||The synthetic peptide Vitr-p-13(YPIVGQELLGAIK-NH2), derived from the bacterial dimeric Vitreoscilla haemoglobin (VHb) in the position 95-107, is characterized by a preeminent "statistical coil" conformation in water as demonstrated by CD experiments and long time-scale MD simulations. In particular, Vitr-p-13 does not spontaneously adopt an alpha-helix folding in Water, but it is rather preferentially found in beta-hairpin-like conformations. Long time-scale MD simulations have also shown that Vitrp-13 displays a "topological-trigger" which initiates alpha-helix folding within residues 7-10, exactly like seen in the temporins, a group of linear, membrane-active antimicrobial peptides of similar length. At variance with temporins, in Vitr-p-13 such a process is energetically very demanding (+ 10 kJ/mol) in water at 300 K, and the peptide was found to be unable to bind model membranes in vitro and was devoid of antimicrobial activity. The present results,, compared with previous studies on similar systems, strengthen the hypothesis of the requirement Of a partial folding when still in aqueous environment to allow a peptide to interact with cell-membranes and eventually exert membrane perturbation-related antibiotic effects on target microbial cells.