The individual tyrosines of proteins: their spectra may or may not differ from those in water or other solvents

The overall derivative spectrum of a protein is the sum of the individual d erivative spectra just as the overall ultraviolet spectrum of a protein is the sum of its component parts, The RNase and DNA binding protein Sso7d has two tyrosines and one tryptophan. We used two mutant forms of the protein to show that the individual aromatics contribute derivative spectra that ca n be explained on the basis of their environments. We used mutant forms of iso-l-cytochrome c to estimate the contributions of the single tryptophan a nd three of the five tyrosines to the overall derivative spectrum. The tryp tophan spectrum is not exceptional. The comparable tyrosine spectra are mor e complex. The derivative spectrum of individual tyrosines does not corresp ond to that expected on the basis of concentration. This is a reflection of two factors: (1) the extent to which mutations are sensed distally through the introduction and compression of packing defects; and (2) the extent to which electronic transitions of tyrosine are influenced by nearby atoms. T his influence could take the form of tyrosine residing in an area where the dielectric coefficient is not uniform; it could also result from tyrosine bumping into neighboring atoms with lower frequency than it does in solutio n, (C) 1999 Elsevier Science B.V, All rights reserved.