Using spectroscopic and microscopic methods to probe the structural stability of human adenovirus type 4

Adenovirus serotype 4 (Ad4) is a major cause of Ad-associated human diseases. Ad4 is also considered to be a potential delivery vector for gene therapy. In this study, multiple spectroscopic techniques together with transmission electron microscopy (TEM) were employed to probe viral stability and to improve pharmaceutical formulations of Ad4-based vaccines and DNA carriers. Perturbations of secondary, tertiary and quaternary structure of Ad4 proteins induced by elevated temperatures over a wide pH range (3-8) were analyzed using circular dichroism, UV absorption and intrinsic and extrinsic fluorescence spectroscopy as well as static and dynamic light scattering. The spectroscopic results obtained indicate a decrease in Ad4 stability as pH increases from 4 to 8, similar to the behavior reported previously for Ad2 and Ad5, although the Ad4 virion appears to possess slightly more tolerance to thermal stress. An empirical phase diagram (EPD) approach was used to summarize the data in the form of a colored map. In addition, the different physical states of Ad4 identified by the EPD were confirmed by TEM images. The results obtained in this study reveal both structural similarities among three commonly employed Ad subtypes (2, 4 and 5) as well as unique properties of Ad4.