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Solvent effects on formation of the starch-iodine complex
598
CARBOHYDRATE
RESEARCH
Note
Solvent effects
on formation
of the starch-iodine
complex
WALTER T SMITH,JR AND GARY T SMITH Chemrstry Department,
Unrverslty of Kentucky,
(Received January 20th. 1969; 111ksed
Lexmgton,
Kentucky
40506 (U S A )
form, March ISth, 1969)
~ODUCTION
Ono et al ’ found that in methyl sulfoxrde-water solutions contammg less than 28 moles of water per liter a posmve starch-iodme test could not be obtained In order to study the factors mvolved m determmmg whether a posrtlve test is obtamed m the presence of nonaqueous solvents we have investigated the solvent reqturements necessary to obtam a positwe test in aqueous soluttons of the followmg solvents. methyl sulfoxlde, N-methylacetarnide, N,iV-dimethylformanude, acetone, p-dloxane, methanol, and ethanol DISCUSSION
No general minimum requirement of water necessary for a positive test was found The percentage of water necessary for a posltrve test vanes from 39% wrth N-methylacetarmde to 68% (v/v) with acetone (Table I) These data mdrcate that the cntical factor 1s not some mmmum proportion of water The water reqmrement IS not directly related to either the dipole moment or the dlelectnc constant of the nonaqueous solvent used The i r spectrum of an aqueous solutlon of the starchiodine complex was smular to that of the starch solution alone TABLE
I
Solvent
Mrmmum % of water (v/u) ro groe cobr
Drpole moment, p
N-Methylacetamlde
39
37
Methanol Ethanol Methyl sulfoxlde N,N-Dmethylformmude pDloxane
47 52 53 58
1.7 17 39 38
58 68
0 29
Acetone
The effects of the different solvents on the amounts of water required for color formatron can be interpreted m terms of the loose “deformed hehx” of Ono and Corbohyd
Res,
10 (1969) 598-600
NOTE
599
its conversron mto a rrgrd hehx when for-matron of a complex takes place. It can be assumed that the loose helix in aqueous solution IS hydrated and that the same IS true of the ngid hehx of the complex This picture is further supported by the viscosity studres of Banks and Greenwood2, which indxcate that the amylose molecule behaves as a random co11 in methyl sulfoxide solutton In soluttons containing solvents other than water the other solvent competes with water for the solvation sites on the loose coil. In the tight or rigid con there is room between one turn of the corl and the next turn for only a small molecule (such as water) to solvate, and hence some water IS required On the other hand, in the loose coil there 1s adequate room between turns of the coil for larger molecules to solvate the amylose, and rf these larger molecules have sufficrent solvatmg power (as might be indicated by then havmg a high dipole moment), the proper stenc fit of the larger solvatmg molecule between turns of the corl should actually tend to stabtie the amylose molecule in the loose co11 To the extent that the loose co11 is stabilized by the solvatmg molecule it wrll be that much more drfficult for the deformed hehx to change over to the ngrd hehx and therefore, that much more difficult to form the colored complex. Our data, although limrted, would mdrcate that there may be some optimum size for the solvent molecule which grves the best stabrhzatron of the deformed helrx Srze can not be considered alone smce obviously the dipole moment has an nnportant effect on solvatmg power It 1s possible to interpret the results in terms of the size and drpolar character of the various solvent molecules Thus the molecule of acetone @ = 2 9) would appear to be of a near optimum size for fitting into the deformed hehx The combmation of suitable srze with a hrgh dipole moment makes acetone a solvent that requires a mm.rmum of 68% of water to permrt formation of the ngrd hehx and the colored complex The other solvents havmg hrgh dipole moments, methyl sulfoxrde and NJV-drmethylformamrde, reqmre 53 and 58% of water, respectively, for the complex to form These molecules are a little larger than acetone and may not fit into the deformed hehx in a manner that would stabrlize the deformed hehx as well as acetone would Methanol and ethanol, whrle smaller, do not have such large dipole moments and hence do not solvate the starch as well as most of the other solvents studred Two of the solvents, p-dioxane and N-methylacctamide, require