The pilot taxiied the aircraft for departure, back tracking to the north-west end of the runway. While turning right to line up for takeoff, the left main landing gear collapsed, allowing the left propeller to strike the ground, stopping the engine. The eye bolt attaching the landing gear side brace strut to the spar fitting was found to have suffered a corrosion fatigue failure allowing the gear to collapse inwards, ie. in the direction of retraction. As this did not effect the downlock latching mechanism or micro switch at the strut elbow joint, all three green gear down lights in the cockpit remained on. The operator had previously experienced this problems with simliar type aircraft, and as the bolt is difficult to see during inspections he instigated a program to replace it with a modified type. This aircraft had only recently been acquired and the bolts would have been replaced at the first service by the operator.