Moreover, some studies hypothesized that the flexibility is inversely proportional to antigenic propensity [9, 51] : a relatively high positive correlation (i.e. Spearman correlation = 0.61 and p-value = 0.018) was found between the flexibility and the minimum concentration needed to inhibit the E.Coli growth with antimicrobial peptides. As a matter of fact, a small flexibility may be related to a compact structure, which could favor antigenic propensity. This result confirms the appropriateness of the scale used for its calculation [9] : this is defined as the ratio between the frequency of the residue in antigenic determinants (experimentally identified) and the frequency of the amino acid on the surface (predicted by using the average of hydrophilicity, accessibility and flexibility values reported by Parker et al. [11] ). N Finally, the high values of both SRCs and regression coefficients, as shown in Table 2 (see columns 2 and 4), demonstrate that some amino acids, like F and Y, favour the peptide reactivity.