special comment p-Droxane has a zero dipole moment because thrs observed moment rs the resultant of two opposmg drpoles. If we consider that only one of these dipoles would be involved in solvatmg the starch molecule, then It grves a better prcture of the true situatron to consrder only one-half of thep-droxane molecule and to approxxmate its solvating power as being somewhat hke methyl ether b = 1.3). On this basis, p-droxane appears to fit well mto the deformed hehx and its drpole (for one end of the molecule) IS sufficrent to grve good stabrhzation of the deformed helix. The low percentage of water reqmred to allow formation of a complex in N-methylacetarmde indrcates erther a low degree of stabihzation of the deformed hehx or, alternatively, that the rigid hehx necessary for formation of the complex Curbohyd-Res , 10 (1969) 598-600
600
NOTE
can be partly solvated by N-methylacetamide Evidence favors the latter view. This solvent is known to form strong hydrogen bonds and may be doing so either with the rigid coil directly or as the second shell of solvating molecules around the co& with water molecules forming the first shell. A comparison of the visible spectra of the starch-iodme complexes in the various solvents supports the idea that the molecules of N-methylacetamide are involved drrectly in the starch-iodme complex in that solvent. The complex formed in the other solvents studied shows a single maximum at 546596 run (depending on the solvent) while the complex in N-methylat&amide shows three maxima at 449,546, and 675 mu ExPERlMEN-l-AL
Solutrons of starch (1% w/v, 2 4 ml) with various amounts of added organic solvent were tested for color formation by the ad&tron of 1 drop of iodine soluuon (prepared from 60 g of potassmm iodide and 40 g of iodine in 1 liter of soluuon) Any formation of a blue color was taken as a posxtive test. Visrble spectra were taken in l-cm cells on a Perkm-Elmer Model 202 spectrophotometer. The i r spectrum was taken in a 0.05 mm AgCl cell with a Beckmann Model IR-8 spectrophotometer REFERENCES 1 S ONO, T. WATANABE,K OGAWA, AND N OKAZAKI,Bull Sot. Chem. Japan, 38 (1965) 643 BANKSAND C T GREENWOOD.Carbohyd Res, 7 (1968) 414
2 W
Carbohyd
Res,
10 (1969) 598-600
CARBOHYDRATE
RESEARCH
Note
Solvent effects
on formation
of the starch-iodine
complex
WALTER T SMITH,JR AND GARY T SMITH Chemrstry Department,
Unrverslty of Kentucky,
(Received January 20th. 1969; 111ksed
Lexmgton,
Kentucky
40506 (U S A )
form, March ISth, 1969)
~ODUCTION
Ono et al ’ found that in methyl sulfoxrde-water solutions contammg less than 28 moles of water per liter a posmve starch-iodme test could not be obtained In order to study the factors mvolved m determmmg whether a posrtlve test is obtamed m the presence of nonaqueous solvents we have investigated the solvent reqturements necessary to obtam a positwe test in aqueous soluttons of the followmg solvents. methyl sulfoxlde, N-methylacetarnide, N,iV-dimethylformanude, acetone, p-dloxane, methanol, and ethanol DISCUSSION
No general minimum requirement of water necessary for a positive test was found The percentage of water necessary for a posltrve test vanes from 39% wrth N-methylacetarmde to 68% (v/v) with acetone (Table I) These data mdrcate that the cntical factor 1s not some mmmum proportion of water The water reqmrement IS not directly related to either the dipole moment or the dlelectnc constant of the nonaqueous solvent used The i r spectrum of an aqueous solutlon of the starchiodine complex was smular to that of the starch solution alone TABLE
I
Solvent
Mrmmum % of water (v/u) ro groe cobr
Drpole moment, p
N-Methylacetamlde
39
37
Methanol Ethanol Methyl sulfoxlde N,N-Dmethylformmude pDloxane
47 52 53 58
1.7 17 39 38
58 68
0 29
Acetone
The effects of the different solvents on the amounts of water required for color formatron can be interpreted m terms of the loose “deformed hehx” of Ono and Corbohyd
Res,
10 (1969) 598-600
NOTE
599
