Preheat oven to 375 degrees; grease a large cookie sheet. In a large bowl, combine flour, baking powder, baking soda, salt and cinnamon; cut in butter until crumbly. In a small bowl, combine 1/2 cup sugar, currants and orange peel. Stir into flour mixture. In a medium bowl, with a wire whisk, stir together the sour cream, egg yolk and lemon juice until smooth. Stir into flour fixture; mixture will be dry. Knead about 5 to 8 times to combine all ingredients. Divide dough in half. Pat each half into a 6-inch circle. Place both circles on cookie sheet; cut each into 6 wedges. In a small bowl, beat egg white with fork until frothy. Brush the top of each scone with egg white; sprinkle with 2 tablespoons sugar. Bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean and scones are lightly browned. Cool on a wire rack. To serve, separate into individual scones.