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REACTION

OF

NINHYDRIN

WITH

DIPEPTIDES

DISCUSSION

It has been shown

that the ratios

of the ninhydrin

color yields of

vary

from 1.6

to 5, depending

and

the length

of the reaction

diastereoisomeric

dipeptides

can

centration

of

methyl

Cellosolve

on the period.

two con- It

is possible that diastereoisomers

significant of other

differences in peptides which

color have

yields may be obtained two or more optically

from active

amino acid necessarily nents.

residues. be a valid

Thus, a racemic mixture

standard

for

the

color

of diastereoisomers yield of either of

may not its compo-

Since an hydrin, the from alanyl is liberated color yields by Dowmont

amino group bound in a fact that nearly 2 moles

peptide linkage cannot of the colored product

react with nin- can be obtained

and leucyl dipeptides during the extended

indicates reaction

that the C-terminal period at 100” (pH

amino acid

5).

Molar

of 1.5 to 1.6 for L-Ala-L-Ala

and

Fruton

(7).

On

the

and other

L-Ala-L-Ser

hand,

in

have been recorded

the

case

of

glycyl

and

free two seryl react dipeptides, apparently only the these amino groups which are initially with ninhydrin. Thus dipeptides be separated into can , basis of their is evidently distinct categories of the N-terminal on the residue maximal color the determining yields. factor. The nature These dif-

ferences in molar color yields may arise from differences

in stability

of the

products from the oxidative dipeptides. The fact that the reaction

deamination

of ninhydrin

of

the

a-amino

groups

with certain dipeptides

of these

can yield

nearly 2 moles of colored

product

presented

a unique

problem

when

this

reaction

was used to measure the extent

by pancreatic acids) react

carboxypeptidase. with ninhydrin much

Since more

the hydrolytic

products

(amino

rapidly

than

dipeptides,

it was

of hydrolysis

of these dipeptides

possible

to

decrease

the

blanks

from

these

dipeptides

and obtain

repro-

(25 volume

per

cent) and

hydrolysis

can

be verified

ducible

results

by using a

relatively

low methyl

a

15 minute

reaction

by

some

other

assay

Cellosolve concentration period. The calculated such as form01 titration.

When

nearly

complete

reaction

of dipeptides

with

ninhydrin

was desired,

a high period

methyl Cellosolve have been used.

concentration

and a relatively

lengthy

reaction

I am grateful

to Dr. M. A. Mitx for his interest

in this work.

The reaction been studied. tion of ninhydrin

of ninhydrin

Several-fold with the

with

various

differences in diastereoisomeric

dipeptides

at 100” (pH

5) has

rates were observed pair, n-Leu-L-Tyr

in the reac- and L-Leu-

L-Tyr.

Nearly

equal rates

are approached

when

the concentration

of

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