DFT conformational study of cysteine in gas phase and aqueous solution

Different conformers of cysteine in gas phase are investigated at the DFT B 3LYP/6-31G* and B3LYP/6-311 + + G** levels. The effect of the solvent is si mulated by using the Onsager and polarizable continuum (PCM) models within the self-consistent reaction field method (SCRF) at the B3LYP/6-31G* level. Specifically, five neutral forms, two anions and one zwitterion were analy sed. Both, in gas phase and solution the most stable normal form has the ca rboxyl group directed toward the amino group. In accord with the experiment , the PCM model predicts that the most stable structure in solution is a zw itterion, a species that does not exist or has a very small stability in ga s phase. A major stabilization in solution is also predicted for the zwitte rionic form of anionic cysteine. Thus the PCM model renders correct stabili ty order of the different conformers in solution, while the Onsager model d oes not, which is due to the underestimation of the electrostatic contribut ions to the solute-solvent interaction for the zwitterions. (C) 2000 Elsevi er Science B.V. All rights reserved.