Direct synthesis of beta-mannans. A hexameric [-> 3)-beta-D-Man-(1 -> 4)-beta-D-Man-(1](3) subunit of the antigenic polysaccharides from Leptospira biflexa and the octameric (1 -> 2)-linked beta-D-mannan of the Candida albicans phospholipomannan. X-ray crystal structure of a protected tetramer