Return exact neutral roots during isoelectric bisection

pyteomics/electrochem.py

@@ -313,11 +313,17 @@ def pI(sequence, pI_range=(0.0, 14.0), precision_pI=0.01, **kwargs):
     left_x, right_x = pI_range
     left_y = _charge_for_dict(peptide_dict, [left_x], pK)[0]
     right_y = _charge_for_dict(peptide_dict, [right_x], pK)[0]
+    if left_y == 0:
+        return left_x
+    if right_y == 0:
+        return right_x
     while (right_x - left_x) > precision_pI:
         if left_y * right_y > 0:
             return left_x if abs(left_y) < abs(right_y) else right_x
         middle_x = (left_x + right_x) / 2.0
         middle_y = _charge_for_dict(peptide_dict, [middle_x], pK)[0]
+        if middle_y == 0:
+            return middle_x
         if middle_y * left_y < 0:
             right_x = middle_x
             right_y = middle_y

tests/test_electrochem.py

@@ -72,6 +72,25 @@ def test_pI_precision(self):
             self.assertTrue(
                 abs(pI('PEPTIDE', precision_pI=precision) - pI_best) < precision)
 
+    def test_pI_exact_root(self):
+        # Only the two terminal groups ionize, so the root is their mean pKa.
+        expected = (2.34 + 9.69) / 2.0
+        for sequence in ('AAA', ['H-', 'A', 'A', 'A', '-OH'],
+                         {'H-': 1, 'A': 3, '-OH': 1}):
+            for bounds in ((2.34, 9.69), (expected, 14.0), (0.0, expected)):
+                with self.subTest(sequence=sequence, bounds=bounds):
+                    self.assertEqual(pI(sequence, pI_range=bounds), expected)
+
+    def test_pI_exact_root_custom_pK(self):
+        self.assertEqual(
+            pI('AAA', pK={'H-': [(9.0, 1)], '-OH': [(5.0, -1)]}),
+            7.0)
+        self.assertEqual(
+            pI('AAA', pK={'H-': [(9.0, 1)], '-OH': [(5.0, -1)]},
+               pI_range=(5.0, 7.0),
+               pK_nterm={'H-': {'A': [(7.0, 1)]}}),
+            6.0)
+
     def test_charge_input(self):
         for i in range(0, 14):
             self.assertAlmostEqual(