http://en.wikipedia.org/wiki/String_Quartets_Nos._1_–_6,_Opus_18_(Beethoven)
http://en.wikipedia.org/wiki/String_Quartets_Nos._7_–_9,_Opus_59_–_Rasumovsky…
http://en.wikipedia.org/wiki/Late_String_Quartets_(Beethoven)

The Three Beethoven String Quartets No. 7 to 9, op. 59. For the chemo nerds their keys are F, E & C.

Beethoven String Quartet No.10 Op.74 and No.11 Op.95

Beethoven String Quartet Op.133 Grosse Fugue

Three Variations on a Theme by Roy Harris, String Quartet Number 2 by Gunther Schuller and String Quartet Number 4 by Andrew Imbrie