Homocysteine-Thiolactone - Homocysteine in Protein Structure/Function and Human Disease - page 19

Reaction 3.1 The hydriodic
acid-dependent conversion of
methionine to Hcy-
thiolactone (Reprinted from
[68])

N H 2

NH 2 . HI

O

HI, 120 o C

OH O

-CH 3 I

S

S

CH 3

Fig. 3.1 Absorption spectra
of L -Hcy-thiolactone

N H 2

1.0

HCl
(0.2 mM in water, 25 C).
D , L -Hcy (0.2 mM), shown for
comparison, does not
appreciably absorb UV light
above 220 nm (Reproduced
from [68])

O

0.8

S

0.6

NH 3 +

COO -

HS

220 240 260 280 300

Wavelength (nm)

3.1.2 Acid-Dependent Cyclization of Homocysteine

In the presence of hydrochloric acid, L -Hcy undergoes intramolecular condensation
to L -Hcy-thiolactone (Reaction 3.2 ) [4]. The rate of ring closure depends on the acid
concentration and temperature. For example, in 0.1 N, 0.6 N, or 6 N hydrochloric
acid at 100 C, 50 % of ring closure condensation occurs in 3 h, 15 min, or 5 min,

respectively [68]. Because Hcy-thiolactone, in contrast to Hcy, absorbs ultraviolet
light (Fig. 3.2 ), the conversion to Hcy-thiolactone has been exploited in the first
spectrophotometric assay for Hcy [189]. The acid-dependent conversion to Hcy-
thiolactone followed by its quantification by high-performance liquid chromatog-
raphy (HPLC) is now used as a convenient procedure for the determination of Hcy
in biological samples [64, 79, 93-95, 190].

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3.2 Physicochemical Properties

3.2.1 UV Spectrum

Differences in physicochemical properties of Hcy-thiolactone and Hcy are
highlighted in Table 3.1 . Similar to other thioesters, Hcy-thiolactone absorbs
ultraviolet light with a maximum at 240 nm and 5,000 M 1 cm 1 in water

(Fig. 3.2 ) [68]. The ability to absorb at 240 nm is also a characteristic property of a
deprotonated sulfhydryl group [191, 192]. Monitoring absorbance at 240 nm
facilitates studies of nucleophilic [73] and electrophilic [84] reactions involving
Hcy-thiolactone as well as quantification of Hcy-thiolactone in biological samples
[64, 67, 190, 193, 194].

ε ¼