**Sara Signorelli, Salvatore Cannistraro \* and Anna Rita Bizzarri**

Biophysics & Nanoscience Centre, DEB, Università della Tuscia, 01100 Viterbo, Italy; signorellis@unitus.it (S.S.); bizzarri@unitus.it (A.R.B.)

**\*** Correspondence: cannistr@unitus.it; Tel.: +39-0761-357136

Received: 15 May 2019; Accepted: 20 June 2019; Published: 24 June 2019

**Abstract:** Raman spectroscopy, which is a suitable tool to elucidate the structural properties of intrinsically disordered proteins, was applied to investigate the changes in both the structure and the conformational heterogeneity of the DNA-binding domain (DBD) belonging to the intrinsically disordered protein p53 upon its binding to Azurin, an electron-transfer anticancer protein from *Pseudomonas aeruginosa*. The Raman spectra of the DBD and Azurin, isolated in solution or forming a complex, were analyzed by a combined analysis based on peak inspection, band convolution, and principal component analysis (PCA). In particular, our attention was focused on the Raman peaks of Tyrosine and Tryptophan residues, which are diagnostic markers of protein side chain environment, and on the Amide I band, of which the deconvolution allows us to extract information about α-helix, β-sheet, and random coil contents. The results show an increase of the secondary structure content of DBD concomitantly with a decrease of its conformational heterogeneity upon its binding to Azurin. These findings suggest an Azurin-induced conformational change of DBD structure with possible implications for p53 functionality.

**Keywords:** Raman spectroscopy; p53; intrinsically disordered protein; blue copper protein Azurin; protein–protein interaction; Amide I band deconvolution; principal component analysis
