Partners HealthCare System Inc., the state's largest hospital and physicians network, has signed a letter of intent to acquire Neighborhood Health Plan, a Boston-based nonprofit that insures more than 240,000 mostly low-income residents across Massachusetts. Under the deal, which would give Partners a foothold in the health insurance business, no money would change hands. But Partners would contribute an unspecified sum to provide grants to more than 50 community health centers affiliated with Neighborhood Health. Partners would also work with the insurer and health centers to provide better medical care more efficiently to the urban poor in Boston and other cities, the parties said. "We are making a critical long-term commitment to take care of this population," said Gary L. Gottlieb, MD, CEO of Partners, which owns nine hospitals in the Boston area. "As we look at the downward pressures on Medicaid dollars and other subsidized products, we are very concerned and want to make sure there are not barriers to our institutions. We want to make sure care is accessible to everyone."