Intramolecular hydrogen bonding of (+)-biotin and biotin derivatives in organic solvents

The intramolecular hydrogen bonding of (+)-biotin and biotin derivatives in mixtures of CDCl 3 and DMSO-d 6 was investigated by 1 H NMR spectroscopy. The dynamic changes observed in the chemic al shifts for the 1-NH and 3-NH protons with changes in solvent composition and temperature confirmed the presence of an intramolecular hydrogen bond between the valeryl side chain hydrogen bond acceptor and the 3-NH proton in a range of biotin derivatives


Introduction
(+)-Biotin, 1, is involved as an enzyme cofactor in a variety of carboxylase, decarboxylase and transcarboxylase reactions. 1,2The specificity inherent in the reversible carboxylation of biotin involving the electrophilic substitution of the 1-NH proton by the carboxylate group has long been known. 3,4Previous reports have demonstrated the more rapid rate of exchange for the 1-NH proton compared to the 3-NH proton in both biotin itself and biotin methyl ester by measurements involving the rate of D 2 O exchange in DMSO-d 6 and NMR saturation transfer experiments. 4,5,6,7This increased rate of exchange was attributed to an intramolecular hydrogen bond formed between the carboxylate group in the valeryl side chain and the 3-NH proton and NOESY measurements confirmed the presence of a conformation similar to that shown in Figure 1. 5 A number of 1 H and 13 C NMR studies on biotin has been reported and a complete assignment for the observed resonances is available. 5,8,9evious NMR studies on biotin have been conducted at concentrations = 17 mM 5 and so both inter-and intramolecular hydrogen bonding might be expected in solution, depending on the solvent. 10In addition, we were not aware of any published data for simple biotin derivatives at low concentration that demonstrated temperature dependent NMR chemical shifts supporting the proposed intramolecular hydrogen bonding between 3-NH and the carboxyl carbonyl on the valeryl side chain.In this paper we report on the effect of solvent composition in the mixed solvent CDCl 3 /DMSO-d 6 on the chemical shifts of the 1-NH and 3-NH protons and on the temperature dependence of the 1-NH and 3-NH protons in biotin and its derivatives in dilute solutions.

Preparation of biotin derivatives
In order to investigate the intramolecular hydrogen bonding of biotin a series of derivatives, 2-8, was prepared using literature procedures.
Nmr spectra of (+)-biotin (+)-Biotin, 1, does not dissolve significantly in CDCl 3 and so DMSO-d 6 was used for the preparation of homogeneous and stable solutions.The assignments for the 1 2).
Previous 1 H NMR data for biotin in DMSO-d 6 showed that the 3-NH proton occurred further downfield compared to the 1-NH proton. 5,8,9The 2D-COSY spectrum for a 5mM solution of biotin in 10% DMSO-d 6 and 90% CDCl 3 at rt, however, indicated that the 1NH proton was further downfield, rather than the expected 3-NH (Figure 3).The 2DROESY NMR spectrum of biotin under these condition displayed cross peaks between H3a and methylene protons H4', between H4 and H4' and between 3-NH and H4' (Figure 4).Given the apparent reversal in the assignments for the 3-NH and 1-NH protons in 10% DMSO-d 6 and 90% CDCl 3, we investigated the effect of solvent composition on biotin 1 H NMR signals and the results for 3-NH and 1-NH are summarized in Figure 5.It is apparent that the addition of increasing proportions of DMSO-d 6 to CDCl 3 causes a dramatic downfield shift in the ureido protons.When the proportion of DMSO-d 6 in the mixed solvent was 4.0%, the 3-NH proton was downfield from 1-NH, suggesting that 3NH was involved in intramolecular hydrogen bonding.As the proportion of DMSO-d 6 was increased the resonance for 3-NH moved upfield, whereas the resonance for 1-NH moved downfied, consistent with a reduction in intramolecular hydrogen bonding for 3NH.At 6.0% DMSO-d 6 content the chemical shifts of both 1-NH and 3-NH became coincident and as the proportion of DMSO-d 6 was increased further, both 1-NH and 3NH moved downfield.Figure 5 shows that the resonance for 1-NH moved further downfield than that of 3-NH for DMSO-d 6 content between 6.6% and 20.0%.When the content of DMSOd 6 was 26.7%, the chemical shifts for 1-NH and 3-NH again became coincident, and increasing the content of DMSO-d 6 above 33.3%caused 3-NH to be further downfield than 1-NH.Previous NMR assignments for biotin have usually assumed that 3-NH resonated further downfield compared to 1-NH and the present results have highlighted the need to determine the effect of solvent composition on the chemical shift carefully for protons capable of hydrogen bonding.
At room temperature, no intermolecular amide-amide hydrogen bonding was detected in a 1mM solution of peptide in halogenated solvents, such as CD 2 Cl 2 and CDCl 3 . 10When polar organic solvents were used, such as CH 3 CN, the concentration of peptide could be raised to 10mM before significant intermolecular hydrogen bonding was detected. 10Therefore, the solution of biotin at 5mM in a mixed solvent of CDCl 3 and DMSO-d 6 would be expected to be substantially free of intermolecular hydrogen bonding.

