Developers Nathan Szanton and Robert C.S. Monks are expected to welcome Gov. John Baldacci and other officials to the opening of 53 Danforth St., a 43-unit apartment building, on Wednesday. Thirty of the apartments are set aside for families with incomes at or below 60 percent of the area median income. The building also has green design features, including a solar thermal hot water heater, automatic hall lights that shut off after a minute with no motion, super-insulated roof and walls and low-flow shower heads. There is also a “carbon counter” in the lobby, which shows the amount of carbon that is not released into the atmosphere thanks to the building’s green features.