Influence of tyrosine on the electronic circular dichroism of helical peptides

Studies of short helical peptides can provide insights into amino acid interactions in proteins and their role in protein folding. A common method of estimating helical structure is electronic circular dichroism (CD) in the far ultraviolet. It is known that aromatic side chains such as tyrosine may influence CD in this region, but this residue has desirable fluorescent proper-ties and is nevertheless often incorporated into peptides. To investigate the relationship between the conformation of tyrosine and its contribution to the CD, we have calculated the CD of some short alanine-based helical peptides from first principles based on molecular dynamics simulations. The calculations are complemented by some analysis of static models. These theoretical studies estimate that a tyrosine residue may contribute up to +/-5000 deg cm(2) dmol(-1) to the mean residue ellipticity at 220 nm, with the most typical value being 1000 deg cm 2 dmol(-1). If uncorrected, this would lead to an underestimate of the helicity of the peptide of 5-20%. For a tyrosine side chain in the trans rotameric state, there is a discernible relationship between its precise orientation and its contribution to CD at 220 nm. Experiments on tyrosine-containing peptides using CD should be interpreted bearing these findings in mind and, preferably, with an independent approach for probing the conformational states of tyrosine.