Thin-layer and gas chromatographic and mass spectral evidence lead to the conclusion that methionine sulfoxide is converted to TFAM-methionine in the course of derivative formation. It is of interest that the yield of TFAM-methionine from methionine sulfoxide is calculated to be 5 2 z of theoretical (based upon gas Chromatographic peak areas) over a wide range of concentration of starting material. Allowing for the error in the method, this could imply that methionine sulfoxide is converted to methionine via a disproportionation reaction similar to that observed for cystine monoxide (7) and for a number of sulfoxides (8). Direct deoxygenation (9), however, cannot be overruled on the basis (7) W. E. Savige and J. A. Maclaren, “The Chemistry of Organic Sulfur Compounds,” Vol. 2, Pergamon Press, New York, N. Y., 1966, p 367. (8) H. H. Szmant, “The Chemistry of Organic Sulfur Compounds,” Vol. 1, Pergamon Press, New York, N. Y., 1961, p 154. (9) D. N. Jones, M. J. Green, and M. A. Saeed, Chem. Commun., 674 (1967).
of our results. At the same time the failure to find a gas chromatographic peak for the N-ac yl ester of methionine sulfone may be because sulfones are prone to elimination reactions at high temperatures and produce the corresponding vinyl compounds (4). In the analysis of amino acids by ion exchange chromatography, inadvertent oxidation of methionine can be recognized by the appearance of separate peaks for methionine sulfoxide and sulfone (IO). This would not appear to be the case for gas chromatography of N-acyl esters, and care should be taken in recognition of this fact, both with sample preparation and treatment prior to analysis. RECEIVED for review May 8, 1968. Accepted June 6, 1968. Work supported in part by grant NsG-496 to Massachusetts Institute of Technology from the National Aeronautics and Space Administration. (10) P. B. Hamilton, ANAL.CHEM.,35, 2055 (1963).
Spectrophotometric Method for Estimating Alkyl Ester HydroIys is R. S. Roy Science College, Mosul Unioersity, Mosul, Iraq ESTERS UNDERGO hydrolysis ( I ) . Both alkyl esters and alkyl acids exhibit maximum absorbance at 204 mp, but the molar absorptivity of an ester is higher than that of the corresponding acid (2). This difference in molar absorptivities can be utilized in determining the concentrations required for estimating the velocity constant of an ester and its acid during hydrolysis at any interval. THEORY. The hydrolysis of an ester may be represented by the following equation: RCOOR’
+ H20 e RCOOH + R’OH
Table I. Absorbance and k Values for Methyl Acetate Hydrolysis at 22” C = 46.7, e, = 34.0, u = O.O3M, [HCI] = 0.15M Time (min) 10 40 90 Absorbance 1.385 1.360 1.290 k (min-1) x 10-3 3.9 3.0 3.6 and
(1)
where R and R’ are alkyl groups.
If
= the molar absorptivity of the ester at 204 mp ea = the molar absorptivity of the acid at 204 mp A = the absorbance of the ester solution (1-cm cell) at
time t a = the concentration of the ester at time 0 ce = the concentration of the ester at time i
Values of ca and ce can be calculated from Equations 4 and 5 , respectively. For calculating k , the velocity constant of hydrolysis, insertion of ce in the kinetic equation for a reaction of the first order (S), gives
ca = the concentration of the acid at time t it follows that the absorbance of the solution containing both ester and acid (during hydrolysis at time t ) is given by
A A because Ce = a
= (U
Cere
+
CaEa
- C J E ~ + CaEa
(2)
(3)
- ea.
On rearrangement, it can be shown that
The rate of hydrolysis of an alkyl ester can be followed by determining the absorbance of the ester solution at 204 mp from time to time. This general method can be applied to all alkyl esters in the absence of absorbance of other constituents interfering with ester absorption band. Results obtained for methyl acetate hydrolysis using a Unicam SP 800 spectrometer and presented in Table I have given a value for k of 0.0035 min-l at 22 “C which agrees with the values obtained by the usual titration method.
-~
RECEIVED for review October 5 , 1967. Accepted November 17, 1967.
(1) B. Capon and B. C. Ghosh, J . Chem. SOC.,1966,472. (2) A. E. Gillam and E. S. Stern, “An Introduction to Electronic Absorption Spectroscopy in Organic Chemistry,” Edward Arnold (Publishers) Ltd., London, 1962, pp 56 and 61.
(3) E. A. Moelwyn-Hughes, “Physical Chemistry,” Pergamon Press, 1964, p 1115.
(4) ~
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ANALYTICAL CHEMISTRY
