Before revision:  This article is to present very strong evidences (mainly at 300 K room temperature, and other temperatures such as 350 K, 450 K, 500 K) to support this salt bridge result and at the same time we emphasize that all our numerical experiments are completely reproducible .
After revision:  This article analyses the NMR and X-ray structures and gives an answer to the above question: the salt bridge presents at pH 6.5 in the X-ray structure is simply gone at pH 4.5 in the NMR structure is simply due to the different pH values that impact electrostatics at the salt bridge and hence also impact the structures. Moreover, some molecular dynamics simulation results of the X-ray structure are reported in this article to reveal the secrets of the structural stability of rabbit prion protein .