Comparative thermal stabilities of recombinant adenoviruses and hexon protein

Differential scanning calorimetry was used to identify the thermal stability profile of the replication deficient and protein IX deleted recombinant adenovirus type 5 that contains the p53 transgene (rAd/p53) in phosphate buffered saline (vPBS) or 10% glycerol (TRIS/phosphate buffer). The wildtype adenovirus (Ad/WT) and purified hexon protein (major capsid protein) were also evaluated in 10% glycerol (TRIS/phosphate buffer) as controls. The thermal profile of rAd/p53 revealed three endothermic transitions (T1, T2 and T3) occurring between 25 °C and 90 °C. T1, which occurred at 46.7 °C in vPBS and 49.4 °C in TRIS/PO4 10% glycerol buffer, was irreversible following repeated scanning and attributed to the degradation of the intact vector. The latter two endothermic transitions, T2 and T3, occurring at 69 °C and 78 °C, respectively, corresponded with the two transitions of purified hexon in temperature and amount of heat absorbed. The thermal profile of Ad/WT revealed four endothermic transitions at 51.5 °C (T1), 70.5 °C (T2A), 73.6 °C (T2B), and 77.4 °C (T3). The higher temperature of degradation as well as additional transition was attributed to the presence of protein IX associated with the hexon. The positions and excess molar heat capacities of the intact rAds were found to be affected by pH, glycerol, vector concentration and the presence or absence of protein IX in the capsid. Irreversibility of T1 implied that the degradation of the intact virus may follow first-order kinetics. The thermal scan rate dependence of T1 further confirmed that degradation of the intact virus may be first-order. The apparent activation energies for the degradation of the intact vectors were determined from the scan rate dependence of T1 and shown to be affected by protein IX in the capsid and solution conditions. Analysis of rAd samples incubated at 45 °C by Field Emission Electron Microscopy (FESEM) confirmed that loss of single particles was first-order. Although aggregates were observed in the samples, degradation appeared to be the dominant reaction leading to disappearance of single virions from the aqueous matrix. Based on thermal and FESEM analysis, an empirical model was proposed that accounted for the disappearance of single rAd particles. At or near T1, degradation of rAd particles followed a unidirectional, pseudo-first order reaction. However, at lower temperatures, disappearance of single virions resulted from competing irreversible degradation and aggregation reactions.