NMR Study of the Protolysis Kinetics in Simple Amino Acids. I. Sarcosine Hydrochloride and its Ester

The mean lifetime of the amino group τ of acidic aqueous solutions of sarcosine hydrochloride and sarcosine methylester hydrochloride were measured using the NMR technique at 21±1°C. The experimental results show: (1) 1/τ is first order in respect to both sarcosine and its ester concentration. (2) 1/τ depends strongly on 1/aH+ in very acidic solutions and becomes almost independent in dilute acid solution. The interpretation is given in terms of an exchange of protons between the amino acid or its ester with a water molecule which is hydrogen bonded to the amino group. The exchange mechanism involves two steps:[Complex chemical formula][Complex chemical formula]Values of the rate constants for sarcosine hydrochloride are: kI′=110(sec−1), k−I′=3.2×1010(sec−1M), kH=8.3×109(sec−1);and for sarcosine methyl ester are: kI′=120(sec−1), k−I′=3.2×1010(sec−1M), kH=1.9×1010(sec−1).