Effects of hydrogen bonding and temperature upon the near ultraviolet circular dichroism and absorption spectra of tyrosine and O-methyl tyrosine derivatives.

To gain information about the properties of tyrosyl residues buried within proteins, the circular dichroism (CD) and absorption spectra of tyrosine derivatives have been investigated in nonpolar solvents. N-Stearyl-L-tyrosine n-hexyl ester dissolved in methylcyclohexane (O-O band at 283 nm) was used to measure the effects of hydrogen-bonding agents. Adding low concentrations of dioxane, N, N-dimethylacetamide, 1-butanol, or methanol causes a lto 4-nm red shift in the absorption spectrum and a 10 to 25% increase in the dipole strength. The results with N, N-dimethylacetamide suggest that a hydrogen bond between a tyrosyl hydroxy group and a carbonyl oxygen of the peptide backbone may be one mechanism for producing a large red shift in proteins. CD spectra were recorded after the hydroxy group of N-stearyl-L-tyrosine n-hexyl ester had been hydrogen bonded. Dioxane and N, N-dimethylacetamide cause the CD spectra to red shift and intensify to the same extent as do the absorption spectra. Evidently hydrogen bonding to these compounds does not alter the conformation of this tyrosine derivative. In contrast, hydrogen bonding of N-stearyl-L-tyrosine n-hexyl ester to butanol or methanol causes a 50% loss of rotatory strength, suggesting an altered conformation. The dependence upon alcohol concentration is the same for both the CD and absorption alterations (halfmaximal effect at 30 mM alcohol). Evidence is presented