The invention relates generally to the detection of ions by ion selective compounds. More particularly, the invention relates to the detection of ammonium ions.
Many receptor molecules (called xe2x80x9cionophoresxe2x80x9d) are capable of binding the ammonium cation. The ionophore nonactin has been widely used in ion selective electrodes (ISE) for the determination of ammonium ions in biological fluid samples. 
However, nonactin has poor selectivity for ammonium ions over potassium ions. Also, nonactin is relatively expensive.
Thus, there is a need in the art for ionophores with superior or at least comparable selectivity for ammonium to that of nonactin, but which are not as cost prohibitive.
The invention provides an ammonium ion selective matrix or ion selective electrode (ISE) containing as an ionophore a compound having the following general structure (I): 
where (a) R is independently or in combination hydrogen or a alkyl, alkoxyalkyl, alkoxyacyl, alkylamino, alkylnitroso, alkylcyano, alkylthio, alkylsulfinyl, alkylsulfonyl, alkenyl, alkoxyalkenyl, alkenylacyl, alkenylamino, alkenylnitroso, alkenylcyano, alkenylthio, alkenylsulfinyl, alkylsulfonyl, alkynyl, alkoxyalkynyl, alkynylacyl, alkynylamino, alkynylnitroso, alkynylcyano, alkynylthio, alkynylsulfinyl, alkynylsulfonyl, and substituted or unsubstituted aryl groups with or with out heteroatoms; and (b) Rxe2x80x2 is either nitrogen or oxygen. Other suitable groups and further descriptions of the groups described here can be determined by one of skill in the art by reference to an organic chemistry textbook (for example, Bruice P Y, Organic Chemistry, Third Edition (Prentice-Hall, 2001) or Wade L W, Organic Chemistry, Fourth Edition, (Prentice-Hall, 1999)). Isomers of ionophore I of different stereo configurations can be included in the ion selective matrix or electrode of the invention. The ionophore used in the matrix or electrode of the invention has superior or at least comparable ammonium ion selectivity to that of nonactin. Advantageously, the ionophore is easier and less expensive to produce than nonactin.
The invention also provides methods of making and using an ion selective matrix or electrode containing ionophore I.
The invention also provides a compound having the following structure (II): 
where R is hydrogen or a alkyl, alkoxyalkyl, alkoxyacyl, alkylamino, alkylnitroso, alkylcyano, alkylthio, alkylsulfinyl, alkylsulfonyl, alkenyl, alkoxyalkenyl, alkenylacyl, alkenylamino, alkenylnitroso, alkenylcyano, alkenylthio, alkenylsulfinyl, alkylsulfonyl, alkynyl, alkoxyalkynyl, alkynylacyl, alkynylamino, alkynylnitroso, alkynylcyano, alkynylthio, alkynylsulfinyl, and alkynylsulfonyl groups and substituted or unsubstituted aryl groups with or with out heteroatoms. All isomers having a difference in stereo configuration are included in the scope of the compound of the invention. The compound of the invention (compound II) may usefully be included as an ionophore in the ammonium ion selective electrode of the invention.
In a particular embodiment, the compound of the invention has the following structure (1). 
The invention also provides a useful method for synthesizing the compound of the invention from block compounds. This method works especially well when the compound II contains ester linkages and the blocks containing ester linkages within compound II are first synthesized in solution, thus making easier the use of solid phase synthesis.
The following structure is used to illustrate and generalize the method. 
where Rxe2x80x2 is independently or in combination hydrogen or an alkyl, alkoxyalkyl, alkoxyacyl, alkylamino, alkylnitroso, alkylcyano, alkylthio, alkylsulfinyl, alkylsulfonyl, alkenyl, alkoxyalkenyl, alkenylacyl, alkenylamino, alkenylnitroso, alkenylcyano, alkenylthio, alkenylsulfinyl, alkylsulfonyl, alkynyl, alkoxyalkynyl, alkynylacyl, alkynylamino, alkynylnitroso, alkynylcyano, alkynylthio, alkynylsulfinyl, alkynylsulfonyl, and substituted or unsubstituted aryl groups with or with out heteroatoms. P is any protection group that can be employed which is compatible to the particular solid phase resin in use. Examples of protection groups are the well known Fmoc, Z, Boc, Trt and the like which can be found in any commercial catalog for peptide synthesis such as Catalog and Peptide Synthesis Handbook (Calbiochem-Novabiochem Corp; San Diego, Calif.).
In one embodiment, compound 1 can be synthesized from the block compounds B1 and B2: 