Kranthi Kumar Namila’s favorite structure is:

N-acetyl L-cysteine


Source: CrystEngComm, 2013, 15, 6498


A novel polymorph of N-acetyl-L-cysteine (NAC) is discovered three decades after the first report on the X-ray crystal structure of this bioactive compound. The crystal structure of the new orthorhombic polymorph (form II in the P212121 space group) is characterized by X-ray diffraction and compared to the triclinic structure of form I (NALCYS02 and NALCYS10 in the P1 space group). Both polymorphs contain a C(7) chain of COOH…OLC–CH3 hydrogen bonds, except that the COOH group is rotated by 180u in form II
to make an auxiliary C–H…O interaction with the methyl group in a R2 2(8) ring motif. The known form I contains weak S–H…OLC–OH and N–H…S hydrogen bonds whereas the new form II has only N–H…S interactions.—Kranthi Kumar Namila

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