its conversron mto a rrgrd hehx when for-matron of a complex takes place. It can be assumed that the loose helix in aqueous solution IS hydrated and that the same IS true of the ngid hehx of the complex This picture is further supported by the viscosity studres of Banks and Greenwood2, which indxcate that the amylose molecule behaves as a random co11 in methyl sulfoxide solutton In soluttons containing solvents other than water the other solvent competes with water for the solvation sites on the loose coil. In the tight or rigid con there is room between one turn of the corl and the next turn for only a small molecule (such as water) to solvate, and hence some water IS required On the other hand, in the loose coil there 1s adequate room between turns of the coil for larger molecules to solvate the amylose, and rf these larger molecules have sufficrent solvatmg power (as might be indicated by then havmg a high dipole moment), the proper stenc fit of the larger solvatmg molecule between turns of the corl should actually tend to stabtie the amylose molecule in the loose co11 To the extent that the loose co11 is stabilized by the solvatmg molecule it wrll be that much more drfficult for the deformed hehx to change over to the ngrd hehx and therefore, that much more difficult to form the colored complex. Our data, although limrted, would mdrcate that there may be some optimum size for the solvent molecule which grves the best stabrhzatron of the deformed helrx Srze can not be considered alone smce obviously the dipole moment has an nnportant effect on solvatmg power It 1s possible to interpret the results in terms of the size and drpolar character of the various solvent molecules Thus the molecule of acetone @ = 2 9) would appear to be of a near optimum size for fitting into the deformed hehx The combmation of suitable srze with a hrgh dipole moment makes acetone a solvent that requires a mm.rmum of 68% of water to permrt formation of the ngrd hehx and the colored complex The other solvents havmg hrgh dipole moments, methyl sulfoxrde and NJV-drmethylformamrde, reqmre 53 and 58% of water, respectively, for the complex to form These molecules are a little larger than acetone and may not fit into the deformed hehx in a manner that would stabrlize the deformed hehx as well as acetone would Methanol and ethanol, whrle smaller, do not have such large dipole moments and hence do not solvate the starch as well as most of the other solvents studred Two of the solvents, p-dioxane and N-methylacctamide, require special comment p-Droxane has a zero dipole moment because thrs observed moment rs the resultant of two opposmg drpoles. If we consider that only one of these dipoles would be involved in solvatmg the starch molecule, then It grves a better prcture of the true situatron to consrder only one-half of thep-droxane molecule and to approxxmate its solvating power as being somewhat hke methyl ether b = 1.3). On this basis, p-droxane appears to fit well mto the deformed hehx and its drpole (for one end of the molecule) IS sufficrent to grve good stabrhzation of the deformed helix. The low percentage of water reqmred to allow formation of a complex in N-methylacetarmde indrcates erther a low degree of stabihzation of the deformed hehx or, alternatively, that the rigid hehx necessary for formation of the complex Curbohyd-Res , 10 (1969) 598-600
600
NOTE
can be partly solvated by N-methylacetamide Evidence favors the latter view. This solvent is known to form strong hydrogen bonds and may be doing so either with the rigid coil directly or as the second shell of solvating molecules around the co& with water molecules forming the first shell. A comparison of the visible spectra of the starch-iodme complexes in the various solvents supports the idea that the molecules of N-methylacetamide are involved drrectly in the starch-iodme complex in that solvent. The complex formed in the other solvents studied shows a single maximum at 546596 run (depending on the solvent) while the complex in N-methylat&amide shows three maxima at 449,546, and 675 mu ExPERlMEN-l-AL
Solutrons of starch (1% w/v, 2 4 ml) with various amounts of added organic solvent were tested for color formation by the ad&tron of 1 drop of iodine soluuon (prepared from 60 g of potassmm iodide and 40 g of iodine in 1 liter of soluuon) Any formation of a blue color was taken as a posxtive test. Visrble spectra were taken in l-cm cells on a Perkm-Elmer Model 202 spectrophotometer. The i r spectrum was taken in a 0.05 mm AgCl cell with a Beckmann Model IR-8 spectrophotometer REFERENCES 1 S ONO, T. WATANABE,K OGAWA, AND N OKAZAKI,Bull Sot. Chem. Japan, 38 (1965) 643 BANKSAND C T GREENWOOD.Carbohyd Res, 7 (1968) 414
2 W
Carbohyd
Res,
10 (1969) 598-600