Han S L, Stimson E R, Maxfield F R, Leach S J, Scheraga H A
Int J Pept Protein Res. 1980 Sep;16(3):183-90. doi: 10.1111/j.1399-3011.1980.tb02951.x.
The state of ionization of [Leu5]-enkephalin in the crystal and in solution was studied with the aid of Raman, i.r., and n.m.r. techniques. The results indicate that, in the crystal, enkephalin adopts the dipolar ion form. In water near neutral pH, enkephalin also exists in the dipolar ion form whereas in DMSO-d6 it is a mixture of the dipolar ion and uncharged species. The pK's of alpha-amino and alpha-carboxyl groups are probably closer to each other in DMSO than in H2O.
