Notwithstanding any special or general law to the contrary, the land owned by the commonwealth under the jurisdiction of the department of mental retardation located in the city of New Bedford at 124 Maple Street,  identified on city maps as parcel ID number 45/237 and commonly known as “the Reed Estate” with the exception of the portion of said land that is occupied by its current structure,  shall be preserved, under the purposes and authority of Article XCVII of the Massachusetts Constitution, with a conservation restriction.  Said land shall not be used for any other purpose or otherwise disposed of except by laws enacted by two-thirds vote, taken by yeas and nays, of each branch of the general court.