Temperature dependent 2nd derivative absorbance spectroscopy of aromatic amino acids as a probe of protein dynamics

Proteins display a broad peak in 250-300 nm region of their UV spectrum containing multiple overlapping bands arising from the aromatic rings of phenylalanine, tyrosine, and tryptophan residues. Employing high resolution 2(nd) derivative absorbance spectroscopy, these overlapping absorption bands can be highly resolved and therefore provide a very sensitive measure of changes in the local microenvironment of the aromatic side chains. This has traditionally been used to detect both subtle and dramatic (i.e., unfolding) conformational alterations of proteins. Herein, we show that plots of the temperature dependent 2(nd) derivative peak positions of aromatic residues have measurable slopes before protein unfolding and that these slopes are sensitive to the dielectric properties of the surrounding microenvironment. We further demonstrate that these slopes correlate with hydration of the buried aromatic residues in protein cores and can therefore be used as qualitative probes of protein dynamics.