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May 5 , 1955INTERACTIONS OF POLAR GASES WITH SOLID PROTEINS 2579 stant since both have approximately the same distance between the chlorine and nitrogen. The 5-, 6and 7-chloroquinolines are the strongest bases in the series for in these the chlorine has the least effect upon the electron density at the nitrogen. The 2-chloroquinoline is such a weak base that a constant could not be determined. ExperimentalAbsorption Spectra.-The spectra were determined as was reported in the previou? paper3 using a Beckman Model DU quartz spectrophotometer. The concentrations were approximately 0.0002 molar.Dissociation Constants.-All pH measurements were made with a Leeds and Northrup pH indicator, Model No.7664 with glass and saturated calomel electrodes. All measurements were made in a constant temperature room at 25'.Materials.-The 4-, 6-, 7and 8-methylquinolines and the 2-and 6-chloroquinolines were Eastman Kodak Co. products and were redistilled before use. The 2and 3methylquinolines and 3-, 4-, 5-, 7and 8-chloroquinolines were prepared in this Laboratory. OrganicStudies of the stoichiometric binding of HC1, NHs, BF3 and CHsNH2 by solid proteins have been extended t o a varied series of proteins and concomitantly to some model organic compounds for purposes of comparison. As reported earlier HCl is bound to arginine, histidine and lysine. NHB is hardly bound at all while CHsNH2 binds less than the expected number of free acid groups. BF3 can add to even weak Lewis bases and its binding is strongly pressure dependent. In all cases but especially with BF3, there is a very pronounced effect of particle dispersity indicating diffusion controlled reactions. Strong and weak basic groups can be distinguished in all cases by techniques of back titration. The evidence points to a Zwitterion structure for the residues such that the strong bases are present as RCOO-l, the corresponding acids being RNHt+l. A smaller base strength of the peptide group is found in proteins compared to that in Nylon and is compatible with the a-helix structure for proteins.