A study on the positive ellipticity in the circular dichroism of ribonuclease A

The small positive ellipticity near 239 nm in the CD spectrum of RNase has been investigated as a function of pH. Theoretical calculations using CD parameters representing buried or exposed tyrosine residues have been carried out. A comparison of the theoretical calculations with experimental data suggests that the changes in the band's intensity, as a function of pH, arise mainly from electronic transitions associated with the tyrosine residues. The buried tyrosine residues are the major contributors to the ellipticity in this region at neutral pH. At higher pH contributions from exposed residues are also observed.