The Q4 lease by law firm Arnold & Porter brings 601 Massachusetts to more than 75% leased.

BOSTON-Thanks in part to expenses that were less than expected, and revenues and interest income that were greater than expected, Boston Properties Inc. beat its own guidance on fourth-quarter funds from operations, the REIT said Tuesday evening. Q4 FFO was $1.27 pershare, compared to guidance of $1.22 to $1.24 per share.

FFO for the year ended Dec. 31, 2012 was $741.4 million, or $4.94 per share basic and $4.90 per share diluted. This compares to FFO for the year prior of $711.0 million, or $4.88 per share basic and $4.84 per share diluted.

A pair of sizeable law firm leases at projects now under development were among the highlight of the quarter. At BXP’s forthcoming 601 Massachusetts Ave. mixed-use project in Washington, DC, Arnold & Porter signed on for about 376,000 square feet, more than half the 478,000 square feet that will be developed there.

Construction of the project is scheduled to commence in Q2 of this year, and the law firm expects to move into the completed building in Q4 2015, according to BXP. 601 Massachusetts is currently more than 75% leased.

At BXP’s 250 W. 55th St. in Midtown Manhattan, the legal tenant that signed on the dotted line in Q4 was Kaye Scholer, which will move from its longtime base of 425 Park Ave. into 246,000 square feet in the new construction. Kaye Scholer will take floors 3 through 20 inaninitial 20-year lease at 250 W. 55th. A mid-2014 move-in is expected; the 980,000-square-foot office tower is currently 46% leased.

The REIT said Tuesday evening it’s looking at Q1 FFO in the range of $1.19 to $1.21 per diluted share, and $5.06 to $5.18 per share for the full year. A conference call to discuss earnings and the ’13 outlook was slated for 10 a.m. Wednesday.