Variable temperature 1 H NMR of biotin derivatives
The temperature dependence of amide protons, the ∆δNH/∆T value, provides an indication of the efficacy of intramolecular hydrogen bonding.Non-hydrogen bonded amide protons generally show a small temperature dependence, < 3 ppb/K. 10,11In order to confirm the degree of intramolecular hydrogen bonding of biotin and its derivatives (1-9), the temperature dependence was determined at 1 mM in CDCl 3 and shown in Table 1 (2 % DMSO-d 6 and 98% CDCl 3 for biotin).
The ∆δNH/∆T value for both 1-NH and 3-NH of compound 9 was 3.8 ppb/K, indicating insignificant intramolecular hydrogen bonding and confirming that 9 would be a suitable standard for comparing the remaining biotin derivatives.As the data in Table 1 shows, for compounds 2-8, the ∆δNH/∆T values for 3-NH were much higher than 3 ppb/K, indicating that in each case 3-NH was involved in intramolecular hydrogen bonding.The ∆δNH/∆T values for 1-NH were higher than expected for 1-8, as previous studies had indicated that favourable conformations involving intramolecular hydrogen bonding between 3-NH and the valeryl carbonyl were possible but no suggestion of intramolecular hydrogen bonding involving 1-NH. 5The observed chemical shifts are averaged over the numerous conformations, including those involving extended chains and intramolecular hydrogen bonding.
These data do indicate that weak intramolecular bonding is possible between acceptors in the valeryl side chain and the donor on 1-NH.
The geometry of biotin ester 6 in CDCl 3 at 20mM was probed with a homonuclear NOESY experiment.Irradiation of H3a revealed the following NOE enhancements, H3' (0.46%) and H4' (0.48%); irradiation of H4 revealed the following NOE enhancements H3' (1.94%), H4' (2.10%) and H5' (1.31%); suggesting a conformation analogous to that shown for biotin in Figure 1.The 2D-ROESY spectrum of biotin ester 6 in CDCl 3 at room temperature is shown in Figure 6 with cross peaks observed between 3-NH and H3', H4' and H5'.In conclusion, we have shown that there is a need to be cautious in the assignment of 1 H NMR spectra for compounds in different solvents when intramolecular hydrogen bonding is possible and confirmed that intramolecular hydrogen bonding between 3NH and the valeryl side chain is possible in a range of biotin derivatives.In addition, the 1-NH of biotin and its derivatives is also capable of intramolecular hydrogen bonding but to a lesser extent than the previously reported 3-NH.

Experimental Section
General Procedures.Melting points were recorded on a Reichert hot stage apparatus.
(0.010g, 0.032mmol) was dissolved in acetone (2mL) and anhydrous sodium iodide (0.096g, 0.64mmol) was added.The mixture was stirred overnight at room temperature.The solvent was removed with a stream of nitrogen and the residue was submitted to flash column separation with 8% MeOH in CH 2 Cl 2 as an eluant to give 9 as a light yellow solid in 74% (0.008g

Figure 1 .
Figure 1.Folded conformation of biotin with intramolecular hydrogen bonding.

Figure 5 . 1 H
Figure 5. 1 H NMR of 3-NH and 1-NH of biotin in mixtures of DMSO-d 6 and CDCl 3.

Table 1 .
Temperature dependence, ∆δNH/∆T in ppb/K, of 1-9 at 1mM in CDCl 3 15 and13C NMR spectra were recorded on a Varian 600 spectrometer in CDCl 3 and DMSO-d 6 as solvent with TMS as an internal standard.Chemical shifts were reproducible within ±0.002 ppm.CDCl 3 was dried over 4Å molecular sieves for at least one month before use.2D-COSY,ROESY and NOESY NMR were recorded on a Varian 600 NMR spectrometer.Vaable temperature NMR experiments were performed under the automatic control of the attachments on the Bruker ACP-300 and Varian 600 spectrometers and the temperature was maintained within a ±0.02 o C. Compounds 212, 3 13 , 413, 513, 614, 715and 815were prepared by literature procedures.