophobic environment seems to be a necessary requir ement for the long lifetime component to be present. However, NATA dis solved in butanol does not exhibit the long lifetime component, while the peptide dissolved in the same solvent under conditions which prese rve its helical structure does show the long lifetime. Thus, a regular secondary structure for the polypetide chain to which the tryptophan residue belongs seems to be a second necessary requirement for the lon g lifetime component to be present. The long lifetime component may th erefore be seen in the context of protein substates.