The pilot reported that the start-up, taxi and initial takeoff roll were normal for the air medical flight in night visual conditions. On the takeoff roll at approximately 90 knots, a white-tailed deer appeared and was moving from the right to the left between the runway and a taxiway. The pilot reported there was no time to safely abort the takeoff before impacting the deer. The airplane struck the deer with the right main landing gear leg, the airplane became airborne, and the pilot was able to return the airplane to the runway surface. The pilot then noticed he had no right brake authority and used reverse thrust to maintain runway centerline. Shortly after, the right main landing gear collapsed. The airplane came to a stop upright on the grass between the runway and a taxiway, impacting runway signage in the process. The right-wing leading edge and spar along with the right aileron sustained substantial damage from the runway signage and ground impact. The pilot reported there were no preimpact mechanical malfunctions or failures with the airframe or the engines that would have precluded normal operation.