Experimental study on aggregation of model dipeptide molecules. V. Stereoselective association of leucine dipeptides

Infrared spectroscopy and ¹H nmr have been used to elucidate the association modes of leucine dipeptides in carbon tetrachloride solution. Two stereoselective types of aggregates have been evidenced. Homochiral molecules are associated in oligomeric aggregates and accommodate a β-parallel-like structure which was characterized by x-ray diffraction. Heterochiral molecules are paired in centrosymmetrical dimers; the latter aggregation mode restricted to the dimer stage predominates in racemic solutions. A theoretical model proposed to account for this aggregation process is consistent with the experimental nmr data. Both homo- and heterochiral association constants are estimated from vapor pressure and nmr experiments.