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Weak hydrogen-bonding interactions in Met rotamers in TM helices. (A) The six Met(−60,−60 or 180,X) rotamers can make two weak hydrogen bonds to carbonyl O atoms. Met(−60,−60,180) is shown with the two C-H⋯O bonds indicated by dotted lines. (B) The second most-frequent Met rotamer in TM helices, Met(180,180,60), has two potential C-H⋯O bonds. One bond, the Hβ-O bond, is the same as in panel A, but the other is a novel bond using the methyl Hɛ atom.
