I have a problem with this question, why is tris buffer used in the sonication of bacteria cells.
TRIS buffers from pH 7 to 9 approximately and, basically, most people use it because it's cheap, readily available and it buffers in a range that is similar to the average physiological pH found in bacterial cytoplasm. Also, it is fairly stable in solution for months and even years, while others like MES or HEPES quickly degrade over a few weeks.
One of the most common procedures used as a first purification step of bacterial cell lysate is nickel affinity chromatography, which requires buffers with pH above 7.6 and this is where TRIS buffer comes in, as it buffers in that range.
Inertia. It is way more time and/or cost-effective to follow a recipe someone used successfully than to reinvent the wheel. And as Antonio points out, Tris is readily available, not hygroscopic, stable, non-toxic, etc. so...
That said, Tris is far from ideal. It is not exactly cheap when you are dealing with large buffer volumes (100 L or more), its effective buffering range is relatively narrow, pH is very temperature-dependent, and its primary amine interferes with a number of reagents (from DEPC to Ni-NTA, where it reduces the shelf-life of the columns).
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