EFFECT OF CYSTEINE ON CHROMOGEN FORMATION BY HEXOSES AND PENTOSES IN O-CRESOL-SULFURIC ACID REACTION

The condensation product formed by furaldehyde with L-cysteine in 75% sulfuric add, is heat-labile compared to the product formed by hydroxy methylfuraldehyde (HMF). In the single-step, o-cresol : sulfuric acid reaction, cysteine enhanced chromogen formation for both furaldehyde a nd HMF at lower concentrations, but selectively inhibited that of fura ldehyde at concentrations higher than 5.0 mg. In the two-stage assay ( o-cresol added to the cooled solution containing the sugar, cysteine a nd sulfuric acid), cysteine enhanced chromogen formation by HMF and he xoses, but decreased color intensity for furaldehyde and pentoses sign ificantly even at 5.0 mg level. Additional heating of the solution con taining the sugar, cysteine and sulfuric acid at 100 degrees for 5 or 10 minutes and measurement of color formed with added o-cresol after 2 4 hours, showed that chromogen formation was abolished with ribose. Ar abinose and xylose yielded feeble chromogens. With mannose, color inte nsity was decreased only by 19% due to additional heating. Other hexos es behaved similar to mannose. This observation can be exploited for t he selective estimation of hexoses in presence of pentoses.